From Discover Magazine: “Unbound and Out: Boosted by Black Holes, Stars Speed Off, Leaving Clues Behind”

From Discover Magazine

November 2, 2018
Stephen Ornes

Astronomers say the galactic center is home to a black hole (illustration shown) with as much mass as 4 million suns. Its entourage likely includes clusters of stars — many of them orbiting each other in two- or three-star systems — as well as smaller black holes. (Credit: NASA/Dana Berry/SkyWorks Digital)

In April, the European Space Agency released the second massive trove of data from Gaia, a spinning, scanning satellite that for nearly five years has been spying on a billion stars.

ESA GAIA Release 2 map

ESA/GAIA satellite

Its goal is to produce a three-dimensional stellar map, enabling a new age of precision astronomy. Like other stargazers, Warren Brown of the Harvard-Smithsonian Center for Astrophysics has plunged headfirst into Gaia’s data. He’s hoping to find space oddities.

He has found some notable ones before. In 2005, Brown identified a young star speeding at 850 kilometers per second through the Milky Way’s lonely hinterland, called the halo.

MIlky Way Halo NASA ESA STScI

The star is traveling so fast that it’s unbound, which means that eventually, it will escape the galaxy. Brown coined the term “hypervelocity star” to refer to this breed of superfast stellar travelers.

Brown suspects that the star was flung by the enormous black hole that lies at the center of the Milky Way, SGR A*.

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

The black hole, about 4 million times the mass of the sun, is so powerful that astronomers classify it as supermassive. Black holes are usually thought of as pulling things toward themselves, but they can also act like cosmic slingshots, Brown says. And their ammo can be as big as stars. Once shot, tossed stars may get a one-way ticket out of the galaxy’s grasp.

Since that initial discovery, surveys by Brown and by other astronomers have identified more than 20 unbound, hypervelocity stars of various origins zipping around, including one traveling away from our galaxy that was probably ejected from the Large Magellanic Cloud, a dwarf galaxy companion of the Milky Way [MNRAS].

Large Magellanic Cloud. Adrian Pingstone December 2003

Discussing these discoveries and their implications in the 2015 Annual Review of Astronomy and Astrophysics, Brown explains that, beyond their own interesting origin tales, such exotic stars may also be useful as tools.

Knowable Magazine spoke with Brown about what it takes to escape the galaxy, what Gaia tells us about space oddities and how stellar travelers can help reveal clues about one of the most fundamental mysteries in astronomy — the invisible dark matter that holds the Milky Way together but remains impossible to detect directly.

Milky Way Dark Matter Halo Credit ESO L. Calçada

This conversation has been edited for clarity and length.

Where do hypervelocity stars come from?

The fastest ones we’ve found all seem to point back to the galactic center. The measurements aren’t definitive, but with Gaia’s data, I found that the fastest stars are best explained by galactic center ejection. However, I also found that half [of known high-speed stars] did not come from the galactic center. I think that’s cool. There’s a mix of things going on in the Milky Way.

How do you think a star would get ejected from the center of the galaxy?

You have to have at least three things, and one of them has to be a supermassive black hole. If you have a supermassive black hole, then you have a lot of energy, and there are a lot of stars around it that interact.

Then if you have a binary — two stars orbiting each other — approaching a black hole, the gravitational tidal field is so extreme it can pull the pair of stars apart. The capture or ejection depends on the direction of each star’s motion relative to the black hole. Physicists call this a three-body exchange: One star exchanges partners — it gets captured and loses energy. The other escapes, and gains all that energy and just shoots out. That’s the slingshot.

It’s a conservation of energy problem.

In 1988, theorist Jack G. Hills at Los Alamos National Laboratory predicted that stars could be ejected from the Milky Way after an interaction with the black hole at the galactic center. Here’s how it works: A binary star system — two stars spinning around each other – approach a black hole. The closer star gets captured, and its energy is transferred to its former companion, which travels outward so fast that it can escape the gravitational pull of the galaxy. (Credit: Adapted from W.R. Brown/AR Astronomy & Astrophysics 2015/Knowable Magazine)

How do you find a hypervelocity star?

The single answer is speed. They’re not orbiting with everything else in the Milky Way. They’re unbound, and they’re never coming back. That’s what makes them different. There are 100 billion stars that look like every other star, that you don’t care about. It’s very much a needle in a haystack.

When we designed our [2008] survey, which I think is fair to say is the only successful survey of unbound stars in the galaxy, we were looking for young stars — blue stars, hot stars — at very large distances from the center, where they shouldn’t exist, unless they were ejected. And that approach worked, because there are very few young stars out in the outer parts of the Milky Way.

Are you using Gaia to study the hypervelocity stars you already knew about, or are you looking for new discoveries?

Both. A paper we just had accepted was on the 20-some odd, unbound stars found previous to Gaia. We’re also looking at outliers in the Gaia catalog that might be hypervelocity stars. It’s one of these things where we find candidates, but we need follow-up observations to decide.

How does Gaia look at stars?

It’s hard to identify a star other than by its motion. Gaia is trying to measure the tangential motion of the star on the plane of the sky. That’s hard. It’s the product of distance times the angular change over time. In astronomy, you don’t observe distance, you can infer it. And it’s a very small angular change — the angular motion is milliarcseconds in one year, or something. It’s a very tiny angle on the sky that’s changing.

You’ve used Gaia’s data to study halo stars and runaway stars, too. Why are these other space oddities interesting?

Runaway stars were discovered [more than] 50 years ago. They’re interesting because they’re very young, massive stars like the hypervelocity stars we’ve found, but they’re ejected from the disk of the Milky Way — instead of from the center — through binary ejections. Its companion explodes. Well, its former companion explodes, releasing energy. If the star’s direction lines up with the rotation of the galaxy, it suddenly has a speed that can exceed the escape velocity. Those are rare — the ones with those speeds — but they can mimic hypervelocity stars. That’s pretty cool.

Halo stars are normal stars orbiting in the outer parts of the Milky Way. There aren’t a lot of stars way out there. The halo is believed to contain about 1 percent of the Milky Way’s stars, or about 1 billion stars. Halo stars were discovered by Oort and others from the unusual motions of a few stars near the Sun. They orbit in their own way and can appear to have a very different velocity with respect to us. When you’re looking for velocity outsiders, things like halo stars show up. The GAIA Data Release 2 catalog is estimated to have 70 million to 80 million halo stars in its catalog.

