From Discover Magazine: “The Milky Way’s Supermassive Black Hole Erupted With a Violent Flare a Few Million Years Ago”

DiscoverMag

From Discover Magazine

October 9, 2019
Erika K. Carlson

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A flare erupted from the Milky Way’s center some 3.5 million years ago. While Earth wouldn’t be in any danger if it happened today, the light would be clearly visible. (Credit: James Josephides/ASTRO 3D)

Astronomers believe supermassive black holes probably lurk in the centers of most large galaxies. These gargantuan black holes can gather swirling disks of material around them as their gravity attracts stars and gases. In some cases, these disks can emit vast amounts of light and even shoot huge jets of matter into space. The center of such an eventful galaxy is called an active galactic nucleus, or AGN.

Our own Milky Way seems to have a relatively calm center, but astronomers suspect this wasn’t always the case.

Some clues suggest that a flare of energetic radiation burst from our galaxy’s center within the last few million years. In a new study, a team of researchers now describe another piece of evidence that the Milky Way burped out such a flare, with the research also pointing to the supermassive black hole in our galaxy’s center, called Sagittarius A*, or Sgr A*, as being responsible.

The team estimated this event occurred about 3.5 million years ago, give or take a million years. That would mean the Milky Way’s center transitioned from an active to a quiet phase pretty recently in Earth’s history, possibly when early human ancestors were roaming the planet.

The flare would have been visible to the naked eye, shining about 10 times fainter than the full moon across a broad spectrum of light wavelengths.

“It would look like the cone of light from a movie projector as it passes through a smoky theater,” University of Sydney astrophysicist and lead study author Jonathan Bland-Hawthorn said in an email.

The researchers describe their findings in an upcoming paper in The Astrophysical Journal.

Following the Trail

Clues to the Milky Way’s active history include giant bubbles of gas ballooning out from the disk of the galaxy. The bubbles, which emit high-energy X-ray and gamma-ray radiation, could have formed when jets of matter shot out from the galaxy’s center.

The new piece of evidence comes from examining a stream of gas that arcs around the Milky Way. This stream is like a trail that two dwarf galaxies, called the Large and Small Magellanic Clouds, leave as they orbit the Milky Way. The research team studied ultraviolet light coming from this gas trail, called the Magellanic Stream.

The characteristics of the UV light indicate that gases in some sections of the stream are in an excited state. Only a very energetic event, like a beam of radiation from an active galactic nucleus, could have done this, according to Bland-Hawthorn. This means that our own home galaxy had an active galactic nucleus phase in the past.

“I think AGN flickering is what goes on for all of cosmic time,” Bland-Hawthorn said via email. “All galaxies are doing this” — like volcanoes that can lie quietly for long stretches of time but suddenly erupt.

Learning more about the central black hole of our galaxy is an exciting area of research, he added.

“I think Sgr A* is the future of astrophysics, like searching for life signatures around planets,” Bland-Hawthorn said. “I am excited by what we will learn over the next 50 years.”

See the full article here .

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From Science News: “The M87 black hole image showed the best way to measure black hole masses”

From Science News

April 22, 2019
Lisa Grossman

Its diameter suggests the black hole is 6.5 billion times the mass of the sun.

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SUPERMASSIVE SOURCE The gases and stars in galaxy Messier 87, shown in this composite image from the Chandra X-ray telescope and the Very Large Array, gave different numbers for the mass of the galaxy’s supermassive black hole.

NASA/Chandra X-ray Telescope

NRAO/Karl V Jansky Expanded Very Large Array, on the Plains of San Agustin fifty miles west of Socorro, NM, USA, at an elevation of 6970 ft (2124 m)

The measure of a black hole is what it does with its stars.

That’s one lesson astronomers are taking from the first-ever picture of a black hole, released on April 10 by an international telescope team (SN Online: 4/10/19).

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SHADOW SIZE The Event Horizon Telescope captured the first image of M87’s black hole. That image showed that the black hole’s mass is about 6.5 billion times the mass of the sun, close to what astronomers expected based on the galaxy’s stars.

That image confirmed that the mass of the supermassive black hole in the center of galaxy Messier 87 is close to what astronomers expected from how nearby stars orbit — solving a long-standing debate over how best to measure a black hole’s mass.

The black hole in Messier 87, which is located about 55 million light-years from Earth, is the first black hole whose mass has been calculated by three precise methods: measuring the motion of stars, the swirl of surrounding gases and now, thanks to the Event Horizon Telescope imaging project, the diameter of the black hole’s shadow.

EHT map

In 1978, the first mass estimates to track the motions of stars whipping around the great gravitational center found that the stars must be orbiting something containing about 5 billion times the mass of the sun. A more precise estimate in 2011 using a similar stellar technique bumped its heft up to 6.6 billion times the mass of the sun.

Star S0-2 Andrea Ghez Keck/UCLA Galactic Center Group at SGR A*, the supermassive black hole at the center of the milky way

Meanwhile, astronomers in 1994 made another estimate by tracing how gases closer to the black hole than the stars swirl around the behemoth. That technique suggested that the black hole was 2.4 billion solar masses, which was revised in 2013 to 3.5 billion solar masses.

For years, it wasn’t clear which technique got closer to the truth.

Now the EHT picture showing a glowing orange ring of gases and dust around the black hole has solved the conflict. According to Einstein’s general theory of relativity, the diameter of the dark space in the center of the image — the black hole’s shadow — is directly related to its mass.

“Bigger black holes cast bigger shadows,” EHT team member Michael Johnson, an astrophysicist at the Harvard Smithsonian Center for Astrophysics, said April 12 at a talk at MIT. “Easy check, we can see whether one or the other of these [mass measuring methods] is correct.” The shadow of M87’s black hole yielded a diameter of 38 billion kilometers, which let astronomers calculate a mass of 6.5 billion suns [The Astrophysical Journal Letters]— very close to the mass suggested by the motion of stars.

The size of the shadow also negated the idea that the black hole is a wormhole, a theoretical bridge between distant points in spacetime (SN: 5/31/14, p. 16). If M87’s black hole had been a wormhole, theory predicts it should look smaller than it does. “It’s a stunning confirmation” of general relativity, Johnson said. “We instantly rule out all these exotic possibilities.”

The mass confirmation may boost confidence in current simulations for how black holes develop, says Priyamvada Natarajan, a Yale University astrophysicist who was not involved with the EHT project. Most black hole mass estimates already use the stellar technique, in part because it’s easier to track a galaxy’s stars from farther away.

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STARS AND STREAKS Astrophysicists have used both stars and gases to weigh in on the mass of the black hole in the galaxy NGC 4258, shown in this composite image. P.Ogle et al/Caltech/CXC/NASA, R.Gendler, STScI/NASA, Caltech-JPL/NASA, VLA/NRAO/NSF

SGR A* , the supermassive black hole at the center of the Milky Way. NASA’s Chandra X-Ray Observatory

Two other black holes whose masses have been measured in multiple ways, the Milky Way’s Sagittarius A* [Astronomy and Astrophysics] and the galaxy NGC 4258’s black hole, also suggest the star method works better. “These three cases now offer renewed faith in our current method,” Natarajan says.

That faith won’t solve the most pressing black hole problems, such as how black holes grew so big so fast in the early universe — at least not right away (SN Online: 3/16/18). The gas versus star measurement of the M87 black hole mass differed by only a factor of two, which is not enough to explain how it got so massive in the first place. A black hole could double its mass in about a million years, at most.

“What we don’t know is how we get supermassive black holes within a billion years,” says Hannalore Gerling-Dunsmore, a former Caltech physicist who is joining the University of Colorado Boulder later this year. She was not on the EHT team. “Once you’re already that big, what’s a million years between friends?”

See the full article here .


NSF press conference on the EHT Messier 87 Black Hole project


European Research Council press conference on the EHT Messier 87 Black Hole project


Katie Bouman on the EHT Messier 87 Black Hole project at Caltech


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From Nautilus: “First Black-Hole Image: It’s Not Looks That Count”

Nautilus

From Nautilus

Apr 11, 2019
Sabine Hossenfelder

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FIRST LOOK: The Event Horizon Telescope measures wavelength in the millimeter regime, too long to be seen by eye, but ideally suited to the task of imaging a black hole: The gas surrounding the black hole is almost transparent at this wavelength and the light travels to Earth almost undisturbed. Since we cannot see light of such wavelength by eye, the released telescope image shows the observed signal shifted into the visible range.Event Horizon Telescope Collaboration.

“The Day Feynman Worked Out Black-Hole Radiation on My Blackboard”
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After a few minutes, Richard Feynman had worked out the process of spontaneous emission, which is what Stephen Hawking became famous for a year later.Wikicommons.

The Italian 14th-century painter, Giotto di Bondone, when asked by the Pope to prove his talent, is said to have swung his arm and drawn a perfect circle. But geometric perfection is limited by the medium. Inspect a canvas closely enough, and every circle will eventually appear grainy. If perfection is what you seek, don’t look at man-made art, look at the sky. More precisely, look at a black hole.

Looking at a black hole is what the Event Horizon Telescope has done for the past 12 years. Yesterday, the collaboration released the long-awaited results from its first full run in April 2017. Contrary to expectation, their inaugural image is not, as many expected, Sagittarius A*, the black hole at the center of the Milky Way. Instead, it is the supermassive black hole in the elliptic galaxy Messier 87, about 55 million light-years from here. This black hole weighs in at 6.5 billion times the mass of our sun, and is considerably larger than the black hole in our own galaxy [1,000 times the size of SGR A*]. So, even though the Messier 87 black hole is a thousand times farther away than Sagittarius A*, it still appears half the size in the sky.

The Event Horizon Telescope (EHT) is not less remarkable than the objects it observes. With a collaboration of 200 people, the EHT uses not a single telescope, but a global network of nine telescopes. Its sites, from Greenland to the South Pole and from Hawaii to the French Alps, act in concert as one. Together, the collaboration commands a telescope the size of planet Earth, staring at a tiny patch in the northern sky that contains the Messier-87 black hole.

