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  • richardmitnick 11:00 am on February 7, 2019 Permalink | Reply
    Tags: Abraham (Avi) Loeb, , , , , Black Hole Initiative, Black Hole Institute, , , Infrared results beautifully complemented by observations at radio wavelengths, , , , S-02, SGR A and SGR A*, , The development of high-resolution infrared cameras revealed a dense cluster of stars at the center of the Milky Way   

    From Nautilus: “How Supermassive Black Holes Were Discovered” 


    From Nautilus

    February 7, 2019
    Mark J. Reid, CfA SAO

    Astronomers turned a fantastic concept into reality.

    An Introduction to the Black Hole Institute

    Fittingly, the Black Hole Initiative (BHI) was founded 100 years after Karl Schwarzschild solved Einstein’s equations for general relativity—a solution that described a black hole decades before the first astronomical evidence that they exist. As exotic structures of spacetime, black holes continue to fascinate astronomers, physicists, mathematicians, philosophers, and the general public, following on a century of research into their mysterious nature.

    Pictor A Blast from Black Hole in a Galaxy Far, Far Away

    This computer-simulated image of a supermassive black hole at the core of a galaxy. Credit NASA, ESA, and D. Coe, J. Anderson

    The mission of the BHI is interdisciplinary and, to that end, we sponsor many events that create the environment to support interaction between researchers of different disciplines. Philosophers speak with mathematicians, physicists, and astronomers, theorists speak with observers and a series of scheduled events create the venue for people to regularly come together.

    As an example, for a problem we care about, consider the singularities at the centers of black holes, which mark the breakdown of Einstein’s theory of gravity. What would a singularity look like in the quantum mechanical context? Most likely, it would appear as an extreme concentration of a huge mass (more than a few solar masses for astrophysical black holes) within a tiny volume. The size of the reservoir that drains all matter that fell into an astrophysical black hole is unknown and constitutes one of the unsolved problems on which BHI scholars work.

    We are delighted to present a collection of essays which were carefully selected by our senior faculty out of many applications to the first essay competition of the BHI. The winning essays will be published here on Nautilus over the next five weeks, beginning with the fifth-place finisher and working up to the first-place finisher. We hope that you will enjoy them as much as we did.

    —Abraham (Avi) Loeb
    Frank B. Baird, Jr. Professor of Science, Harvard University
    Chair, Harvard Astronomy Department
    Founding Director, Black Hole Initiative (BHI)

    In the 1700s, John Michell in England and Pierre-Simon Laplace in France independently thought “way out of the box” and imagined what would happen if a huge mass were placed in an incredibly small volume. Pushing this thought experiment to the limit, they conjectured that gravitational forces might not allow anything, even light, to escape. Michell and Laplace were imagining what we now call a black hole.

    Astronomers are now convinced that when massive stars burn through their nuclear fuel, they collapse to near nothingness and form a black hole. While the concept of a star collapsing to a black hole is astounding, the possibility that material from millions and even billions of stars can condense into a single supermassive black hole is even more fantastic.

    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

    Yet astronomers are now confident that supermassive black holes exist and are found in the centers of most of the 100 billion galaxies in the universe.

    How did we come to this astonishing conclusion? The story begins in the mid-1900s when astronomers expanded their horizons beyond the very narrow range of wavelengths to which our eyes are sensitive. Very strong sources of radio waves were discovered and, when accurate positions were determined, many were found to be centered on distant galaxies. Shortly thereafter, radio antennas were linked together to greatly improve angular resolution.

    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)

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

    CfA Submillimeter Array Mauna Kea, Hawaii, USA,4,207 m (13,802 ft) above sea level

    These new “interferometers” revealed a totally unexpected picture of the radio emission from galaxies—the radio waves did not appear to come from the galaxy itself, but from two huge “lobes” symmetrically placed about the galaxy. Figure One shows an example of such a “radio galaxy,” named Cygnus A. Radio lobes can be among the largest structures in the universe, upward of a hundred times the size of the galaxy itself.

    Figure One: Radio image of the galaxy Cygnus A. Dominating the image are two huge “lobes” of radio emitting plasma. An optical image of the host galaxy would be smaller than the gap between the lobes. The minimum energy needed to power some radio lobes can be equivalent to the total conversion of 10 million stars to energy! Note the thin trails of radio emission that connect the lobes with the bright spot at the center, where all of the energy originates. NRAO/AUI

    How are immense radio lobes energized? Their symmetrical placement about a galaxy clearly suggested a close relationship. In the 1960s, sensitive radio interferometers confirmed the circumstantial case for a relationship by discovering faint trails, or “jets,” tracing radio emission from the lobes back to a very compact source at the precise center of the galaxy. These findings motivated radio astronomers to increase the sizes of their interferometers in order to better resolve these emissions. Ultimately this led to the technique of Very Long Baseline Interferometry (VLBI), in which radio signals from antennas across the Earth are combined to obtain the angular resolution of a telescope the size of our planet!