Why do you want better measures on unbound stars?

Good measures on the trajectory of hypervelocity stars tell you about how these things were ejected. Was it a single black hole or a binary black hole? It’s fun to think about. The really interesting work is not just in studying the stars themselves but learning what you can do with them and how to use them as tools.

How can a star be useful?

Hypervelocity stars are the ultimate test particle for the gravitational potential of the Milky Way, which is the pull of all the Milky Way’s matter: its stars, gas and dark matter [the invisible matter thought to hold galaxies together]. The gravitational pull varies with position [in the galaxy] because all the matter is distributed across hundreds of thousands of light years of space.

How can hypervelocity stars map the gravitational potential?

If we’re right about where the stars come from, then their arc out of the galaxy tells you the potential of the Milky Way.

We look at the stars at different moments in time. We look at where the star is today, the specific direction its path is following. We can ask: How much does that differ from a straight line to the center? If you know exactly where the star comes from, then any deviation in the measurement of its position tells you how everything else is affecting its path.

In September, after searching Gaia’s data for hypervelocity stars — like the ones predicted by Jack Hills and first discovered by Warren Brown — astronomers not only found stars headed out of the galaxy (shown in red) but also, to their surprise, fast stars traveling toward the galactic center (yellow). These inbound travelers may have been ejected from other galaxies, and are now passing through the Milky Way. (Credit: ESA; Marchetti et al. 2018; NASA/ESA Hubble)

Imagine the simple case that the galaxy was a perfectly spherical ball. These hypervelocity stars launch in the center and follow a straight line out, but they get pulled down by the pull of the galaxy. The stars in the galactic disk will pull on the star and decelerate it.

How is dark matter distributed in the galaxy?

No one knows, but theoretical simulations predict that the dark matter is not spherical, but distributed with a different length in every direction, like an American football. It’s mostly in the exterior of the galaxy, farther out from the sun.

No one can see the distribution of dark matter directly, but it seems different than that of ordinary matter. Hypervelocity stars can test this, if you can measure their trajectories well enough. These stars are going off in different directions, and in principle each star is a completely independent tracer.

Gaia is still in the midst of its mission. What do you want to see in five years, after the final data release, and in future missions?

Its measurements get better with time, and every star gets measured 70, 80, 100 times. What we have currently is a lot of very good evidence that, taken together, says you have to have stars ejected by a black hole to explain the observations. Presumably, at the end, we’ll have three times better measurements, which means we’ll have three times smaller error bars. Some of the candidates will probably go away, but the end-of-mission Gaia measurements should definitively tell us that these hypervelocity stars are ejected by our galactic center black hole. If they do come from the galactic center, then they can tell us what stars in that region are like. Ironically, hypervelocity stars are easier for us to see than stars that are still in the center of the galaxy, because there’s so much dust and stars in between.

Gaia is not the final piece of evidence, though. We’ll still need spectroscopy to determine the nature of each star. Is it a white dwarf? A main sequence star? An old evolved star?

How else can Gaia’s data help you study hypervelocity stars?

Presumably, we’ll also see stars that we didn’t know about.

See the full article here .

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

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.

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.”

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/Submillimeter-wave Astronomy (ARO/SMT)

Atacama Pathfinder EXperiment

Combined Array for Research in Millimeter-wave Astronomy (CARMA)

Atacama Submillimeter Telescope Experiment (ASTE)

Caltech Submillimeter Observatory (CSO)

Institut de Radioastronomie Millimetrique (IRAM) 30m

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

Large Millimeter Telescope Alfonso Serrano

Submillimeter Array Hawaii SAO

ESO/NRAO/NAOJ ALMA Array, Chile

South Pole Telescope SPTPOL

NSF CfA Greenland telescope

Greenland Telescope

Future Array/Telescopes

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

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.

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.

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 COSMOS Magazine: “Signs of mergers may help us prove supermassive black holes exist”

From COSMOS Magazine

28 October 2018
Martin Krause

Black holes with masses billions of times that of the sun have long been theorised. Now, research takes astronomy closer to proving the contention.

Visible light image of the radio galaxy Hercules A obtained by the Hubble Space Telescope superposed with a radio image taken by the Very Large Array of radio telescopes in New Mexico, USA. NASA

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)

Observations of nature tend to throw up unexpected results and new mysteries – whether you’re investigating the rain forest or outer space. When radio astronomy took off in the 1950s, we had no idea that it would lead to the discovery that galaxies including our own seem to have terrifyingly large black holes at their centre – millions to billions of times the mass of the sun.

A few decades later, we still haven’t been able to prove that these beasts – dubbed supermassive black holes – actually exist. But our new research, published in the Monthly Notices of the Royal Astronomical Society, could one day help us do so.

Early radio astronomers discovered that some galaxies emit radio waves (a type of electromagnetic radiation). They knew that galaxies sometimes collide and merge, and naturally wondered whether this could have something to do with the radio emission. Better observations, however, refuted this idea over the years.

They also discovered that the radio waves were emitted as narrow jets, meaning that the power came from a tiny region in the nucleus. The radio power was indeed huge – often surpassing the luminosity of all the stars in the galaxy taken together. Various suggestions were made as to how such a huge amount of energy could be produced, and it was in the 1970s that scientists finally proposed [Astronomy and Astrophysics] that a supermassive black hole could be the culprit. The objects are nowadays known as quasars.

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

Theoretical models estimated that these objects would have a mass of an entire small galaxy concentrated in a space comparable to Earth’s orbit around the sun. But because only some galaxies produce energetic outbursts, it was unclear how common supermassive black holes would be. With the advent of the Hubble Space Telescope in 1990, the centres of nearby galaxies that did not emit radio bursts could finally be investigated. Did they contain supermassive black holes too?

It turned out that many did – astronomers saw signs of gravitating masses influencing the matter around it without emitting any light. Even the Milky Way showed evidence of having a supermassive black hole at the centre, now known as Sgr A*.