Event Horizon Telescope Array

Arizona Radio Observatory
Arizona Radio Observatory/Submillimeter-wave Astronomy (ARO/SMT)

ESO/APEX
Atacama Pathfinder EXperiment

CARMA Array no longer in service
Combined Array for Research in Millimeter-wave Astronomy (CARMA)

Atacama Submillimeter Telescope Experiment (ASTE)
Atacama Submillimeter Telescope Experiment (ASTE)

Caltech Submillimeter Observatory
Caltech Submillimeter Observatory (CSO)

IRAM 30m Radio telescope, on Pico Veleta in the Spanish Sierra Nevada,, Altitude 2,850 m (9,350 ft)


Institut de Radioastronomie Millimetrique (IRAM) 30m

James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA
James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA

Large Millimeter Telescope Alfonso Serrano
Large Millimeter Telescope Alfonso Serrano

CfA Submillimeter Array Mauna Kea, Hawaii, USA, Altitude 4,080 m (13,390 ft)

Submillimeter Array Hawaii SAO

ESO/NRAO/NAOJ ALMA Array
ESO/NRAO/NAOJ ALMA Array, Chile [recently added]

South Pole Telescope SPTPOL
South Pole Telescope SPTPOL [recently added]

Future Array/Telescopes

NOEMA (NOrthern Extended Millimeter Array) will double the number of its 15 meter antennas of its predecessor from six to twelve, located in the French Alpes on the wide and isolated Plateau de Bure at an elevation of 2550 meters

NSF CfA Greenland telescope


Greenland Telescope

ARO 12m Radio Telescope, Kitt Peak National Observatory, Arizona, USA, Altitude 1,914 m (6,280 ft)


ARO 12m Radio Telescope

In theory, black holes are regions of space where the gravitational pull is so large that everything, including light, becomes trapped for eternity. The surface of the trapping region is called the “event horizon.” It has no substance; it is a property of space itself. In the simplest case, the event horizon is a sphere—a perfect sphere, made of nothing.

In reality, it’s complicated. Astrophysicists have had evidence for the existence of black holes since the 1990s, but so far all observations have been indirect—inferred from the motion of visible stars and gas, leaving doubt as to whether the dark object really possesses the defining event horizon. It turned out difficult to actually see a black hole. Trouble is, they’re black. They trap light. And while Stephen Hawking proved that black holes must emit radiation due to quantum effects, this quantum glow is far too feeble to observe.

But much like the prisoners in Plato’s cave, we can see black holes by observing the shadows they cast. Black holes attract gas from their environment. This gas collects in a spinning disk, and heats up as it spirals into the event horizon, pushing around electric charges. This gives rise to strong magnetic fields that can create a “jet,” a narrow, directed stream of particles leaving the black hole at almost the speed of light. But whatever strays too close to the event horizon falls in and vanishes without a trace.

At the same time black holes bend rays of light, bend them so strongly, indeed, that looking at the front of a black hole, we can see part of the disk behind it. The light that just about manages to escape reveals what happens nearby the horizon. It is an asymmetric image that the astrophysicists expect, brighter on the side of the black hole where the material surrounding it moves toward us, and darker where it moves away from us. The hot gas combined with the gravitational lensing creates the unique observable signature that the EHT looks out for.

The experimental challenge is formidable. The network’s telescopes must synchronize their data-taking using atomic clocks. Weather conditions must be favorable at all locations simultaneously. Once recorded, the amount of data is so staggeringly large, it must be shipped on hard disks to central locations for processing.

The theoretical challenges are not any lesser. Black holes bend light so much that it can wrap around the horizon multiple times. The resulting image is too complicated to capture in simple equations. Though the math had been known since the 1920s, it wasn’t until 1978 that physicists got a first glimpse of what a black hole would actually look like. In that year, the French astrophysicist Jean-Pierre Luminet programmed the calculation on an IBM 7040 using punchcards. He drew the image by hand.

Today, astrophysicists use computers many times more powerful to predict the accretion of gas onto the black hole and how the light bends before reaching us. Still, the partly turbulent motion of the gas, the electric and magnetic fields created by it, and the intricacies of the particle’s interactions are not fully understood.

The EHT’s observations agree with expectation. But this result is more than just another triumph of Einstein’s theory of general relativity. It is also a triumph of the astronomers’ resourcefulness. They joined hands and brains to achieve what they could not have done separately. And while their measurement settles a long-standing question—yes, black holes really do have event horizons!—it is also the start of further exploration. Physicists hope that the observations will help them understand better the extreme conditions in the accretion disk, the role of magnetic fields in jet formation, and the way supermassive black holes affect galaxy formation.

When the Pope received Giotto’s circle, it was not the image itself that impressed him. It was the courtier’s report that the artist produced it without the aid of a compass. This first image of a black hole, too, is remarkable not so much for its appearance, but for its origin. A black sphere, spanning 40 billion kilometers, drawn on a background of hot gas by the greatest artist of all: Nature herself.

See the full article here .

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Welcome to Nautilus. We are delighted you joined us. We are here to tell you about science and its endless connections to our lives. Each month we choose a single topic. And each Thursday we publish a new chapter on that topic online. Each issue combines the sciences, culture and philosophy into a single story told by the world’s leading thinkers and writers. We follow the story wherever it leads us. Read our essays, investigative reports, and blogs. Fiction, too. Take in our games, videos, and graphic stories. Stop in for a minute, or an hour. Nautilus lets science spill over its usual borders. We are science, connected.

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From European Southern Observatory: “Most Detailed Observations of Material Orbiting close to a Black Hole”

ESO 50 Large

From European Southern Observatory

31 October 2018

Oliver Pfuhl
Max Planck Institute for Extraterrestrial Physics
Garching bei München, Germany
Tel: +49 89 30 000 3295
Email: pfuhl@mpe.mpg.de

Jason Dexter
Max Planck Institute for Extraterrestrial Physics
Garching bei München, Germany
Tel: +49 89 30 000 3324
Email: jdexter@mpe.mpg.de

Thibaut Paumard
CNRS Researcher
Observatoire de Paris, France
Tel: +33 145 077 5451
Email: thibaut.paumard@obspm.fr

Xavier Haubois
ESO Astronomer
Santiago, Chile
Tel: +56 2 2463 3055
Email: xhaubois@eso.org

IR Group Secretariat
Max Planck Institute for Extraterrestrial Physics
Garching bei München, Germany
Tel: +49 89 30000 3880
Email: ir-office@mpe.mpg.de

Hannelore Hämmerle
Public Information Officer, Max Planck Institute for Extraterrestrial Physics
Garching bei München, Germany
Tel: +49 89 30 000 3980
Email: hannelore.haemmerle@mpe.mpg.de

Calum Turner
ESO Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6670
Email: pio@eso.org

ESO’s GRAVITY instrument confirms black hole status of the Milky Way centre.

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This visualisation uses data from simulations of orbital motions of gas swirling around at about 30% of the speed of light on a circular orbit around the black hole. Credit:
ESO/Gravity Consortium/L. Calçada

ESO’s exquisitely sensitive GRAVITY instrument has added further evidence to the long-standing assumption that a supermassive black hole lurks in the centre of the Milky Way. New observations show clumps of gas swirling around at about 30% of the speed of light on a circular orbit just outside its event horizon — the first time material has been observed orbiting close to the point of no return, and the most detailed observations yet of material orbiting this close to a black hole.

ESO GRAVITY insrument on The VLTI, interferometric instrument operating in the K band, between 2.0 and 2.4 μm. It combines 4 telescope beams and is designed to peform both interferometric imaging and astrometry by phase referencing. Credit: MPE/GRAVITY team

ESO’s GRAVITY instrument on the Very Large Telescope (VLT) Interferometer has been used by scientists from a consortium of European institutions, including ESO [1], to observe flares of infrared radiation coming from the accretion disc around Sagittarius A*, the massive object at the heart of the Milky Way.

SgrA* NASA/Chandra

Sgr A* from ESO VLT

SGR A* , the supermassive black hole at the center of the Milky Way. NASA’s Chandra X-Ray Observatory

The observed flares provide long-awaited confirmation that the object in the centre of our galaxy is, as has long been assumed, a supermassive black hole. The flares originate from material orbiting very close to the black hole’s event horizon — making these the most detailed observations yet of material orbiting this close to a black hole.

While some matter in the accretion disc — the belt of gas orbiting Sagittarius A* at relativistic speeds [2] — can orbit the black hole safely, anything that gets too close is doomed to be pulled beyond the event horizon. The closest point to a black hole that material can orbit without being irresistibly drawn inwards by the immense mass is known as the innermost stable orbit, and it is from here that the observed flares originate.

“It’s mind-boggling to actually witness material orbiting a massive black hole at 30% of the speed of light,” marvelled Oliver Pfuhl, a scientist at the MPE. “GRAVITY’s tremendous sensitivity has allowed us to observe the accretion processes in real time in unprecedented detail.”

These measurements were only possible thanks to international collaboration and state-of-the-art instrumentation [3]. The GRAVITY instrument which made this work possible combines the light from four telescopes of ESO’s VLT to create a virtual super-telescope 130 metres in diameter, and has already been used to probe the nature of Sagittarius A*.

Earlier this year, GRAVITY and SINFONI, another instrument on the VLT, allowed the same team to accurately measure the close fly-by of the star S2 as it passed through the extreme gravitational field near Sagittarius A*, and for the first time revealed the effects predicted by Einstein’s general relativity in such an extreme environment.

Star SO-2 Keck/UCLA Galactic Center Group

ESO/SINFONI

ESO SINFONI

During S0-2’s close fly-by, strong infrared emission was also observed.

“We were closely monitoring S0-2, and of course we always keep an eye on Sagittarius A*,” explained Pfuhl. “During our observations, we were lucky enough to notice three bright flares from around the black hole — it was a lucky coincidence!”

This emission, from highly energetic electrons very close to the black hole, was visible as three prominent bright flares, and exactly matches theoretical predictions for hot spots orbiting close to a black hole of four million solar masses [4]. The flares are thought to originate from magnetic interactions in the very hot gas orbiting very close to Sagittarius A*.

Reinhard Genzel, of the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, who led the study, explained: “This always was one of our dream projects but we did not dare to hope that it would become possible so soon.” Referring to the long-standing assumption that Sagittarius A* is a supermassive black hole, Genzel concluded that “the result is a resounding confirmation of the massive black hole paradigm.”

Notes

[1] This research was undertaken by scientists from the Max Planck Institute for Extraterrestrial Physics (MPE), the Observatoire de Paris, the Université Grenoble Alpes, CNRS, the Max Planck Institute for Astronomy, the University of Cologne, the Portuguese CENTRA – Centro de Astrofisica e Gravitação and ESO.

[2] Relativistic speeds are those which are so great that the effects of Einstein’s Theory of Relativity become significant. In the case of the accretion disc around Sagittarius A*, the gas is moving at roughly 30% of the speed of light.