    GMVA The Global VLBI Array

    Radio images made from VLBI observations soon revealed that the sources at the centers of radio galaxies are “microscopic” by galaxy standards, even smaller than the distance between the sun and our nearest star.

    When astronomers calculated the energy needed to power radio lobes they were astounded. It required 10 million stars to be “vaporized,” totally converting their mass to energy using Einstein’s famous equation E = mc2! Nuclear reactions, which power stars, cannot even convert 1 percent of a star’s mass to energy. So trying to explain the energy in radio lobes with nuclear power would require more than 1 billion stars, and these stars would have to live within the “microscopic” volume indicated by the VLBI observations. Because of these findings, astronomers began considering alternative energy sources: supermassive black holes.

    Given that the centers of galaxies might harbor supermassive black holes, it was natural to check the center of our Milky Way galaxy for such a monster. In 1974, a very compact radio source, smaller than 1 second of arc (1/3600 of a degree) was discovered there. The compact source was named Sagittarius A*, or Sgr A* for short, and is shown at the center of the right panel of Figure 2. Early VLBI observations established that Sgr A* was far more compact than the size of our solar system. However, no obvious optical, infrared, or even X-ray emitting source could be confidently identified with it, and its nature remained mysterious.

    Figure Two: Images of the central region of the Milky Way. The left panel shows an infrared image. The orbital track of star S2 is overlaid, magnified by a factor of 100. The orbit has period of 16 years, requires an unseen mass of 4 million times that of the sun, and the gravitational center is indicated by the arrow. The right panel shows a radio image. The point-like radio source Sgr A* (just below the middle of the image) is precisely at the gravitational center of the orbiting stars. Sgr A* is intrinsically motionless at the galactic center and, therefore, must be extremely massive.Left panel: R. Genzel; Right panel: J.-H. Zhao

    Star S0-2 Andrea Ghez Keck/UCLA Galactic Center Group

    Andrea’s Favorite star SO-2

    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

    SGR A and SGR A* from Penn State and NASA/Chandra

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

    Meanwhile, the development of high-resolution infrared cameras revealed a dense cluster of stars at the center of the Milky Way. These stars cannot be seen at optical wavelengths, because visible light is totally absorbed by intervening dust. However, at infrared wavelengths 10 percent of their starlight makes its way to our telescopes, and astronomers have been measuring the positions of these stars for more than two decades. These observations culminated with the important discovery that stars are moving along elliptical paths, which are a unique characteristic of gravitational orbits. One of these stars has now been traced over a complete orbit, as shown in the left panel of Figure Two.

    Many stars have been followed along partial orbits, and all are consistent with orbits about a single object. Two stars have been observed to approach the center to within the size of our solar system, which by galaxy standards is very small. At this point, gravity is so strong that stars are orbiting at nearly 10,000 kilometers per second—fast enough to cross the Earth in one second! These measurements leave no doubt that the stars are responding to an unseen mass of 4 million times that of the sun. Combining this mass with the (astronomically) small volume indicated by the stellar orbits implies an extraordinarily high density. At this density it is hard to imagine how any type of matter would not collapse to form a black hole.

    The infrared results just described are beautifully complemented by observations at radio wavelengths. In order to identify an infrared counterpart for Sgr A*, the position of the radio source needed to be precisely transferred to infrared images. An ingenious method to do this uses sources visible at both radio and infrared wavelengths to tie the reference frames together. Ideal sources are giant red stars, which are bright in the infrared and have strong emission at radio wavelengths from molecules surrounding them. By matching the positions of these stars at the two wavebands, the radio position of Sgr A* can be transferred to infrared images with an accuracy of 0.001 seconds of arc. This technique placed Sgr A* precisely at the position of the gravitational center of the orbiting stars.

    How much of the dark mass within the stellar orbits can be directly associated with the radio source Sgr A*? Were Sgr A* a star, it would be moving at over 10,000 kilometers per second in the strong gravitational field as other stars are observed to do. Only if Sgr A* is extremely massive would it move slowly. The position of Sgr A* has been monitored with VLBI techniques for over two decades, revealing that it is essentially stationary at the dynamical center of the Milky Way. Specifically, the component of Sgr A*’s intrinsic motion perpendicular to the plane of the Milky Way is less than one kilometer per second. By comparison, this is 30 times slower than the Earth orbits the sun. The discovery that Sgr A* is essentially stationary and anchors the galactic center requires that Sgr A* contains over 400,000 times the mass of the sun.

    Recent VLBI observations have shown that the size of the radio emission of Sgr A* is less than that contained within the orbit of Mercury. Combining this volume available to Sgr A* with the lower limit to its mass yields a staggeringly high density. This density is within a factor of less than 10 of the ultimate limit for a black hole. At such an extreme density, the evidence is overwhelming that Sgr A* is a supermassive black hole.