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

At this point, astronomers became increasingly convinced that supermassive black holes were a reality and could plausibly explain the extreme energetic outbursts from some galaxies.

However, there is no definitive proof yet. That is despite the fact that some supermassive black holes emit jets – these come from the surroundings of the black hole rather than the black hole itself. So how do you prove the existence of something completely dark? A black hole as defined by Einstein’s theory of general relativity is a region of space bounded by a horizon – a surface from inside of which no light or material object can ever escape. So, it’s a pretty difficult task for astronomers: they need to see something that emits nothing.

For smaller black holes the size of a stellar mass, a proof was indeed found: when two such objects merge, they emit gravitational waves, a tiny wobbling of space that was for the first time registered in 2015. The detection proved that black holes exist, that they sometimes form pairs and that they indeed merge. This was a tremendous success, honoured with the Nobel prize in 2017.

We also have a good understanding of where normal sized black holes come from – they are what is left after a star much more massive than the sun has arrived at the end of its lifetime. But both the existence and the origin of supermassive black holes are shrouded in mystery.

Spinning black holes

We have now found indications that many of the radio jets produced by supermassive black holes may in fact be the result of these objects forming pairs, orbiting each other. We did this by comparing the observed radio maps of their regions with our computer models.

The presence of a second black hole would make the jets produced by the first one change direction in a periodic way over hundreds of thousands of years. We realised that the cyclic change in jet direction would cause a very specific appearance in radio maps of the galaxy centre.

Lobes are created by the jets depositing energy to surrounding particles. Author provided.

We found evidence of such a pattern in about 75% of our sample of “radio galaxies” (galaxies that emit radio waves), suggesting that supermassive black hole pairs are the rule, not the exception. Such pairs are actually expected to form after galaxies merge. Each galaxy contains a supermassive black hole, and since they are heavier than all the individual stars, they sink to the centre of the newly formed galaxy where they first form a close pair and then merge under emission of gravitational waves.

While our observation provides an important piece of evidence for the existence of pairs of supermassive black holes, it’s not a proof either. What we observe are still the effects that the black holes somehow cause indirectly. Just like with normal black holes, a full proof of the existence of supermassive black hole pairs requires detection of gravitational waves emitted by them.

Current gravitational wave telescopes can only detect gravitational waves from stellar mass black holes.

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

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)

The reason is that they orbit around one another much faster, which leads to the production of higher frequency gravitational waves that we can detect. The next generation of instruments will however be able to register low frequency gravitational waves as well – potentially from supermassive black hole pairs.

ESA/eLISA the future of gravitational wave research

This would finally prove their existence – half a century after they were first proposed. It’s an exciting time to be a scientist.

See the full article here .

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From NASA/DLR SOFIA: “Magnetic Fields May Be the Key to Black Hole Activity”

From NASA/DLR SOFIA

NASA SOFIA GREAT [German Receiver for Astronomy at Terahertz Frequencies]

NASA SOFIA High-resolution Airborne Wideband Camera-Plus HAWC+ Camera

NASA/SOFIA Forcast

Artist’s conception of the core of Cygnus A, including the dusty donut-shaped surroundings, called a torus, and jets launching from its center. Magnetic fields are illustrated trapping the dust in the torus. These magnetic fields could be helping power the black hole hidden in the galaxy’s core by confining the dust in the torus and keeping it close enough to be gobbled up by the hungry black hole.
Credits: NASA/SOFIA/Lynette Cook

Collimated jets provide astronomers with some of the most powerful evidence that a supermassive black hole lurks in the heart of most galaxies. Some of these black holes appear to be active, gobbling up material from their surroundings and launching jets at ultra-high speeds, while others are quiescent, even dormant. Why are some black holes feasting and others starving? Recent observations from the Stratospheric Observatory for Infrared Astronomy, or SOFIA, are shedding light on this question.

SOFIA data indicate that magnetic fields are trapping and confining dust near the center of the active galaxy, Cygnus A, and feeding material onto the supermassive black hole at its center.

The unified model, which attempts to explain the different properties ­of active galaxies, states that the core is surrounded by a donut-shaped dust cloud, called a torus. How this obscuring structure is created and sustained has never been clear, but these new results from SOFIA indicate that magnetic fields may be responsible for keeping the dust close enough to be devoured by the hungry black hole. In fact, one of the fundamental differences between active galaxies like Cygnus A and their less active cousins, like our own Milky Way, may be the presence or absence of a strong magnetic field around the black hole.

Although celestial magnetic fields are notoriously difficult to observe, astronomers have used polarized light — optical light from scattering and radio light from accelerating electrons — to study magnetic fields in galaxies. But optical wavelengths are too short and the radio wavelengths are too long to observe the torus directly. The infrared wavelengths observed by SOFIA are just right, allowing scientists, for the first time, to target and isolate the dusty torus.

SOFIA’s new instrument, the High-resolution Airborne Wideband Camera-plus (HAWC+), is especially sensitive to the infrared emission from aligned dust grains. This has proven to be a powerful technique to study magnetic fields and test a fundamental prediction of the unified model: the role of the dusty torus in the active-galaxy phenomena.

“It’s always exciting to discover something completely new,” noted Enrique Lopez-Rodriguez, a scientist at the SOFIA Science Center, and the lead author on the report of this new discovery. “These observations from HAWC+ are unique. They show us how infrared polarization can contribute to the study of galaxies.”

Two images of Cygnus A layered over each other to show the galaxy’s jets glowing with radio radiation (shown in red). Quiescent galaxies, like our own Milky Way, do not have jets like this, which may be related to magnetic fields. The yellow image shows background stars and the center of the galaxy shrouded in dust when observed with visible light. The area SOFIA observed is inside the small red dot in the center.
Credits: Optical Image: NASA/STSiC Radio Image: NSF/NRAO/AUI/VLA

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)

Recent observations of the heart of Cygnus A made with HAWC+ show infrared radiation dominated by a well-aligned dusty structure. Combining these results with archival data from the Herschel Space Observatory, the Hubble Space Telescope and the Gran Telescopio Canarias, the research team found that this powerful active galaxy, with its iconic large-scale jets, is able to confine the obscuring torus that feeds the supermassive black hole using a strong magnetic field.