[3] GRAVITY was developed by a collaboration consisting of the Max Planck Institute for Extraterrestrial Physics (Germany), LESIA of Paris Observatory–PSL/CNRS/Sorbonne Université/Univ. Paris Diderot and IPAG of Université Grenoble Alpes/CNRS (France), the Max Planck Institute for Astronomy (Germany), the University of Cologne (Germany), the CENTRA–Centro de Astrofísica e Gravitação (Portugal) and ESO.

[4] The solar mass is a unit used in astronomy. It is equal to the mass of our closest star, the Sun, and has a value of 1.989 × 1030 kg. This means that Sgr A* has a mass 1.3 trillion times greater than the Earth.

More information

This research was presented in a paper entitled Detection of Orbital Motions Near the Last Stable Circular Orbit of the Massive Black Hole SgrA*, by the GRAVITY Collaboration, published in the journal Astronomy & Astrophysics on 31 October 2018.

The GRAVITY Collaboration team is composed of: R. Abuter (ESO, Garching, Germany), A. Amorim (Universidade de Lisboa, Lisbon, Portugal), M. Bauböck (Max Planck Institute for Extraterrestrial Physics, Garching, Germany [MPE]), J.P. Berger (Univ. Grenoble Alpes, CNRS, IPAG, Grenoble, France [IPAG]; ESO, Garching, Germany), H. Bonnet (ESO, Garching, Germany), W. Brandner (Max Planck Institute for Astronomy, Heidelberg, Germany [MPIA]), Y. Clénet (LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, Univ. Paris Diderot, Meudon, France [LESIA])), V. Coudé du Foresto (LESIA), P. T. de Zeeuw (Sterrewacht Leiden, Leiden University, Leiden, The Netherlands; MPE), C. Deen (MPE), J. Dexter (MPE), G. Duvert (IPAG), A. Eckart (University of Cologne, Cologne, Germany; Max Planck Institute for Radio Astronomy, Bonn, Germany), F. Eisenhauer (MPE), N.M. Förster Schreiber (MPE), P. Garcia (Universidade do Porto, Porto, Portugal; Universidade de Lisboa Lisboa, Portugal), F. Gao (MPE), E. Gendron (LESIA), R. Genzel (MPE; University of California, Berkeley, California, USA), S. Gillessen (MPE), P. Guajardo (ESO, Santiago, Chile), M. Habibi (MPE), X. Haubois (ESO, Santiago, Chile), Th. Henning (MPIA), S. Hippler (MPIA), M. Horrobin (University of Cologne, Cologne, Germany), A. Huber (MPIA), A. Jimenez Rosales (MPE), L. Jocou (IPAG), P. Kervella (LESIA; MPIA), S. Lacour (LESIA), V. Lapeyrère (LESIA), B. Lazareff (IPAG), J.-B. Le Bouquin (IPAG), P. Léna (LESIA), M. Lippa (MPE), T. Ott (MPE), J. Panduro (MPIA), T. Paumard (LESIA), K. Perraut (IPAG), G. Perrin (LESIA), O. Pfuhl (MPE), P.M. Plewa (MPE), S. Rabien (MPE), G. Rodríguez-Coira (LESIA), G. Rousset (LESIA), A. Sternberg (School of Physics and Astronomy, Tel Aviv University, Tel Aviv, Israel, Center for Computational Astrophysics, Flatiron Institute, New York, USA), O. Straub (LESIA), C. Straubmeier (University of Cologne, Cologne, Germany), E. Sturm (MPE), L.J. Tacconi (MPE), F. Vincent (LESIA), S. von Fellenberg (MPE), I. Waisberg (MPE), F. Widmann (MPE), E. Wieprecht (MPE), E. Wiezorrek (MPE), J. Woillez (ESO, Garching, Germany), S. Yazici (MPE; University of Cologne, Cologne, Germany).

See the full article here .


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ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre EEuropean Extremely Large Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

ESO La Silla HELIOS (HARPS Experiment for Light Integrated Over the Sun)

ESO 3.6m telescope & HARPS at Cerro LaSilla, Chile, 600 km north of Santiago de Chile at an altitude of 2400 metres.

ESO 2.2 meter telescope at La Silla, 600 km north of Santiago de Chile at an altitude of 2400 metres.

ESO/Cerro LaSilla, 600 km north of Santiago de Chile at an altitude of 2400 metres.

ESO VLT Platform at Cerro Paranal elevation 2,635 m (8,645 ft)


ESO VLT 4 lasers on Yepun

Glistening against the awesome backdrop of the night sky above ESO_s Paranal Observatory, four laser beams project out into the darkness from Unit Telescope 4 UT4 of the VLT.

ESO/NTT at Cerro La Silla, Chile, at an altitude of 2400 metres



ESO/Vista Telescope at Cerro Paranal, with an elevation of 2,635 metres (8,645 ft) above sea level

ESO/NRAO/NAOJ ALMA Array in Chile in the Atacama at Chajnantor plateau, at 5,000 metres

ESO/E-ELT,to be on top of Cerro Armazones in the Atacama Desert of northern Chile. located at the summit of the mountain at an altitude of 3,060 metres (10,040 ft).

ESO/APEX high on the Chajnantor plateau in Chile’s Atacama region, at an altitude of over 4,800 m (15,700 ft)

Leiden MASCARA instrument, La Silla, located in the southern Atacama Desert 600 kilometres (370 mi) north of Santiago de Chile at an altitude of 2,400 metres (7,900 ft)

Leiden MASCARA cabinet at ESO Cerro la Silla located in the southern Atacama Desert 600 kilometres (370 mi) north of Santiago de Chile at an altitude of 2,400 metres (7,900 ft)

ESO Next Generation Transit Survey at Cerro Paranel, 2,635 metres (8,645 ft) above sea level

SPECULOOS four 1m-diameter robotic telescopes 2016 in the ESO Paranal Observatory, 2,635 metres (8,645 ft) above sea level

ESO TAROT telescope at Paranal, 2,635 metres (8,645 ft) above sea level

ESO ExTrA telescopes at Cerro LaSilla at an altitude of 2400 metres

#astronomy, #astrophysics, #basic-research, #cosmology, #eso-gravity, #eso-vlti, #most-detailed-observations-of-material-orbiting-close-to-a-black-hole, #sgr-a, #star-s2-keck-ucla-galactic-center-group

From The New York Times: “Trolling the Monster in the Heart of the Milky Way”

New York Times

From The New York Times

Oct. 30, 2018
Dennis Overbye

In a dark, dusty patch of sky in the constellation Sagittarius, a small star, known as S2 or, sometimes, S0-2, cruises on the edge of eternity. Every 16 years, it passes within a cosmic whisker of a mysterious dark object that weighs some 4 million suns, and that occupies the exact center of the Milky Way galaxy.

Star S0-2 Keck/UCLA Galactic Center Group

For the last two decades, two rival teams of astronomers, looking to test some of Albert Einstein’s weirdest predictions about the universe, have aimed their telescopes at the star, which lies 26,000 light-years away. In the process, they hope to confirm the existence of what astronomers strongly suspect lies just beyond: a monstrous black hole, an eater of stars and shaper of galaxies.

For several months this year, the star streaked through its closest approach to the galactic center, producing new insights into the behavior of gravity in extreme environments, and offering clues to the nature of the invisible beast in the Milky Way’s basement.

One of those teams, an international collaboration based in Germany and Chile, and led by Reinhard Genzel, of the Max Planck Institute for Extraterrestrial Physics, say they have found the strongest evidence yet that the dark entity is a supermassive black hole, the bottomless grave of 4.14 million suns.

ESO VLT at Cerro Paranal in the Atacama Desert, •ANTU (UT1; The Sun ),
•KUEYEN (UT2; The Moon ),
•MELIPAL (UT3; The Southern Cross ), and
•YEPUN (UT4; Venus – as evening star).
elevation 2,635 m (8,645 ft) from above Credit J.L. Dauvergne & G. Hüdepohl atacama photo

ESO VLT 4 lasers on Yepun

The evidence comes in the form of knots of gas that appear to orbit the galactic center. Dr. Genzel’s team found that the gas clouds circle every 45 minutes or so, completing a circuit of 150 million miles at roughly 30 percent of the speed of light. They are so close to the alleged black hole that if they were any closer they would fall in, according to classical Einsteinian physics.

Astrophysicists can’t imagine anything but a black hole that could be so massive, yet fit within such a tiny orbit.

The results provide “strong support” that the dark thing in Sagittarius “is indeed a massive black hole,” Dr. Genzel’s group writes in a paper that will be published on Wednesday under the name of Gravity Collaboration, in the European journal Astronomy & Astrophysics.

“This is the closest yet we have come to see the immediate zone around a supermassive black hole with direct, spatially resolved techniques,” Dr. Genzel said in an email.

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Reinhard Genzel runs the Max Planck Institute for Extraterrestrial Physics in Munich. He has been watching S2, in the constellation Sagittarius, hoping it will help confirm the existence of a supermassive black hole.Credit Ksenia Kuleshova for The New York Times.

The work goes a long way toward demonstrating what astronomers have long believed, but are still at pains to prove rigorously: that a supermassive black hole lurks in the heart not only of the Milky Way, but of many observable galaxies. The hub of the stellar carousel is a place where space and time end, and into which stars can disappear forever.

The new data also help to explain how such black holes can wreak havoc of a kind that is visible from across the universe. Astronomers have long observed spectacular quasars and violent jets of energy, thousands of light-years long, erupting from the centers of galaxies.

Roger Blandford, the director of the Kavli Institute for Particle Astrophysics and Cosmology at Stanford University, said that there is now overwhelming evidence that supermassive black holes are powering such phenomena.

“There is now a large burden of proof on claims to the contrary,” he wrote in an email. “The big questions involve figuring out how they work, including disk and jets. It’s a bit like knowing that the sun is a hot, gaseous sphere and trying to understand how the nuclear reactions work.”

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Images of different galaxies — some of which have evocative names like the Black Eye Galaxy, bottom left, or the Sombrero Galaxy, second left — adorn a wall at the Max Planck Institute.Credit Ksenia Kuleshova for The New York Times.

Sheperd Doeleman, a radio astronomer at the Harvard-Smithsonian Center for Astrophysics, called the work “a tour de force.” Dr. Doeleman studies the galactic center and hopes to produce an actual image of the black hole, using a planet-size instrument called the Event Horizon Telescope.