    These discoveries are elegant for their directness and simplicity. Orbits of stars provide an absolutely clear and unequivocal proof of a great unseen mass concentration. Finding that the compact radio source Sgr A* is at the precise location of the unseen mass and is motionless provides even more compelling evidence for a supermassive black hole. Together they form a simple, unique demonstration that the fantastic concept of a supermassive black hole is indeed a reality. John Michell and Pierre-Simon Laplace would be astounded to learn that their conjectures about black holes not only turned out to be correct, but were far grander than they ever could have imagined.

    Mark J. Reid is a senior astronomer at the Center for Astrophysics, Harvard & Smithsonian. He uses radio telescopes across the globe simultaneously to obtain the highest resolution images of newborn and dying stars, as well as black holes.

    See the full article here .


    Please help promote STEM in your local schools.

    Stem Education Coalition

    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.

  • richardmitnick 12:31 pm on January 22, 2019 Permalink | Reply
    Tags: , , , , , , Our Galaxy's Supermassive Black Hole Could Be Pointing a Relativistic Jet Right at Us, , SGR A and SGR A*   

    From Science Alert: “Our Galaxy’s Supermassive Black Hole Could Be Pointing a Relativistic Jet Right at Us” 


    From Science Alert

    22 JAN 2019

    A black hole simulation (Bronzwaer/Davelaar/Moscibrodzka/Falcke/Radboud University)

    Things are officially getting exciting. New science has just come in from the collaboration to photograph Sagittarius A*, the supermassive black hole at the centre of the Milky Way, and it’s ponying up the secrets at our galaxy’s dusty heart.

    SGR A and SGR A* from Penn State and NASA/Chandra

    The image below is the best picture yet of Sgr A* (don’t worry, there’s more to come from the Event Horizon Telescope), and while it may look like just a weird blob of light to you, astrophysicists studying the radio data can learn a lot from what they’re looking at – and they think they’ve identified a relativistic jet angled towards Earth.

    EHT map

    Because the image taken of the region is the highest resolution yet – twice as high as the previous best – the researchers were able to precisely map the properties of the light around the black hole as scattered by the cloud.

    “The galactic centre is full of matter around the black hole, which acts like frosted glass that we have to look through,” astrophysicist Eduardo Ros of the Max Planck Institute for Radio Astronomy in Germany told New Scientist.

    Using very long baseline interferometry to take observations at a wavelength of 3.5 millimetres (86 GHz frequency), a team of astronomers has used computer modelling to simulate what’s inside the thick cloud of plasma, dust and gas surrounding the black hole.

    Above: The bottom right image shows Sgr A* as seen in the data. The top images are simulations, while the bottom left is Sgr A* with the scattering removed.
    (S. Issaoun, M. Mościbrodzka, Radboud University/ M. D. Johnson, CfA)

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

    GMVA The Global VLBI Array

    It revealed that Sgr A*’s radio emission comes from a smaller region than previously thought.

    Most of it is coming from an area just 300 milllionth of a degree of the night sky, with a symmetrical shape. And, since black holes don’t emit detectable radiation on their own, the source is most likely one of two things.

    “This may indicate that the radio emission is produced in a disk of infalling gas rather than by a radio jet,” said astrophysicist Sara Issaoun of Radboud University in The Netherlands.

    “However, that would make Sgr A* an exception compared to other radio emitting black holes. The alternative could be that the radio jet is pointing almost at us.”

    Active black holes are surrounded by a swirling cloud of material that’s falling into it like water down a drain. As this material is swallowed by the black hole, it emits jets of particles from its rotational poles at velocities approaching light speed.

    We’re not quite sure how this happens, but astronomers believe that material from the inner part of the accretion disc is channelled towards and launched from the poles via magnetic field lines.

    Since Earth is in the galactic plane, having a jet pointed in our direction would mean that the black hole is oriented quite strangely, as if it’s lying on its side. (Nearby galaxy Centaurus A, for instance, has jets shooting perpendicular to the galactic plane.)

    But this orientation has been hinted at before. Last year the GRAVITY Collaboration described flares around Sgr A* consistent with something orbiting it face-on from our perspective – like looking at the Solar System from above.

    This means the long-awaited picture of the shadow of a black hole will – hopefully – be breathtakingly detailed.

    Meanwhile, studying data such as these help build a comprehensive picture of how these mysterious cosmic objects work.

    “Understanding how black holes work … takes more than the picture of its shadow (although incredible in its own right),” Issaoun wrote on Facebook. “It takes observations at many different wavelengths (radio, X-ray, infrared etc) to piece together the entire story, so every piece counts!”

    The team’s paper has been published in The Astrophysical Journal..

    So “Maybe this is true after all,” said Radboud University astronomer Heino Falcke, “and we are looking at this beast from a very special vantage point.”

    Hopefully, when the Event Horizon Telescope releases the first images of Sgr A*’s event horizon – something we are expecting very soon – they will reveal more. And, in case you were starting to get worried, the 1.4-millimetre wavelength (230 GHz) will reduce the light scattering by a factor of 8.

    See the full article here .

    See also here .


    Please help promote STEM in your local schools.

    Stem Education Coalition

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