ESA/Herschel spacecraft active from 2009 to 2013

Gran Telescopio Canarias at the Roque de los Muchachos Observatory on the island of La Palma, in the Canaries, Spain, sited on a volcanic peak 2,267 metres (7,438 ft) above sea level

The results of this study were published in the July 10th issue of The Astrophysical Journal Letters.

Cygnus A is in the perfect location to learn about the role magnetic fields play in confining the dusty torus and channeling material onto the supermassive black hole because it is the closest and most powerful active galaxy. More observations of different types of galaxies are necessary to get the full picture of how magnetic fields affect the evolution of the environment surrounding supermassive black holes. If, for example, HAWC+ reveals highly polarized infrared emission from the centers of active galaxies but not from quiescent galaxies, it would support the idea that magnetic fields regulate black hole feeding and reinforce astronomers’ confidence in the unified model of active galaxies.

See the full article here .

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SOFIA is a Boeing 747SP jetliner modified to carry a 100-inch diameter telescope. SOFIA is a joint project of NASA and the German Aerospace Center (DLR). The aircraft is based at and the program is managed from NASA Armstrong Flight Research Center’s facility in Palmdale, California. NASA’s Ames Research Center, manages the SOFIA science and mission operations in cooperation with the Universities Space Research Association (USRA) headquartered in Columbia, Maryland, and the German SOFIA Institute (DSI) at the University of Stuttgart.

From NASA Goddard Space Flight Center via Manu Garcia of IAC: “New Simulation Sheds Light on Spiraling Supermassive Black Holes”

From Manu Garcia, a friend from IAC.

The universe around us.
Astronomy, everything you wanted to know about our local universe and never dared to ask.

From NASA Goddard Space Flight Center

Oct. 2, 2018
Jeanette Kazmierczak
jeanette.a.kazmierczak@nasa.gov
NASA’s Goddard Space Flight Center, Greenbelt, Md.

This animation rotates 360 degrees around a frozen version of the simulation in the plane of the disk. Credit: NASA’s Goddard Space Flight Center

A new model is bringing scientists a step closer to understanding the kinds of light signals produced when two supermassive black holes, which are millions to billions of times the mass of the Sun, spiral toward a collision. For the first time, a new computer simulation that fully incorporates the physical effects of Einstein’s general theory of relativity shows that gas in such systems will glow predominantly in ultraviolet and X-ray light.

Just about every galaxy the size of our own Milky Way or larger contains a monster black hole at its center. Observations show galaxy mergers occur frequently in the universe, but so far no one has seen a merger of these giant black holes.

“We know galaxies with central supermassive black holes combine all the time in the universe, yet we only see a small fraction of galaxies with two of them near their centers,” said Scott Noble, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “The pairs we do see aren’t emitting strong gravitational-wave signals because they’re too far away from each other. Our goal is to identify — with light alone — even closer pairs from which gravitational-wave signals may be detected in the future.”

A paper describing the team’s analysis of the new simulation was published Tuesday, Oct. 2, in The Astrophysical Journal.

Gas glows brightly in this computer simulation of supermassive black holes only 40 orbits from merging. Models like this may eventually help scientists pinpoint real examples of these powerful binary systems. Credits: NASA’s Goddard Space Flight Center

Scientists have detected merging stellar-mass black holes — which range from around three to several dozen solar masses — using the National Science Foundation’s Laser Interferometer Gravitational-Wave Observatory (LIGO).

Gravitational waves are space-time ripples traveling at the speed of light. They are created when massive orbiting objects like black holes and neutron stars spiral together and merge.

Black holes heading toward a merger. Precise laser interferometry can detect the ripples in space-time generated when two black holes collide. LIGO-Caltech-MIT-Sonoma State Aurore Simonn

Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

Supermassive mergers will be much more difficult to find than their stellar-mass cousins. One reason ground-based observatories can’t detect gravitational waves from these events is because Earth itself is too noisy, shaking from seismic vibrations and gravitational changes from atmospheric disturbances. The detectors must be in space, like the Laser Interferometer Space Antenna (LISA) led by ESA (the European Space Agency) and planned for launch in the 2030s.

ESA/NASA eLISA space based, the future of gravitational wave research

Observatories monitoring sets of rapidly spinning, superdense stars called pulsars may detect gravitational waves from monster mergers. Like lighthouses, pulsars emit regularly timed beams of light that flash in and out of view as they rotate. Gravitational waves could cause slight changes in the timing of those flashes, but so far studies haven’t yielded any detections.

But supermassive binaries nearing collision may have one thing stellar-mass binaries lack — a gas-rich environment. Scientists suspect the supernova explosion that creates a stellar black hole also blows away most of the surrounding gas. The black hole consumes what little remains so quickly there isn’t much left to glow when the merger happens.

Supermassive binaries, on the other hand, result from galaxy mergers. Each supersized black hole brings along an entourage of gas and dust clouds, stars and planets. Scientists think a galaxy collision propels much of this material toward the central black holes, which consume it on a time scale similar to that needed for the binary to merge. As the black holes near, magnetic and gravitational forces heat the remaining gas, producing light astronomers should be able to see.

“It’s very important to proceed on two tracks,” said co-author Manuela Campanelli, director of the Center for Computational Relativity and Gravitation at the Rochester Institute of Technology in New York, who initiated this project nine years ago. “Modeling these events requires sophisticated computational tools that include all the physical effects produced by two supermassive black holes orbiting each other at a fraction of the speed of light. Knowing what light signals to expect from these events will help modern observations identify them. Modeling and observations will then feed into each other, helping us better understand what is happening at the hearts of most galaxies.”

The new simulation shows three orbits of a pair of supermassive black holes only 40 orbits from merging. The models reveal the light emitted at this stage of the process may be dominated by UV light with some high-energy X-rays, similar to what’s seen in any galaxy with a well-fed supermassive black hole.

Three regions of light-emitting gas glow as the black holes merge, all connected by streams of hot gas: a large ring encircling the entire system, called the circumbinary disk, and two smaller ones around each black hole, called mini disks. All these objects emit predominantly UV light. When gas flows into a mini disk at a high rate, the disk’s UV light interacts with each black hole’s corona, a region of high-energy subatomic particles above and below the disk. This interaction produces X-rays. When the accretion rate is lower, UV light dims relative to the X-rays.