Event Horizon Telescope Array

Arizona Radio Observatory
Arizona Radio Observatory/Submillimeter-wave Astronomy (ARO/SMT)

ESO/APEX
Atacama Pathfinder EXperiment

CARMA Array no longer in service
Combined Array for Research in Millimeter-wave Astronomy (CARMA)

Atacama Submillimeter Telescope Experiment (ASTE)
Atacama Submillimeter Telescope Experiment (ASTE)

Caltech Submillimeter Observatory
Caltech Submillimeter Observatory (CSO)

IRAM NOEMA interferometer
Institut de Radioastronomie Millimetrique (IRAM) 30m

James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA
James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA

Large Millimeter Telescope Alfonso Serrano
Large Millimeter Telescope Alfonso Serrano

CfA Submillimeter Array Hawaii SAO
Submillimeter Array Hawaii SAO

ESO/NRAO/NAOJ ALMA Array
ESO/NRAO/NAOJ ALMA Array, Chile

South Pole Telescope SPTPOL
South Pole Telescope SPTPOL

NSF CfA Greenland telescope

Greenland Telescope

Future Array/Telescopes

Plateau de Bure interferometer
Plateau de Bure interferometer

The study is also a major triumph for the European Southern Observatory, a multinational consortium with headquarters in Munich and observatories in Chile, which had made the study of S2 and the galactic black hole a major priority. The organization’s facilities include the Very Large Telescope [shown above], an array of four giant telescopes in Chile’s Atacama Desert (a futuristic setting featured in the James Bond film “Quantum of Solace”), and the world’s largest telescope, the Extremely Large Telescope, now under construction on a mountain nearby.

ESO/E-ELT,to be on top of Cerro Armazones in the Atacama Desert of northern Chile. located at the summit of the mountain at an altitude of 3,060 metres (10,040 ft).

Einstein’s bad dream

Black holes — objects so dense that not even light can escape them — are a surprise consequence of Einstein’s general theory of relativity, which ascribes the phenomenon we call gravity to a warping of the geometry of space and time. When too much matter or energy are concentrated in one place, according to the theory, space-time can jiggle, time can slow and matter can shrink and vanish into those cosmic sinkholes.

Einstein didn’t like the idea of black holes, but the consensus today is that the universe is speckled with them. Many are the remains of dead stars; others are gigantic, with the masses of millions to billions of suns. Such massive objects seem to anchor the centers of virtually every galaxy, including our own. Presumably they are black holes, but astronomers are eager to know whether these entities fit the prescription given by Einstein’s theory.

Andrea Ghez, astrophysicist and professor at the University of California, Los Angeles, who leads a team of scientists observing S2 for evidence of a supermassive black hole UCLA Galactic Center Group

Although general relativity has been the law of the cosmos ever since Einstein devised it, most theorists think it eventually will have to be modified to explain various mysteries, such as what happens at the center of a black hole or at the beginning of time; why galaxies clump together, thanks to unidentified stuff called dark matter; and how, simultaneously, a force called dark energy is pushing these clumps of galaxies apart.

Women in STEM – Vera Rubin

Fritz Zwicky discovered Dark Matter when observing the movement of the Coma Cluster

Coma cluster via NASA/ESA Hubble

But most of the real work was done by Vera Rubin

Fritz Zwicky from http:// palomarskies.blogspot.com


Astronomer Vera Rubin at the Lowell Observatory in 1965, worked on Dark Matter (The Carnegie Institution for Science)


Vera Rubin measuring spectra, worked on Dark Matter (Emilio Segre Visual Archives AIP SPL)


Vera Rubin, with Department of Terrestrial Magnetism (DTM) image tube spectrograph attached to the Kitt Peak 84-inch telescope, 1970. https://home.dtm.ciw.edu

Dark Energy Survey


Dark Energy Camera [DECam], built at FNAL


NOAO/CTIO Victor M Blanco 4m Telescope which houses the DECam at Cerro Tololo, Chile, housing DECam at an altitude of 7200 feet

The existence of smaller black holes was affirmed two years ago, when the Laser Interferometer Gravitational-Wave Observatory, or LIGO, detected ripples in space-time caused by the collision of a pair of black holes located a billion light-years away.


VIRGO Gravitational Wave interferometer, near Pisa, Italy

Caltech/MIT Advanced aLigo Hanford, WA, USA installation


Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger

ESA/eLISA the future of gravitational wave research

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Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)

But those black holes were only 20 and 30 times the mass of the sun; how supermassive black holes behave is the subject of much curiosity among astronomers.

“We already know Einstein’s theory of gravity is fraying around the edges,” said Andrea Ghez, a professor at the University of California, Los Angeles. “What better places to look for discrepancies in it than a supermassive black hole?” Dr. Ghez is the leader of a separate team that, like Dr. Genzel’s, is probing the galactic center. “What I like about the galactic center is that you get to see extreme astrophysics,” she said.

Despite their name, supermassive black holes are among the most luminous objects in the universe. As matter crashes down into them, stupendous amounts of energy should be released, enough to produce quasars, the faint radio beacons from distant space that have dazzled and baffled astronomers since the early 1960s.

Women in STEM – Dame Susan Jocelyn Bell Burnell

Dame Susan Jocelyn Bell Burnell, discovered pulsars with radio astronomy. Jocelyn Bell at the Mullard Radio Astronomy Observatory, Cambridge University, taken for the Daily Herald newspaper in 1968. Denied the Nobel.

Dame Susan Jocelyn Bell Burnell 2009

Dame Susan Jocelyn Bell Burnell (1943 – ), still working from http://www. famousirishscientists.weebly.com

Astronomers have long suspected that something similar could be happening at the center of the Milky Way, which is marked by a dim source of radio noise called Sagittarius A* (pronounced Sagittarius A-star).

Sgr A* from ESO VLT


SgrA* NASA/Chandra


SGR A* , the supermassive black hole at the center of the Milky Way. NASA’s Chandra X-Ray Observatory

But the galactic center is veiled by dust, making it all but invisible to traditional astronomical ways of seeing.

Seeing in the dark

Reinhard Genzel grew up in Freiburg, Germany, a small city in the Black Forest. As a young man, he was one of the best javelin throwers in Germany, even training with the national team for the 1972 Munich Olympics. Now he is throwing deeper.

He became interested in the dark doings of the galactic center back in the 1980s, as a postdoctoral fellow at the University of California, Berkeley, under physicist Charles Townes, a Nobel laureate and an inventor of lasers. “I think of myself as a younger son of his,” Dr. Genzel said in a recent phone conversation.

In a series of pioneering observations in the early 1980s, using detectors that can see infrared radiation, or heat, through galactic dust, Dr. Townes, Dr. Genzel and their colleagues found that gas clouds were zipping around the center of the Milky Way so fast that the gravitational pull of about 4 million suns would be needed to keep it in orbit. But whatever was there, it emitted no starlight. Even the best telescopes, from 26,000 light years away, could make out no more than a blur.

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An image of the central Milky Way, which contains Sagittarius A*, taken by the VISTA telescope at the E.S.O.’s Paranal Observatory, mounted on a peak just next to the Very Large Telescope.CreditEuropean Southern Observatory/VVV Survey/D. Minniti/Ignacio Toledo, Martin Kornmesser


Part of ESO’s Paranal Observatory, the VLT Survey Telescope (VISTA) observes the brilliantly clear skies above the Atacama Desert of Chile. It is the largest survey telescope in the world in visible light.
Credit: ESO/Y. Beletsky, with an elevation of 2,635 metres (8,645 ft) above sea level

Two advances since then have helped shed some figurative light on whatever is going on in our galaxy’s core. One was the growing availability in the 1990s of infrared detectors, originally developed for military use. Another was the development of optical techniques that could drastically increase the ability of telescopes to see small details by compensating for atmospheric turbulence. (It’s this turbulence that blurs stars and makes them twinkle.)

Glistening against the awesome backdrop of the night sky above ESO_s Paranal Observatory, four laser beams project out into the darkness from Unit Telescope 4 UT4 of the VLT.

These keen eyes revealed hundreds of stars in the galaxy’s blurry core, all buzzing around in a circle about a tenth of a light year across. One of the stars, which Dr. Genzel calls S2 and Dr. Ghez calls S-02, is a young blue star that follows a very elongated orbit and passes within just 11 billion miles of the mouth of the putative black hole every 16 years.

During these fraught passages, the star, yanked around an egg-shaped orbit at speeds of up to 5,000 miles per second, should experience the full strangeness of the universe according to Einstein. Intense gravity on the star’s surface should slow the vibration of light waves, stretching them and making the star appear redder than normal from Earth.

This gravitational redshift, as it is known, was one of the first predictions of Einstein’s theory. The discovery of S2 offered astronomers a chance to observe the phenomenon in the wild — within the grip of gravity gone mad, near a supermassive black hole.

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Left, calculations left out at the Max Planck Institute, viewed from above, right.Credit Ksenia Kuleshova for The New York Times

In the wheelhouse of the galaxy

To conduct that experiment, astronomers needed to know the star’s orbit to a high precision, which in turn required two decades of observations with the most powerful telescopes on Earth. “You need twenty years of data just to get a seat at this table,” said Dr. Ghez, who joined the fray in 1995.

And so, the race into the dark was joined on two different continents. Dr. Ghez worked with the 10-meter Keck telescopes, located on Mauna Kea, on Hawaii’s Big Island.


Keck Observatory, Maunakea, Hawaii, USA.4,207 m (13,802 ft), above sea level, showing also NASA’s IRTF and NAOJ Subaru


UCO Keck Laser Guide Star Adaptive Optics

Dr. Genzel’s group benefited from the completion of the European Southern Observatory’s brand new Very Large Telescope [above] array in Chile.

The European team was aided further by a new device, an interferometer named Gravity, that combined the light from the array’s four telescopes.

ESO GRAVITY insrument on The VLTI, interferometric instrument operating in the K band, between 2.0 and 2.4 μm. It combines 4 telescope beams and is designed to peform both interferometric imaging and astrometry by phase referencing. Credit: MPE/GRAVITY team

Designed by a large consortium led by Frank Eisenhauer of the Max Planck Institute, the instrument enabled the telescope array to achieve the resolution of a single mirror 130 meters in diameter. (The name originally was an acronym for a long phrase that included words such as “general,” “relativity,” and “interferometry,” Dr. Eisenhauer explained in an email.)

“All of the sudden, we can see 1,000 times fainter than before,” said Dr. Genzel in 2016, when the instrument went into operation. In addition, they could track the movements of the star S2 from day to day.

Meanwhile, Dr. Ghez was analyzing the changing spectra of light from the star, to determine changes in the star’s velocity. The two teams leapfrogged each other, enlisting bigger and more sophisticated telescopes, and nailing down the characteristics of S2. In 2012 Dr. Genzel and Dr. Ghez shared the Crafoord Prize in astronomy, an award nearly as prestigious as the Nobel. Events came to head this spring and summer, during a six-month period when S2 made its closest approach to the black hole.

“It was exciting in the middle of April when a signal emerged and we started getting information,” Dr. Ghez said.

On July 26, Dr. Genzel and Dr. Eisenhauer held a news conference in Munich to announce that they had measured the long-sought gravitational redshift. As Dr. Eisenhauer marked off their measurements, which matched a curve of expected results, the room burst into applause.

“The road is wide open to black hole physics,” Dr. Eisenhauer proclaimed.

In an email a month later, Dr. Genzel explained that detecting the gravitational redshift was only the first step: “I am usually a fairly sober, and sometimes pessimistic person. But you may sense my excitement as I write these sentences, because of these wonderful results. As a scientist (and I am 66 years old) one rarely if ever has phases this productive. Carpe Diem!”

In early October, Dr. Ghez, who had waited to observe one more phase of the star’s trip, said her team soon would publish their own results.

A monster in the basement

In the meantime, Dr. Genzel was continuing to harvest what he called “this gift from nature.”

The big break came when his team detected evidence of hot spots, or “flares,” in the tiny blur of heat marking the location of the suspected black hole. A black hole with the mass of 4 million suns should have a mouth, or event horizon, about 16 million miles across — too small for even the Gravity instrument to resolve from Earth.

The hot spots were also too small to make out. But they rendered the central blur lopsided, with more heat on one side of the blur than the other. As a result, Dr. Genzel’s team saw the center of that blur of energy shift, or wobble, relative to the position of S2, as the hot spot went around it.

As a result, said Dr. Genzel, “We see a little loop on the sky.” Later he added, “This is the first time we can study these important magnetic structures in a spatially resolved manner just like in a physics laboratory.”

He speculated that the hot spots might be produced by shock waves in magnetic fields, much as solar flares erupt from the sun. But this might be an overly simplistic model, the authors cautioned in their paper. The effects of relativity turn the neighborhood around the black hole into a hall of mirrors, Dr. Genzel said: “Our statements currently are still fuzzy. We will have to learn better to reconstruct reality once we better understand exactly these mirages.”

The star has finished its show for this year. Dr. Genzel hopes to gather more data from the star next year, as it orbits more distantly from the black hole. Additional observations in the coming years may clarify the star’s orbit, and perhaps answer other questions, such as whether the black hole was spinning, dragging space-time with it like dough in a mixer.

But it may be hard for Dr. Genzel to beat what he has already accomplished, he said by email. For now, shrink-wrapping 4 million suns worth of mass into a volume just 45 minutes around was a pretty good feat “for a small boy from the countryside.”

See the full article here .

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From Ethan Siegel: “This Is How We Will Successfully Image A Black Hole’s Event Horizon”

From Ethan Siegel
Oct 10, 2018

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Five different simulations in general relativity, using a magnetohydrodynamic model of the black hole’s accretion disk, and how the radio signal will look as a result. Note the clear signature of the event horizon in all the expected results. (GRMHD SIMULATIONS OF VISIBILITY AMPLITUDE VARIABILITY FOR EVENT HORIZON TELESCOPE IMAGES OF SGR A*, L. MEDEIROS ET AL., ARXIV:1601.06799)

As the Event Horizon Telescope prepares to release its first results, we can expect not just one, but two black hole images.

Event Horizon Telescope Array

Arizona Radio Observatory
Arizona Radio Observatory/Submillimeter-wave Astronomy (ARO/SMT)

ESO/APEX
Atacama Pathfinder EXperiment

CARMA Array no longer in service
Combined Array for Research in Millimeter-wave Astronomy (CARMA)

Atacama Submillimeter Telescope Experiment (ASTE)
Atacama Submillimeter Telescope Experiment (ASTE)

Caltech Submillimeter Observatory
Caltech Submillimeter Observatory (CSO)

IRAM NOEMA interferometer
Institut de Radioastronomie Millimetrique (IRAM) 30m

James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA
James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA

Large Millimeter Telescope Alfonso Serrano
Large Millimeter Telescope Alfonso Serrano

CfA Submillimeter Array Hawaii SAO
Submillimeter Array Hawaii SAO

ESO/NRAO/NAOJ ALMA Array
ESO/NRAO/NAOJ ALMA Array, Chile

South Pole Telescope SPTPOL
South Pole Telescope SPTPOL

NSF CfA Greenland telescope

Greenland Telescope

Future Array/Telescopes

Plateau de Bure interferometer
Plateau de Bure interferometer

What does a black hole actually look like? For generations, scientists argued over whether black holes actually existed or not. Sure, there were mathematical solutions in General Relativity that indicated they were possible, but not every mathematical solution corresponds to our physical reality. It took observational evidence to settle that issue.

Owing to matter orbiting and infalling around black holes, both stellar-mass versions and the supermassive versions, we’ve detected the X-ray emissions characteristic of their existences. We found and measured the motions of individual stars that orbit suspected black holes, confirming the existence of massive objects at the centers of galaxies. If only we could directly image these objects that emit no light themselves, right? Amazingly, that time is here.

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The black hole at the center of the Milky Way, along with the actual, physical size of the Event Horizon pictured in white. The visual extent of darkness will appear to be 5/2 as large as the event horizon itself.(UTE KRAUS, PHYSICS EDUCATION GROUP KRAUS, UNIVERSITÄT HILDESHEIM; BACKGROUND: AXEL MELLINGER)

In theory, a black hole is an object that cannot hold itself up against gravity. Whatever outward forces there are — including radiation, nuclear and electromagnetic forces, or even quantum degeneracy arising from the Pauli exclusion principle — must be equal and opposite to the inward force of gravity, or collapse is inevitable. If you get that gravitational collapse, you will form an event horizon.

An event horizon is the location where the fastest speed attainable, the speed of light, is exactly equal to the speed necessary to escape from the gravity of the object inside. Outside of the event horizon, light can escape. Inside the event horizon, light cannot. It’s for this reason that black holes are expected to be black: the event horizon should describe a dark sphere in space where there should be no light detectable of any type.

We see objects in the Universe that are so consistent with the expectations for a black hole that there are no good theories, at all, for what else they might be. Furthermore, we can calculate how large these event horizons should both physically be for a black hole (proportional to a black hole’s mass) and how large they should appear in General Relativity (about 2.5 times the diameter of the physical extent).

As viewed from Earth, the largest apparent black hole should be Sagittarius A*, which is the black hole at the center of the Milky Way, with an apparent size of approximately 37 micro-arc-seconds. At 4 million solar masses and a distance of around 27,000 light years, it should appear larger than any other. But the second largest one? That’s at the center of Messier 87, over 50 million light years away.

SgrA* NASA/Chandra


SGR A* , the supermassive black hole at the center of the Milky Way. NASA’s Chandra X-Ray Observatory

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The second-largest black hole as seen from Earth, the one at the center of the galaxy Messier 87, is shown in three views here. Despite its mass of 6.6 billion Suns, it is over 2000 times farther away than Sagittarius A*. It may not be resolvable by the EHT if our mass estimates are too large, but if the Universe is kind, we’ll get an image, after all. (TOP, OPTICAL, HUBBLE SPACE TELESCOPE / NASA / WIKISKY; LOWER LEFT, RADIO, NRAO / VERY LARGE ARRAY (VLA); LOWER RIGHT, X-RAY, NASA / CHANDRA X-RAY TELESCOPE)

NASA/ESA Hubble Telescope

NRAO/Karl V Jansky Expanded Very Large Array, on the Plains of San Agustin fifty miles west of Socorro, NM, USA, at an elevation of 6970 ft (2124 m)

NASA/Chandra X-ray Telescope

The reason that black hole is so huge? Because even at that incredible distance, it’s over 6 billion solar masses, meaning it should appear as roughly 3/4ths the size of the Milky Way’s black hole. Black holes are well-known for emitting radiation in the radio portion of the spectrum, as matter accelerates around the event horizon, but this gives us a brilliant way to attempt to view it: through very-long-baseline interferometry in the radio portion of the spectrum.

All we need, to make that happen, is an enormous array of radio telescopes. We need them all over the globe, so that we can take temporally simultaneous measurements of the same objects from locations up to 12,700 kilometers (8,000 miles) away: the diameter of Earth. By taking these multiple images, we can piece together an image — so long as the source we’re imaging is radio-bright enough — as small as 15 micro-arc-seconds in size.

The Event Horizon Telescope (EHT) is exactly such an array, and it’s not only been taking data from all over the world (including in Antarctica) for years, it’s already taken all the images necessary of Sagittarius A* and of Messier 87 you could hope for. All that’s left, now, is to process the data and construct the images for the general public to view.

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Two of the possible models that can successfully fit the Event Horizon Telescope data thus far, as of earlier in 2018. Both show an off-center, asymmetric event horizon that’s enlarged versus the Schwarzschild radius, consistent with the predictions of Einstein’s General Relativity. (R.-S. LU ET AL, APJ 859, 1)

We’ve already taken the data necessary to create the first black hole images ever, so what’s the hold up? What are we poised to learn? And what might surprise us about what the Universe has in store?

In theory, the event horizon should appear as an opaque black circle, letting no light from behind it through. It should display a brightening on one side, as matter accelerates around the black hole. It should appear 250% the size that General Relativity predicts, due to the distortion of spacetime. And it should happen because of a spectacular network of telescopes, in unison, all viewing the same object.

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The Allen Telescope Array is potential capable of detecting a strong radio signal from Proxima b, or of working in concert with other radio telescopes across extremely long baselines to try and resolve the event horizon of a black hole. (WIKIMEDIA COMMONS / COLBY GUTIERREZ-KRAYBILL)

Normally, the resolution of your telescope is determined by two factors: the diameter of your telescope and the wavelength of light you’re using to view it. The number of wavelengths of light that fit across your dish determines the optimal angular diameter you can resolve. Yet if this were truly our limits, we’d never see a black hole at all. You’d need a telescope the diameter of the Earth to view even the closest ones in the radio, where black holes emit the strongest and most reliably.

But the trick of very-long baseline interferometry is to view extremely bright sources, simultaneously, from identical telescopes separated by large distances. While they only have the light-gathering power of the surface area of the individual dishes, they can, if a source is bright enough, resolve objects with the resolution of the entire baseline. For the Event Horizon Telescope, that baseline is the diameter of the Earth.