Based on the simulation, the researchers expect X-rays emitted by a near-merger will be brighter and more variable than X-rays seen from single supermassive black holes. The pace of the changes links to both the orbital speed of gas located at the inner edge of the circumbinary disk as well as that of the merging black holes.

This 360-degree video places the viewer in the middle of two circling supermassive black holes around 18.6 million miles (30 million kilometers) apart with an orbital period of 46 minutes. The simulation shows how the black holes distort the starry background and capture light, producing black hole silhouettes. A distinctive feature called a photon ring outlines the black holes. The entire system would have around 1 million times the Sun’s mass. Credits: NASA’s Goddard Space Flight Center; background, ESA/Gaia/DPAC

“The way both black holes deflect light gives rise to complex lensing effects, as seen in the movie when one black hole passes in front of the other,” said Stéphane d’Ascoli, a doctoral student at École Normale Supérieure in Paris and lead author of the paper. “Some exotic features came as a surprise, such as the eyebrow-shaped shadows one black hole occasionally creates near the horizon of the other.”

The simulation ran on the National Center for Supercomputing Applications’ Blue Waters supercomputer at the University of Illinois at Urbana-Champaign.

U Illinois Urbana-Champaign Blue Waters Cray Linux XE/XK hybrid machine supercomputer

Modeling three orbits of the system took 46 days on 9,600 computing cores. Campanelli said the collaboration was recently awarded additional time on Blue Waters to continue developing their models.

The original simulation estimated gas temperatures. The team plans to refine their code to model how changing parameters of the system, like temperature, distance, total mass and accretion rate, will affect the emitted light. They’re interested in seeing what happens to gas traveling between the two black holes as well as modeling longer time spans.

“We need to find signals in the light from supermassive black hole binaries distinctive enough that astronomers can find these rare systems among the throng of bright single supermassive black holes,” said co-author Julian Krolik, an astrophysicist at Johns Hopkins University in Baltimore. “If we can do that, we might be able to discover merging supermassive black holes before they’re seen by a space-based gravitational-wave observatory.”

See the full article here.

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NASA’s Goddard Space Flight Center is home to the nation’s largest organization of combined scientists, engineers and technologists that build spacecraft, instruments and new technology to study the Earth, the sun, our solar system, and the universe.

Named for American rocketry pioneer Dr. Robert H. Goddard, the center was established in 1959 as NASA’s first space flight complex. Goddard and its several facilities are critical in carrying out NASA’s missions of space exploration and scientific discovery.

NASA/Goddard Campus

From Discover Magazine: “Thousands of Black Holes Form Disks in the Center of the Galaxy”

From Discover Magazine

September 13, 2018
Chelsea Gohd

In this artistic visualization, a supermassive black hole at a galaxy’s center shoots out radiation and high-speed winds. According to a new study, supermassive black holes at a galaxy’s center are surrounded by a disk of black holes and massive stars. (Credit: NASA/JPL-Caltech)

At the center of most galaxies lie supermassive black holes. Their exceptional gravity pulls in thousands of stars and stellar mass black holes, or black holes formed when a massive star collapses due to gravity.

By simulating how objects interact near the supermassive black holes in the center of galaxies, astrophysicists from Eötvös University in Hungary have shown, in a new study, that these black holes form a thick disk around a galaxy’s supermassive black hole.

“Previously it was thought that the orbits of both light and massive stellar objects are distributed [uniformly] around the supermassive black hole,” Ákos Szölgyén, a researcher at Eötvös University who led the study, said in a statement, “we now understand that massive stars and black holes typically segregate into a disk.”

Eötvös University

An artist’s [now iconic] illustration of the black hole Cygnus X-1. New research shows that thick disks of black holes and massive stars likely form at the center of all galaxies, surrounding supermassive black holes. (Credit: NASA/CXC/M.Weiss)

Swarm of Black Holes

In their simulation, Szölgyén and his Ph.D. advisor, Bence Kocsis, incorporated something called vector resonant relaxation. It’s an effect that gravity has on objects orbiting a supermassive black hole. This effect grows over millions of years, making the orbital planes of these objects turn.

Kocsis compared the effect and the behavior of the objects to the movement of bees, “Unlike a swarm of bees around a beehive, stars fly around in the galactic center in a more ordered way: along precessing elliptical trajectories, each confined to a plane, respectively,” he said in the statement. Kocsis continued, describing how the objects shift their orbits slowly over millions of years.

This effect helped the astronomers see that while black holes orbit in a disk, less massive objects like stars form a more spherical distribution, Kocsis added in an email.

Stars usually form in one of two ways at the centers of galaxies. Gas can condense into stars around the supermassive black hole. Or, alternatively, groups of stars called globular clusters can spin into the galaxy’s center, where they’re ripped into the building blocks of new stars by the supermassive black hole. “In both cases, we find a disk of black holes,” Kocsis noted.

That means these black hole disks probably form in all galaxies.

Black Hole Disks and Gravitational Waves

According to Kocsis, this study could also help scientists better understand gravitational waves. As scientists have detected gravitational waves using LIGO and VIRGO, they’ve been surprised to see the rate of black hole mergers is much higher than they expected. “The big question, known as the ‘final AU problem’,” Kocsis explained, is how black holes might get to an AU (or astronomical unit, roughly the distance from the Earth to the sun) that drives them to merge.

According to Kocsis, better understanding black hole disks could help answer this question because these dense environments “may lead to mergers more often.”

This study was published in the journal Physical Review Letters.

See the full article here .

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From Discover Magazine: “Black Holes Flicker as They Stop Gorging Themselves on Matter”

From Discover Magazine

September 7, 2018
Alison Klesman

This artistically enhanced image shows a Hubble Space Telescope view of the active galaxy Arp 220, which houses a feeding supermassive black hole at its center. (Credit: NASA/JPL-Caltech)

NASA/ESA Hubble Telescope

Black holes are by nature difficult to study directly. Because even light cannot escape these massive objects, astronomers must turn to other methods to spot and study them. While information is lost once it crosses a black hole’s event horizon, outside that boundary, it can still escape. A recent study, led by a graduate student in the Department of Astronomy of the Universidad de Chile, has now found that the amount of light emitted from around a black hole is determined by one thing, and one thing only: the rate at which matter is falling into the black hole.