I’m so pleased that the Event Horizon Telescope, and imaging the event horizon of a black hole directly, was the topic of October 3rd’s Perimeter Institute public lecture: Images from the Edge of Spacetime, by Avery Broderick.

The live blog is now complete, having aired initially at 7 PM Eastern Time (4 PM Pacific Time), and you can follow along by watching the video below. Watch the talk any time, and follow along with the live blog that follows!

(All updates, below, will have the timestamps in bold in Pacific time, with screenshots where appropriate from the lecture itself.)


1:26:25
3:50 PM: Welcome! Let’s start the Live Blog a little early, so we can give you a bit of background.

The biggest thing you have to realize, when it comes to imaging a black hole’s event horizon, is that we’re not looking for light, but the absence of light. When you look at a galaxy’s center, you’re going to see a ton of light, coming from all the matter located there. What the event horizon of a black hole gives you, spectacularly, is a shadow: a region wherein any light coming from behind it gets absorbed and swallowed. The key to imaging the event horizon is to see the light, behind the black hole, that emerges from around the horizon itself.

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Some of the possible profile signals of the black hole’s event horizon as simulations of the Event Horizon Telescope indicate. (HIGH-ANGULAR-RESOLUTION AND HIGH-SENSITIVITY SCIENCE ENABLED BY BEAMFORMED ALMA, V. FISH ET AL., ARXIV:1309.3519)

3:54 PM: What’s an incredibly exciting possibility, that hopefully we’ll get to hear more about in this lecture, is what we might see if something is flawed in Einstein’s theory of general relativity. Of course we expect Einstein to be right; general relativity has never led us astray yet, not in any experiment, measurement, or at any level of detail. But if the event horizon is a different size, opacity, or shape than what we predict, or doesn’t even exist at all, that could lead us to a revolution in physics. Quantum gravitational effects, for example, shouldn’t be important here. But if they are… well, that’s part of why we look!

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This multiwavelength view of the Milky Way’s galactic center goes from the X-ray through the optical and into the infrared, showcasing Sagittarius A* and the intragalactic medium located some 25,000 light years away. Using radio data, the EHT will resolve the black hole’s event horizon. (X-RAY: NASA/CXC/UMASS/D. WANG ET AL.; OPTICAL: NASA/ESA/STSCI/D.WANG ET AL.; IR: NASA/JPL-CALTECH/SSC/S.STOLOVY)

3:58 PM: I know we’re all hoping for an answer to the biggest question we have: what does the event horizon look like? That’s why we have the telescope array, after all, doing what it’s doing. But take a look at the multiwavelength image above. We have to see through all of that radiation, and prevent it from being a foreground contaminant, to image the event horizon of the black hole itself.

It’s important to appreciate how much of the Universe we have to see through, as though it were transparent (and it’s not 100% transparent), just to have a shot at the event horizon itself. Today, I’m hoping we learn exactly how we can do this, and why we’re so confident that the EHT will get us there. Remember, the Milky Way’s black hole, and all black holes, are radio-loud objects!

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This four-panel view shows the Milky Way’s central region in four different wavelengths of light, with the longer (submillimeter) wavelengths at top, going through the far-and-near infrared (2nd and 3rd) and ending in a visible-light view of the Milky Way. Note that the dust lanes and foreground stars obscure the center in visible light, but not so much in the infrared. (ESO/ATLASGAL CONSORTIUM/NASA/GLIMPSE CONSORTIUM/VVV SURVEY/ESA/PLANCK/D. MINNITI/S. GUISARD ACKNOWLEDGEMENT: IGNACIO TOLEDO, MARTIN KORNMESSER)

4:01 PM: Before the lecture starts, and it’s about to begin, here’s one last thing: this is the center of the Milky Way in four independent wavelengths. There’s a lot going on there, and we’re looking for an object that’s roughly the size of Jupiter’s orbit around the Sun. Are you not impressed at the ambition of the EHT? You should be impressed!!

4:04 PM: If you’re wondering why we don’t go for a closer black hole than the Milky Way’s center, because there are closer ones, it’s because the size of a black hole is dependent on its mass and its distance. Double the mass means double the radius; double the distance means half the radius. The second most massive black hole in the Milky Way that we’ve ever found is thousands of times less massive than the one at our galaxy’s center, but only about 10–20 times closer. This is why we go for bigger rather than closer!

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Hawking radiation is what inevitably results from the predictions of quantum physics in the curved spacetime surrounding a black hole’s event horizon. This visualization is more accurate than a simple particle-antiparticle pair analogy, since it shows photons as the primary source of radiation rather than particles. However, the emission is due to the curvature of space, not the individual particles, and doesn’t all trace back to the event horizon itself. (E. SIEGEL)

4:08 PM: “Black holes are objects into which things go, and don’t come out.” That is a solid definition of a black hole, which Avery gave… to first order. This should be true for every black hole in our Universe, but give it time. After about 10²⁰ years, perhaps a billion (or ten) times the age of our Universe, they’ll start to radiate, via Hawking radiation, faster than it can absorb any matter that surrounds it. They will shrink, and when they do, that will herald their disappearance.

On long enough timescales, things will come out, albeit not from inside the black hole, but from the curved spacetime outside of it.

4:10 PM: Avery says if you crush the Sun down to 3 km, it would become a black hole. Crush the Earth to 1 cm, and it’s a black hole. Crush a human, and it’s about 10^-11 times the width of a proton. (This is a correction to Avery’s number.)

And crush the Universe down to… approximately the size of the Universe itself, and it’ll be a black hole? Be careful here; the Universe is expanding, and full of dark energy, and that changes the equation tremendously. Our Schwarzschild solution, a great approximation for real black holes, no longer applies here. (I hope Avery gets this right when he gets there!)

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Our galaxy’s supermassive black hole has witnessed some incredibly bright flares, but none was as bright or long-lasting as XJ1500+0134. Owing to events like this and many others, a large amount of Chandra data, over a 19 year time period, exists of the galactic center. (NASA/CXC/STANFORD/I. ZHURAVLEVA ET AL.)

4:14 PM: Looking at supermassive black holes is fantastic; you get to see, in the radio, these massive lobes.

But the image above, that I’ve chosen, is in the X-ray! Black holes are powerful all across the electromagnetic spectrum. We can see their effects because, as Avery correctly notes, the matter expelled from the black holes changes their environments.

4:17 PM: Avery makes the point that the Universe is complicated, but black holes are simple. And this is true, so long as you’re looking at their macro-properties. But there is a huge amount of theoretical motivation to assume that what a black hole is made out of matters! If you made a black hole out of 10⁵⁵ neutrons or 10⁵⁵ antineutrons, there should be a difference. Not in General Relativity, but in terms of information and quantum numbers.

Does this actually matter? We’re not sure, and the EHT won’t teach us that. There are many questions we ought to remember that physics has left to resolve, no matter what the answers the EHT (or any experiment) can give us.

4:20 PM: Avery brings up a fun acronym: ISCO. ISCO stands for innermost stable circular orbit. This is not the event horizon, but rather an orbit about three times the radius of the event horizon. There should be, therefore, an empty hole that’s in between ISCO and the event horizon, where no matter (stably) exists.

The innermost orbit for matter, and for photons, and for even the spacetime that starts to get dragged around (yes, this happens!), all affect what someone viewing the event horizon would actually see. Frame-dragging is a real effect in relativity, and cannot be ignored!

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Countless scientific tests of Einstein’s general theory of relativity have been performed, subjecting the idea to some of the most stringent constraints ever obtained by humanity. Einstein’s first solution was for the weak-field limit around a single mass, like the Sun; he applied these results to our Solar System with dramatic success. We can view this orbit as Earth (or any planet) being in free-fall around the Sun, traveling in a straight-line path in its own frame of reference. (LIGO SCIENTIFIC COLLABORATION / T. PYLE / CALTECH / MIT)

4:24 PM: I think this is a really important point that Avery only glosses over, but is a source of confusion for a lot of people in General Relativity. The curvature of spacetime is not determined by mass. Sure, no less a figure than Wheeler noted that matter tells spacetime how to curve; curved space tells matter how to move, but it’s more than that. The curvature of spacetime is determined by the presence, distribution, and density of both matter and energy. This includes energy of all forms: radiation, kinetic energy, and many quantities other than just mass.

Mass plays a major role, but it isn’t the only important thing as far as affecting spacetime goes.

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A large slew of stars have been detected near the supermassive black hole at the Milky Way’s core. In addition to these stars and the gas and dust we find, we anticipate there to be upwards of 10,000 black holes within just a few light years of Sagittarius A*, but detecting them had proved elusive until now. (S. SAKAI / A. GHEZ / W.M. KECK OBSERVATORY / UCLA GALACTIC CENTER GROUP)

4:27 PM: I want to note something that Avery said at the 0:25 minute mark in his talk, asking if these objects with large masses and X-ray/radio emissions are, in fact, black holes? He then left the question hanging and didn’t answer.

But you know what? Except for crackpots on the internet, pretty much everyone now accepts that these objects are black holes, and it was Andrea Ghez’s group, at UCLA, that answered that question for us. You see stars, by looking in the infrared, orbiting a point of incredible mass, about 4 million solar masses. Yet no light (at least, in the infrared) comes from that mass.

Why? Because there is no explanation for it other than a black hole. That’s a black hole, folks, and with supreme confidence we can look for it with a telescope like the EHT.

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The galaxy NGC 1277, speeding through the Perseus cluster, not only contains predominantly red stars, but red (and not blue) globular clusters, as well as a shockingly large supermassive black hole to go along with its rapid speed through the cluster. (MICHAEL A. BEASLEY, IGNACIO TRUJILLO, RYAN LEAMAN & MIREIA MONTES, NATURE (2018), DOI:10.1038/NATURE25756)

4:31 PM: There is a great graphic and a great conundrum in Avery’s talk. The largest black hole, as seen from Earth, is the one at the Milky Way’s center. The second largest is the one at M87. The fourth largest? The one at the center of Andromeda.

But the third largest is a weirdo: NGC 1277. It’s the size of the Milky Way, but appears to have a >10 billion solar mass black hole. This is controversial, but it’s a tantalizing possibility!

4:34 PM: Why is it so hard to resolve a black hole? Well, many reasons. We talked about resolution earlier, but that’s not the only one.

Not every galaxy is radio-loud, meaning you can’t see the shadow against the radio background if there is no background. (And so, sorry NGC 1277 fans, that’s out.) If a galaxy isn’t radio-transparent, because there’s too much foreground, it won’t be visible either. But if you’re limited by diffraction, which is the nature of your telescope, you can see the wavelength divided by your telescope’s diameter. You’d need a ~12 million meter diameter telescope to get the EHT’s resolution in the radio.