The research, published September 4 in The Astrophysical Journal, was aimed at determining the physical mechanism behind the variability observed from the active black holes at the centers of galaxies (known as active galactic nuclei, or AGN), which are supermassive black holes currently sucking in matter.

In astronomy, this process is known as accretion. Such black holes have accretion disks, which are disks of matter swirling around them as it is funneled inward, like water going down a drain. Outside the event horizon, these disks shine brightly as the material inside is heated by friction, giving off visible light and even more energetic light, such as X-rays. These disks are also variable — astronomers aren’t exactly sure why, but the current understanding is that as clumps of matter interact in the disk or fall into the black hole, it causes changes in the light the disk emits.

The team combined data from the Sloan Digital Sky Survey and the QUEST-La Silla AGN Variability Survey to combine physical properties —the mass and the accretion rate, or the speed at which a black hole is eating — of about 2,000 AGN with information about their variability.

SDSS Telescope at Apache Point Observatory, near Sunspot NM, USA, Altitude 2,788 meters (9,147 ft)

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

What they found was surprising: “Contrary to what was believed, the only important physical property to explain the amplitude of the variability is the AGN accretion rate,” said Paula Sánchez-Sáez, the student who led the study and first author of the paper, in a press release.

Out With The Old

Why is this surprising? “The results obtained in this study challenge the old paradigm that the amplitude of the AGN variability depended mainly on the luminosity of the AGN,” Sánchez-Sáez said. What this means is that previously, astronomers assumed that more luminous (brighter) AGN varied more, while less luminous (dimmer) AGN varied less. This study instead discovered that the rate at which a black hole is eating is the only thing that affects the amount its light varies, regardless of whether it is bright or dim.

But the challenge to previous thinking makes sense, Sánchez-Sáez said, because in the past, it’s been difficult to accurately measure a black hole’s mass, and thus its accretion rate. Only with newer data provided by large surveys can astronomers begin to build up the numbers they need to test their assumptions.

With Black Holes, Less is More

Furthermore, the study revealed a relationship that may seem backwards: “What we detect is that the less they [black holes] swallow, the more they vary,” said Paulina Lira of the Universidad de Chile and the CATA Center for Excellence in Astrophysics, and a co-author on the paper. In scientific terms, the amplitude (amount) of variability is inversely proportional to the accretion rate, or the amount of food a black hole is consuming at any given time.

This initial study was based on variability information from the QUEST-La Silla AGN Variability Survey spanning about five years. Now, the researchers are looking to study the variability of these objects in greater detail, for which they’ll need more data. That means staring at these AGN for longer periods of time — at least 10 years or more. For that, they’ll need to wait for future surveys, such as those proposed with the Large Synoptic Survey Telescope, which is expected to reach full science operations by 2023. This will “extend our light curves to an order of 20 years,” said Lira, providing an even more accurate picture of the black hole’s behavior over longer periods of time.

See the full article here .

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From Instituto de Astrofísica de Canarias – IAC via Manu Garcia: “Discover the causes of the apparent displacement of a supermassive black hole”

From Manu Garcia, a friend from IAC.

The universe around us.
Astronomy, everything you wanted to know about our local universe and never dared to ask.

From Instituto de Astrofísica de Canarias – IAC

Observing the core of Messier 87, HST-1 galaxy.

Messier 87 image with WFC3 HST (2016) with F814W filter. different knots are seen along the jet, including the first node HST-1. Credit: NASA/ESA Hubble.

NASA/ESA Hubble Telescope

NASA/ESA Hubble WFC3

The study by two researchers from Instituto de Astrofísica reveals that the shift observed in the nucleus of the galaxy Messier 87 is not due to a shift of its massive black hole, but variations in light production in the center of the galaxy caused by bursts from a jet, a flow of material relativistic beam as the hole itself emits.

Today it is assumed that all massive galaxies contain a supermassive black hole (SMBH, for its acronym in English) at its core. In recent years galaxies are looking for candidates to present a SMBHs displaced from its equilibrium position. Among the scenarios that can cause this displacement are merging two SMBHs or the existence of a binary system SMBHs, which gives information about galactic evolution and formation frequency and fusion of such objects.

One of the galaxies candidates to present a displaced SMBHs is the giant elliptical Messier 87, containing one of the closest and best-studied active galaxy nuclei (AGN, for its acronym in English). Previous research SMBHs displacement of Messier 87 gave very different results, which was confusing. However, a new study by the student of the University of La Laguna (ULL), Elena López Navas has provided new data suggesting that the SMBHs of this galaxy is in its equilibrium position and shifts found must be variations in the production center or photocentric light caused by bursts from the relativistic jet, a flow of matter that the hole itself expelled outside at speeds near that of light.

Research has been necessary to analyze a large number of high-resolution images of Messier 87 taken at different times and with different instruments installed on the Hubble Space Telescope (HST) and the Very Large Telescope (VLT).

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

“Given these results, we realized that the images showed a shift in the center of the galaxy were taken at a time when M87 was a huge explosion that could be measured in all ranges of the electromagnetic spectrum,” adds Almudena Prieto , co-author and researcher at the Institute of Astrophysics of the Canary Islands (IAC). This outbreak took place between 2003 and 2007 at the node nearest the nucleus known as Messier 87 HST-1 jet. During the duration of the phenomenon, this knot increased its flow coming to shine even more than the core itself. “Temporal analysis of displacement of center of the galaxy shows that indeed the burst is related to the change of the position of photocentric – clarifies the astrophysics, however, after this phenomenon, and the core photocentric meet occupying the same place, so we deduce that the core and the black hole are always in the same location coinciding with the minimum of galactic potential. ”

Displacements found (in milli – arcseconds) against the date of
observation of each analyzed image. An increase of displacement is observed
around 2005, when the maximum emission occurred in the first
knot jet, HST-1. Credit: Elena Lopez.