4:38 PM: So why does Avery, at the 0:36 mark in his talk, say you’d need a 5 km telescope, rather than a 12 million meter telescope, to see the black hole at the galaxy’s center?

Two reasons. Number one, the telescopes he’s talking about are optical/infrared, which have wavelengths that are around 1,000 times shorter than the radio wavelengths the EHT will look at. (This is good; the plane of the Milky Way, which includes the galactic center, is opaque to visible light!)

Number two, you want better resolution than the thing you’re trying to image. Otherwise, it’s just one pixel, and you can’t learn what you want to learn about an event horizon from just one pixel!

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The occultation of Jupiter’s moon, Io, with its erupting volcanoes Loki and Pele, as occulted by Europa, which is invisible in this infrared image. GMT will provide significantly enhanced resolution and imaging. (LBTO)

4:45 PM: His analogy with Fourier series isn’t really doing it for me. If you wonder, “how can you use multiple telescopes to get the resolution I need” to reconstruct an image, it is highly dependent on what you’re looking at. Always, more telescopes covering more area at more locations are better.

But if you only have two telescopes, you can still do incredible thing, like the Large Binocular Telescope Observatory (LBTO) did just a few years ago, when they imaged erupting volcanoes on Jupiter’s Moon, Io, while another of its moons (Europa) eclipsed it. Pretty incredible!

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The amount of computational power and data-writing speed has been the limiting factor in EHT-like studies. Proto-EHT began in 2007, and was capable of doing absolutely none of the science it’s doing today. (PERIMETER INSTITUTE)

4:49 PM: So what took us so long to build the EHT? After all, we’ve had telescopes and planet Earth for a really, really long time, and we’ve been capable of taking these images. But it requires a whole lot of data. Writing down enough (and the right kinds of) data, fast enough, and then bringing them together with enough computational power to analyze them, is only now, for the first time, possible. If we had tried even a decade ago to build and run the EHT, it would not have been possible.

4:51 PM: Avery says the biggest advance was the addition of ALMA to the EHT array. And ALMA is so, so fantastic. A piece of the array is shown, above, but check out below, where ALMA has taken some pretty spectacular, high-resolution images of… well, planets forming around young stars, like nothing else ever has, even up through today.

4:53 PM: And now, at last, at the 0:51 minute mark of the talk, we get the real reason why all this analysis takes so long. There are different atmospheric phase delays that include calibration, calculation, mistakes and re-calculation, of 27 Petabytes of data, from all the different stations.

Computational time is often a joke, but that’s the hold up. He has no images to show, because there are no final-version, mistake-free images available. Early 2019, maybe, is what he says we can look forward to for the first images.

4:54 PM: Be patient, EHT fans! Be pleased they are taking the time to get it right!

4:58 PM: Avery has just made an argument for why black holes must exist, and the objects at the centers of the Milky Way and M87 must be one. (Or, two, more accurately.) If you have stuff that falls onto a central accreting body, it will heat up and shine. But if they run into a hard object that doesn’t have an event horizon, it will heat up and shine upon impact. If you had impact emission, it would show up.

There was no emission, which should theoretically show up in the infrared. The lack of this would push above the infrared limits, and it’s not there!

Bam!

And therefore, black hole. It can’t be large and cool, and isn’t hot enough to be a non-black-hole. QED.

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The second-largest black hole as seen from Earth, the one at the center of the galaxy Messier 87, is around 1000 times larger than the Milky Way’s black hole, but is over 2000 times farther away. The relativistic jet emanating from its central core is one of the largest, most collimated ones ever observed. (ESA/HUBBLE AND NASA)

5:02 PM: So how do you measure the mass of a black hole? You measure the gas orbiting the central black hole; you measure the stars orbiting it. But you get two different numbers, and they disagree. They disagree by about a factor of 2 for M87, and (although most people don’t remember) they used to disagree for the Milky Way in the early 2000s. From X-rays, we estimated around 2.5–2.7 million solar masses, but from stars, we estimate 4 million solar masses.

Who’s right? My bet is on stars because the observations have fewer assumptions to translate into a mass, but the EHT should teach us which (if either) is correct!

5:04 PM: Avery contends these are the two black holes you’d want, ideally, to test black holes. They’re different; one is small and close, the other is large and farther; one is active with a big jet (M87) while the other is quiet; both have a large enough angular size to be resolved with a telescope the size of our planet, etc. And these are good arguments. But I’d still rather have a stellar mass black hole that happened to be within just a few light years to try out. Any help, Alpha Centauri?

(This is the first Perimeter talk I’ve seen that hasn’t been properly budgeted for time, BTW, so I’m sorry if any of you watching are bummed that it’s gone over.)

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The proto-EHT data is consistent with, but only weakly constrains, the black hole properties of our galaxy’s center. (PERIMETER INSTITUTE)

5:08 PM: Avery is talking about the early proto-EHT data, that took these first observations, and showed it was consistent with our models of black holes within General Relativity. But there is really so little that we get; we get info about mass, a little bit about rotation, and a little bit about the surrounding environment. Until we can see the horizon itself, and know its shape, we are very limited in what we can constrain.

Even Avery is disappointed with what we can say with Proto-EHT data.

5:10 PM: What will be very, very cool, that Avery is telling, is that there will be movies, not just images, that are interesting. On timescales of decades, black holes will jitter, similar to how Brownian motion works. Atoms and molecules bounce off of tiny particles under a microscope; that’s Brownian motion. Well, for the black hole at the galactic center, stars orbit and move closer-or-farther from the central black hole, and they gravitationally push it around!

5:12 PM: I would like to point out that this is why it’s so important to make your observations simultaneously in time to one another; you cannot reconstruct a single image from interferometry if you’re not looking at the same object anymore. Like Heraclitus said, “you can’t step into the same river twice.” Well, you can’t look at the same black hole twice, apparently.

That’s deep.

5:13 PM: Okay, for those of you watching, I’m just going to say that if you’re 73 minutes into a 60 minute talk, and you’re just now mentioning things like the “Bardeen-Petterson effect,” someone should start playing the “wrap-it-up” music.

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The supermassive black hole at the center of our galaxy, Sagittarius A*, flares brightly in X-rays whenever matter is devoured. In other wavelengths of light, from infrared to radio, we can see the individual stars in this innermost portion of the galaxy. (X-RAY: NASA/UMASS/D.WANG ET AL., IR: NASA/STSCI)

5:17 PM: Okay, this last thing is cool enough that I should mention here: flares at the center of the Milky Way’s black hole. They happen, and they typically last minutes.

But why? Are they turbulent features in the accretion disk? Or are they arising from the infall of matter, like “hot blobs” in the accretion flow, that flare out when they get accelerated and devoured?

Models of both are continuously being improved, and based not on the event horizon itself, but the luminous signals that come out around the outside of the event horizon, we might be able to tell them apart. Why does our black hole flare? The EHT might teach us.

5:20 PM: So, if you’ve made it this far, you probably watched the whole thing. So how do you sum it up?

Black holes are real.
We can see their effects and learn about it indirectly.
They should have event horizons.
The EHT should create an image of them with the data we have.
It will take lots of time.
And if we observe the light from outside of them, we might learn more about the environment of these black holes, and what causes transient events like flares.

And that’s the end! Q&A time!

5:22 PM: Fun question: what gets ejected from a black hole? What are these jets made out of? Where do they come from?

Avery gives the real answer: we don’t know. We think they’re filled with protons, nuclei, etc., and that’s Avery’s first answer. But they could be just electromagnetic (light) radiation. (Avery says that; most scientists, as I understand it, deem that incredibly unlikely.)

The follow-up is what is the jet’s effect on the black hole? While Avery is assuming equal-and-opposite bipolar jets, that assumption isn’t necessary. It’s like asking what effect a fly has as it splats on your semi truck’s windshield. It’s negligible.

5:25 PM: Avery’s final question is what made him want to study black holes? And the answer is… Star Trek! There’s no better way to end a live-blog than that, so live long and prosper, everyone, and I’ll see you next time!

See the full article here .

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“Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

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From Discover Magazine: “Secrets Of The Strange Stars That Circle Our Supermassive Black Hole”

DiscoverMag

From Discover Magazine

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This artist’s illustration shows the supermassive black hole lurking at the center of our spiral galaxy, the Milky Way. (Credit: NASA/CXC/M.Weiss)

NASA/Chandra X-ray Telescope

High winds are the norm at the center of the Milky Way. Astronomers have now clocked suns orbiting the galactic core at a staggering 3,000 miles (4,800 kilometers) per second. At this rate, Earth would complete its orbit around the sun in a mere three days. What lurks at the galaxy’s core that can accelerate stars to such speeds?

Astronomers have considered various possibilities. Does the center of the galaxy harbor a tight cluster of superdense stellar remnants (neutron stars)? Or perhaps a huge ball of subatomic neutrino particles?

But these and other more exotic possibilities were eliminated in the spring of 2002 when a star called S2 swept down in its highly eccentric orbit and passed within 17 light-hours of the Milky Way’s center — a minuscule distance in galactic terms. In 17 hours, light travels three times the distance between Pluto and the sun.

Only one object is compact enough and has sufficient mass to accelerate stars to such a high speed: a supermassive black hole. Astronomers had suspected that a black hole must lie at the Milky Way’s core, but plotting the orbit of S2 and other stars dramatically strengthened the evidence.

SO-2 Image UCLA Galactic Center Groupe via S. Sakai and Andrea Ghez at Keck Observatory

Our central black hole is small by the standard of what lurks in the hearts of other galaxies. Observations of the giant elliptical galaxy Messier 87 suggest the presence of a black hole 6 billion times more massive than the sun. The interaction of two supermassive black holes probably produces the intense X-rays streaming from the galaxy NGC 6240. The Andromeda Galaxy may harbor a black hole of 140 million solar masses.

Andromeda Galaxy Adam Evans

In comparison, our galaxy’s black hole is paltry — containing about 4 million solar masses. But its nearness means we can study it in detail, including charting the orbits of dozens of stars buzzing around it like bees. The stellar-mass black holes found in some binary star systems are too small to be observed in detail by telescopes anytime soon. So, the best chance of seeing what happens in the bizarre neighborhood around a black hole is to study the one at the Milky Way’s heart. So far, it has not failed to surprise us.

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Bright stars surround the supermassive black hole at the Milky Way’s center. (Credit: NASA/CXC/M.Weiss)

The Inner Realm

The galactic center lies about 26,000 lightyears from Earth toward the constellation Sagittarius. It is a region of the sky where bright stars mingle with dark clouds of gas and dust. The actual center is too obscured to reveal much when astronomers observe it in visible light. What we know of it comes from data collected in infrared and radio wavelengths. These wavelengths can pass through the dust and gas and reach Earth-based telescopes.

Astronomers have long known that the strongest source of radio energy in the sky, after the sun, lies at the galactic center. This broad core region is called Sagittarius A, often abbreviated as Sgr A.

SGR A* , the supermassive black hole at the center of the Milky Way. NASA’s Chandra X-Ray Observatory

Sgr A hosts dozens of individual radio sources. One is called Sagittarius A*, pronounced “Sagittarius A star.” It lies at the very center of the galaxy and coincides with the position of the supermassive black hole. Everything else rotates clockwise (from Earth’s point of view) around this point, making it the dynamic center of the galaxy. And it is a very busy neighborhood.

Surrounding Sgr A* at a distance of several light-years, a shell of dust rotates counterclockwise — opposite to the galaxy’s general rotation. Lying inside the shell, and turning in the same direction, is a small spiral structure with three arms.

Each arm is a stream of hot gas set aglow by nearby stars. The gas flows toward the center of the spiral where Sgr A* lies. Radio images taken a few years apart revealed the spiral is rotating. More recently, a close-up look at Sgr A* with new imaging technology has revealed the amazingly powerful gravity of the object the spiral encircles.

Stellar Raceway

In 2002, a team of astronomers led by Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, published the first scientific paper announcing S2’s 17-light-hour close encounter with Sgr A*. Using the European Southern Observatory’s (ESO) Very Large Telescope (VLT) in Chile, Genzel’s group caught S2 as it rounded Sgr A* at a fantastic speed. The VLT’s adaptive optics reduces atmospheric blurring, allowing astronomers to chart S2’s position more accurately.

ESO VLT at Cerro Paranal in the Atacama Desert, elevation 2,635 m (8,645 ft) from above Credit J.L. Dauvergne & G. Hüdepohl atacama photo

For the previous decade, the astronomers had been plotting S2’s orbit, mostly with ESO’s 3.6-meter New Technology Telescope, also in Chile.


ESO/NTT at Cerro La Silla, Chile, at an altitude of 2400 metres


ESO/Cerro LaSilla, 600 km north of Santiago de Chile at an altitude of 2400 metres.

The orbital positions allowed the researchers to calculate S2’s orbital period around Sgr A* as about 16 years. The orbit is quite eccentric. The star swoops in to within 17 light-hours at its closest approach to Sgr A*, but then sweeps outward to a distance of some 10 light-days at its farthest point. To produce such an orbit requires a compact black hole with about 4 million solar masses.

Genzel and his colleagues were not the only ones tracking S2 and the many other stars zipping around Sgr A*. Astronomer Andrea Ghez’s Galactic Center Group at UCLA has studied S2 and its motions with the 10-meter Keck Telescope in Hawaii.


Keck Observatory, Maunakea, Hawaii, USA.4,207 m (13,802 ft), above sea level, showing also NASA’s IRTF and NAOJ Subaru

In 2000, the team reported evidence that S2’s path is curved — early evidence S2 is orbiting something at the galactic center. The UCLA team later discovered S2’s close orbital distance to Sgr A* at about the same time as Genzel and his colleagues.

Extensive observations in recent years by Genzel, Ghez, and others paint a fascinating picture of the flurry of activity around Sgr A*. One of the most challenging observations astronomers have performed on the galactic center stars is spectroscopy, or separating starlight into its component wavelengths. A spectrum reveals much about a star’s composition, age, and mass.

Gathering enough light from a distant star to take a good spectrum requires tracking the target through a narrow slit for many hours. Any small shift in the slit’s position contaminates the spectrum with light from other sources. Spectroscopy is especially challenging in the crowded star field around Sgr A*, where the density of stars is more than a million times higher than in our stellar neighborhood.

In 2003, Ghez took a spectrum of S2 with the Keck Telescope using its adaptive optics system. The slit trained on the star was only 0.04 inch (1 millimeter) wide. Keeping this narrow gap locked on S2 was like aiming a gun sight on an object the size of a basketball 1,000 miles (1,600 km) away.

The spectrum revealed S2 to be a heavyweight star some 15 times the sun’s mass. Such large stars exhaust their hydrogen supply quickly — in this case, in less than 10 million years. That means S2 must be younger than 10 million years. In addition, the star has a very hot atmosphere, as do other stars orbiting close to Sgr A*. This also indicates a relatively young age.

In short, these stars formed 3 to 6 million years ago. This raises a major problem: Why are such young stars orbiting so close to Sgr A*, a region of intense magnetic fields and strong gravitational forces that would normally prevent star formation?

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Radio astronomy reveals hidden features of the Milky Way’s center, including remnants of supernova explosions and stars forming in vast clouds of gas and dust. (Credit: W.M. Goss/C. Lang/VLA/NRAO)

NRAO/Karl V Jansky VLA, on the Plains of San Agustin fifty miles west of Socorro, NM, USA, at an elevation of 6970 ft (2124 m)

Stellar Masquerade

One possible explanation is that S2 and its companions may be old stars masquerading as young ones — “a phenomenon we understand quite well in Los Angeles,” Ghez once quipped to a science reporter.

In this case, what seem to be young stars are actually the cores of older suns that collided and merged. The collisions could have stripped away the suns’ cool outer layers, exposing their hot interiors. The result would be a cluster of massive stars that appear much younger than they really are.

But there’s a problem with this scenario. A collision violent enough to strip away the outer layers should also annihilate both stars and leave only a trail of hot gas. And so astronomers have proposed alternatives. For example, perhaps the stars formed elsewhere and migrated inward under the black hole’s gravitational pull.

The problem with this explanation is that most active star formation in the Milky Way occurs far from the core, in its spiral arms. It would take the stars too long to migrate as close to the center as S2.

Dense dust clouds do lie closer to Sgr A* than the spiral arms, to within a few dozen light-years. Stars are probably forming inside of them. It’s conceivable that a cluster of young stars could spiral down to within a few light-years of the center — and do so in less than 10 million years.

The problem here is that to get closer to the black hole, the stars would have to shed angular momentum — the quantity that keeps planets in nice safe orbits around stars instead of “falling” directly into them.

One way to lose angular momentum is to bump into other stars. But it’s difficult to imagine how stars could endure this process and migrate to within light-hours of Sgr A* without being destroyed. Besides, the process should leave behind a trail of stars toward Sgr A* for a long distance, something astronomers have not yet seen. Instead, the shell of stars orbiting close to Sgr A* has a definite outer edge.

Star Birth in a Disk

Another possibility is that Sgr A*’s central cluster stars formed within a rotating disk of gas and dust immediately surrounding the black hole. In fact, some observations suggest most stars in the central cluster orbit roughly in the same plane — an arrangement reminiscent of the major planets in our solar system. The planets formed in a disk of gas and dust, so perhaps S2 and its fellow travelers did, too.

However, not all astronomers agree the central cluster has a disklike structure. Another caveat: To spawn stars, the disk would need to be dense enough to withstand the black hole’s tidal forces.

It’s also conceivable that Sgr A*’s companion stars formed in dust clouds circling at high speed within a few light-years of the galactic center. Collisions between the clouds could have spawned shock waves, triggering star formation. As the result of collisions between the clouds, they and the new stars embedded within them could have shed enough momentum to settle into orbits around the black hole. The galactic core’s strong magnetic field would have gradually swept the leftover interstellar dust and gas away from the black hole. What would remain is a disk of young stars in close orbit to Sgr A*.

This scenario explains much of what astronomers see in the galactic core, although not all. UCLA astronomer Brad Hansen thinks he has a viable alternative: Hot young stars now orbit the Milky Way’s central black hole because a second smaller black hole dragged them there.

The process begins in a crowded young star cluster, dozens of light-years from the galactic center. Collisions between big stars in the cluster’s core form an intermediate-sized black hole in the range of 1,000 to 10,000 solar masses. Gradually, the black hole would migrate toward the galactic center, dragging its cargo of “hostage stars” along with it. Hansen argues this is the only way to quickly transport massive young stars into the galactic center from an outside star-birth location.

All the black-hole ferry scenario lacks is hard evidence to support it. If a second black hole orbits the primary black hole in the galactic core, its presence might be detectable. Its tug on Sgr A* might cause a detectable wiggle. Clearly, astronomers still have a lot of work left to fully understand the processes at work in the galactic core.

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Dozens of young stars orbit at high speeds around the galaxy’s central black hole. By plotting the stars’ positions for years, astronomers calculated their orbits and estimated the mass of the black hole they encircle. (Credit: Astronomy: Jay Smith, after Andrea Ghez (UCLA))

Imaging The Black Hole

Fast-moving stars like S2 remain the best evidence that a black hole lies at the heart of the Milky Way. Other support includes periodic bursts of infrared light from Sgr A*. The bursts suggest the black hole spins, completing a turn every 17 minutes. Astronomers have also detected strong radio pulses coming from Sgr A*. This may indicate that packets of ultra-hot gas and dust are falling into the black hole.

But this is all still circumstantial evidence. The definitive proof might come if astronomers could actually image the black hole’s edge or “event horizon,” beyond which no light or matter can escape.

Radio energy passes through the veil of obscuring dust and gas around the galactic center, providing a way to directly image a black hole. By itself, a black hole is essentially invisible. But it would be detectable as a silhouette against the accretion disk of gas spiraling into it. The gas emits energy as it accelerates to high speeds around the black hole.

Light follows a highly curved path near a black hole, making its silhouette appear wider than it actually is. Bright rings or arcs, formed as the black hole bends or “lenses” light from background sources, might protrude from the silhouette’s edges.

In 2008, radio astronomers announced an important milestone in the study of our galaxy’s black hole. By combining the power of three radio telescopes, researchers detected features around Sgr A* as small as 31 million miles (50 million km) across. The study found that radio emission from Sgr A* is offset from the black hole, perhaps because it comes from an accretion disk. Astronomers hope the Event Horizon Telescope — a nearly Earth-sized radio observatory comprising about a dozen separate instruments — will be able to image the black hole’s silhouette in the next few years.

Whatever the result, imaging the Milky Way’s central black hole will put the existence of black holes on a firmer footing and perhaps reveal important new insights about the evolution of galactic cores. A failure to see it will bring into question what we understand about the heart of our own galaxy — including the origins of the highspeed roller derby of young stars whizzing around its center.

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