“In our work we have found that the SMBHs is in a stable over the last 20 years position; On the contrary, what changes is the production center of light or Fotocentro “says Lopez, author of this study, as work Master’s Research in Astrophysics, which has just been published in the journal <em>Monthly Notices of the Royal Astronomical Society</em> (MNRAS).

The new data have caused great interest among the astrophysics community, as the study SMBHs position of M87 is crucial to understanding the evolution of this galaxy and analysis of other AGN jets. “In addition, this research reminds us that we must be cautious when considering variables sources with irregularities such as, in this case, a huge jet,” says Lopez, who is currently conducting a training grant in astrophysical research at the IAC.

Work Master Thesis: E. Lopez Navas (2018 ULL), “Measurement and analysis of the displacement between the Fotocentro and the supermassive black hole in M87“.

Contact:
Elena Lopez Navas, ULL student / IAC: eln_ext@iac.es
Almudena Prieto Escudero, a researcher at the IAC: aprieto@iac.es

See the full article here.

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The Instituto de Astrofísica de Canarias(IAC) is an international research centre in Spain which comprises:

The Instituto de Astrofísica, the headquarters, which is in La Laguna (Tenerife).
The Centro de Astrofísica en La Palma (CALP)
The Observatorio del Teide (OT), in Izaña (Tenerife).
The Observatorio del Roque de los Muchachos (ORM), in Garafía (La Palma).

Roque de los Muchachos Observatory is an astronomical observatory located in the municipality of Garafía on the island of La Palma in the Canary Islands, at an altitude of 2,396 m (7,861 ft)

These centres, with all the facilities they bring together, make up the European Northern Observatory(ENO).

The IAC is constituted administratively as a Public Consortium, created by statute in 1982, with involvement from the Spanish Government, the Government of the Canary Islands, the University of La Laguna and Spain’s Science Research Council (CSIC).

The International Scientific Committee (CCI) manages participation in the observatories by institutions from other countries. A Time Allocation Committee (CAT) allocates the observing time reserved for Spain at the telescopes in the IAC’s observatories.

The exceptional quality of the sky over the Canaries for astronomical observations is protected by law. The IAC’s Sky Quality Protection Office (OTPC) regulates the application of the law and its Sky Quality Group continuously monitors the parameters that define observing quality at the IAC Observatories.

The IAC’s research programme includes astrophysical research and technological development projects.

The IAC is also involved in researcher training, university teaching and outreachactivities.

The IAC has devoted much energy to developing technology for the design and construction of a large 10.4 metre diameter telescope, the ( Gran Telescopio CANARIAS, GTC), which is sited at the Observatorio del Roque de los Muchachos.

Gran Telescopio Canarias at the Roque de los Muchachos Observatory on the island of La Palma, in the Canaries, SpainGran Telescopio CANARIAS, GTC

From ESOblog: “What’s Next for the Heart of the Milky Way”

From ESOblog

This artist´s impression shows the path of the star S2 as it passes very close to the supermassive black hole at the centre of the Milky Way. As it gets close to the black hole the very strong gravitational field causes the colour of the star to shift slightly to the red, an effect of Einstein´s general theory of relativity.
Credit: ESO/M. Kornmesser

Reinhard Genzel on the significance and future of galactic centre research.

10 August 2018

Reinhard Genzel’s team at the Max Planck Institute for Extraterrestrial Physics (MPE) recently found general relativistic effects during the closest approach of the star S2 to the Sagittarius A*, a supermassive black hole at the centre of the Milky Way.

Star S2 Keck/UCLA Galactic Center Group

SgrA* NASA/Chandra

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

Sgr A* from ESO VLT

This discovery is not only a step forward in the research of the galactic centre, but it’s also a great leap in our understanding of physics. In the last of three blog posts, Reinhard Genzel discusses this recent discovery and what future research might look like.

Q: Firstly can you tell us about the observations your team have just completed?

A: The star S2 passed very close to the black hole in the centre of our galaxy, the Milky Way, just a few weeks ago. With our long-term preparations for this event, we were able to gather a lot of high-quality data, not only on the position of the star along its orbit, but also on its velocity. Indeed, over the past decade, we developed a completely novel instrument, GRAVITY, which allows us to study the galactic centre in unprecedented detail and ultra-high precision.

ESO GRAVITY in the VLTI

Q: Some members of team have worked over 16 years to prepare for these observations, since the last time S2 made a close approach to SgrA*. What have we learned in this time and what have you discovered now?

A: The first close approach in 2002 and the first full orbit, were dedicated to proving that there is indeed a supermassive black hole at the centre of our Milky Way. Actually, we now believe that all large galaxies harbour a black hole at their core. With the current observing campaign, we focused on studying the black hole in more detail to find out more about general relativistic effects and the properties of the black hole itself — and we have now found evidence of these effects.

This artist´s impression shows the path of the star S2 as it passes very close to the supermassive black hole at the centre of the Milky Way. As it gets close to the black hole the very strong gravitational field causes the colour of the star to shift slightly to the red, an effect of Einstein´s general theory of relativity. Credit: ESO/M. Kornmesser

Q: Why are your team’s observations of S2 important?

A: The black hole in our Milky Way is close enough that we are able to study individual stars near it — we can do that in no other galaxy. The star S2 is special in that it comes very close to the black hole and it completes its orbit in only 16 years. For the other stars, we can only observe part of their orbits — which also gives us some very interesting information — but in the coming years, only S2 dips so deeply into the gravitational well of the black hole.

Q: What can these observations tell us about general relativity?

A: We are observing an object in a very strong gravitational field, much stronger than anything that can be observed on Earth. We saw general relativistic effects indicated by the orbital precession, an effect we already know from the orbit of Mercury around the Sun, and the gravitational redshift, wherein the starlight changes frequency due to the strong pull of gravity. While other observations have seen general relativistic effects in a few other astronomical systems, our observations of the heart of the Milky Way, for the first time, tested Einstein’s theory in the extreme gravitational field around a massive black hole.

Learn more about the first successful test of Einstein´s General Theory of Relativity near a supermassive black hole in ESOcast 173.

Q: What can these observations tell us about black holes?

A: First of all, it can tell us that black holes really do exist, and that they are not just a theoretical construct. All our observations show that there is a supermassive black hole at the centre of the Milky Way, if Einstein’s general theory of relativity holds. With this new data we can make a strong case that Einstein is right — and with general relativity in place, the only possible explanation is a black hole.

Q: What is it about these observations that make them different from the last time S2 made its closest approach to the galactic centre?

A: To observe the effects that I have mentioned above, we need very accurate data on the orbit of S2 and on its velocity. With the GRAVITY instrument, we now have a hundred-fold improvement in our astrometry, our tracking of stars, compared to the 1990s and about 20 times better data than during the last close flyby. Now, we can even follow the star’s motion from day to day.

This time-lapse view shows images from the GRAVITY instrument on ESO´s Very Large Telescope as it tracks the progress of the star S2 as it made a close passage past the black hole at the centre of the Milky Way in May 2018. Credit: ESO/GRAVITY Collaboration

Q: What are you looking forward to learning about the galactic centre in the future? What do you realistically expect to find out in the next few years?

A: Our first step was to look for one particular post-Newtonian effect, namely, that clocks tick more slowly in a gravitational field. But predictions from the theory of general relativity are far more astonishing. If the black hole has a spin, spacetime itself will rotate, pulling the stars along with it. The unprecedented resolution and sensitivity of our GRAVITY instrument — we hope — will allow us to measure this effect using faint stars at an even closer orbit. Such measurements might also allow us to determine if there are additional massive objects, such as stellar-mass or intermediate-mass black holes, close to the galactic centre as predicted by many theorists. Furthermore, we also hope to see gas orbiting at distances very close to the black hole. We do see gas emission shining up regularly, and we hope to push our instrument a bit further, such that we can see how the emission runs around the black hole – within less than half an hour or so! This would be the full relativistic regime, and correspondingly exciting!

Q: Why should we continue studying the galactic centre? What mysteries are still unsolved?

A: The black hole in the galactic centre is the ideal laboratory to study these extreme objects. Ultimately we want to bring together the theories of quantum mechanics and gravitation, which could lead to new physics. Theorists predict that this should happen close to the event horizon, the point from which leaving a black hole becomes impossible.

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From Chandra: “Finding the Happy Medium of Black Holes”

NASA/Chandra Telescope

From NASA Chandra

August 9, 2018
Press Release
Megan Watzke
Chandra X-ray Center, Cambridge, Mass.
617-496-7998
mwatzke@cfa.harvard.edu

.
Credit: X-ray: NASA/CXC/ICE/M.Mezcua et al.; Infrared: NASA/JPL-Caltech; Illustration: NASA/CXC/A.Hobart
Press Image, Caption, and Videos

Important evidence for populations of intermediate-mass black holes (IMBHs) has been found.

Using data from Chandra and other telescopes, two teams independently discovered IMBHs relatively nearby and billions of light years away.

The detection of X-ray emission by Chandra provides critical evidence that IMBHs have been discovered.

IMBHs may play a significant role in the formation of the very biggest black holes in the early Universe.

This image shows data from a massive observing campaign that includes NASA’s Chandra X-ray Observatory. These Chandra data have provided strong evidence for the existence of so-called intermediate-mass black holes (IMBHs). Combined with a separate study also using Chandra data, these results may allow astronomers to better understand how the very largest black holes in the early Universe formed, as described in our latest press release.

The COSMOS (“cosmic evolution survey”) Legacy Survey has assembled data from some of the world’s most powerful telescopes spanning the electromagnetic spectrum. This image contains Chandra data from this survey, equivalent to about 4.6 million seconds of observing time. The colors in this image represent different levels of X-ray energy detected by Chandra. Here the lowest-energy X-rays are red, the medium band is green, and the highest-energy X-rays observed by Chandra are blue. Most of the colored dots in this image are black holes. Data from the Spitzer Space Telescope are shown in grey.

NASA/Spitzer Infrared Telescope

The inset shows an artist’s impression of a growing black hole in the center of a galaxy. A disk of material surrounding the black hole and a jet of outflowing material are also depicted.

Two new separate studies using the Chandra COSMOS-Legacy survey data and other Chandra data have independently collected samples of IMBHs, an elusive category of black holes in between stellar mass black holes and the supermassive black holes found in the central regions of massive galaxies.

One team of researchers identified 40 growing black holes in dwarf galaxies. Twelve of them are located at distances more than five billion light years from Earth and the most distant is 10.9 billion light years away, the most distant growing black hole in a dwarf galaxy ever seen. Most of these sources are likely IMBHs with masses that are about 10,000 to 100,000 times that of the Sun.

A second team found a separate, important sample of possible IMBHs in galaxies that are closer to Earth. In this sample, the most distant IMBH candidate is about 2.8 billion light years from Earth and about 90% of the IMBH candidates they discovered are no more than 1.3 billion light years away.

They detected 305 galaxies in their survey with black hole masses less than 300,000 solar masses. Observations with Chandra and with ESA’s XMM-Newton of a small part of this sample show that about half of the 305 IMBH candidates are likely to be valid IMBHs.

ESA/XMM Newton

The masses for the ten sources detected with X-ray observations were determined to be between 40,000 and 300,000 times the mass of the Sun.

IMBHs may be able to explain how the very biggest black holes, the supermassive ones, were able to form so quickly after the Big Bang. One leading explanation is that supermassive black holes grow over time from smaller black holes “seeds” containing about a hundred times the Sun’s mass. Some of these seeds should merge to form IMBHs. Another explanation is that they form very quickly from the collapse of a giant cloud of gas with a mass equal to hundreds of thousands of times that of the Sun. There is yet to be a consensus among astronomers on the role IMBHs may play.

A paper describing the COSMOS-Legacy result by Mar Mezcua (Institute for Space Sciences, Spain) and colleagues was published in the August issue of the Monthly Notices of the Royal Astronomical Society and is available online. The paper by Igor Chilingarian (Harvard-Smithsonian Center for Astrophysics) on the closer IMBH sample is being published in the August 10th issue of The Astrophysical Journal and is available online.

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NASA’s Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program for NASA’s Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory controls Chandra’s science and flight operations from Cambridge, Mass.