• Black hole billiards in the centers of g

    From ScienceDaily@1:317/3 to All on Wed Mar 9 21:30:50 2022
    Black hole billiards in the centers of galaxies

    Date:
    March 9, 2022
    Source:
    University of Copenhagen - Faculty of Science
    Summary:
    Researchers provide the first plausible explanation to why one of
    the most massive black hole pairs observed to date by gravitational
    waves also seemed to merge on a non-circular orbit. Their suggested
    solution involves a chaotic triple drama inside a giant disk of
    gas around a super massive black hole in a galaxy far, far away.



    FULL STORY ========================================================================== Researchers provide the first plausible explanation to why one of the
    most massive black hole pairs observed to date by gravitational waves
    also seemed to merge on a non-circular orbit. Their suggested solution,
    now published in Nature, involves a chaotic triple drama inside a giant
    disk of gas around a super massive black hole in a galaxy far, far away.


    ========================================================================== Black holes are one of the most fascinating objects in the Universe,
    but our knowledge of them is still limited -- especially because they do
    not emit any light. Up until a few years ago, light was our main source
    of knowledge about our universe and its black holes, until the Laser Interferometer Gravitational Wave Observatory (LIGO) in 2015 made its breakthrough observation of gravitational waves from the merger of two
    black holes.

    "But how and where in our Universe do such black holes form and
    merge? Does it happen when nearby stars collapse and both turn into
    black holes, is it through close chance encounters in star clusters,
    or is it something else? These are some of the key questions in the
    new era of Gravitational Wave Astrophysics," says Assist. Prof. Johan
    Samsing from the Niels Bohr Institute at the University of Copenhagen,
    lead author of the paper.

    He and his collaborators may have now provided a new piece to the puzzle,
    which possibly solves the last part of a mystery that astrophysicists
    have struggled with for the past few years.

    Unexpected Discovery in 2019 The mystery dates back to 2019, when an
    unexpected discovery of gravitational waves was made by the LIGO and Virgo observatories. The event named GW190521is understood to be the merger
    of two black holes, which not only were heavier than previously thought physically possible, but had in addition produced a flash of light.



    ========================================================================== Possible explanations have since been provided for these two
    characteristics, but the gravitational waves also revealed a third
    astonishing feature of this event -- namely that the black holes did
    not orbit each other along a circle in the moments before merging.

    "The gravitational wave event GW190521 is the most surprising discovery
    to date. The black holes' masses and spins were already surprising,
    but even more surprising was that they appeared not to have a circular
    orbit leading up to the merger," says co-author Imre Bartos, Prof. at
    the University of Florida.

    But why is a non-circular orbit so unusual and unexpected? "This is
    because of the fundamental nature of the gravitational waves emitted,
    which not only brings the pair of black holes closer for them to finally
    merge but also acts to circularize their orbit." explains co-author
    Zoltan Haiman, a Professor at Columbia University.

    This observation made many people around the world, including Johan
    Samsing in Copenhagen, wonder, "It made me start thinking about how
    such non-circular (known as "eccentric") mergers can happen with the surprisingly high probability as the observation suggests," says Johan
    Samsing.



    ==========================================================================
    It Takes Three to Tango A possible answer would be found in the harsh environment in the centers of galaxies harboring a giant black hole
    millions of times the mass of the Sun surrounded by a flat, rotating
    disk of gas.

    "In these environments the typical velocity and density of black holes
    is so high that smaller black holes bounce around as in a giant game of billiards and wide circular binaries cannot exist," points out co-author
    Prof. Bence Kocsis from the University of Oxford.

    But as the group further argued, a giant black hole is not enough,
    "New studies show that the gas disk plays an important role in capturing smaller black holes, which over time move closer to the center and also
    closer to one other.

    This not only implies they meet and form pairs, but also that such a pair
    might interact with another, third, black hole, often leading to a chaotic tango with three black holes flying around, " explains astrophysicist
    Hiromichi Tagawa from Tohoku University, co-author of the study.

    However, all previous studies up to observation of GW190521 indicated that forming eccentric black hole mergers is relatively rare. This naturally
    brings up the question: Why did the already unusual gravitational wave
    source GW190521 also merge on an eccentric orbit? Two Dimensional Black
    Hole Billiards Everything that has been calculated so far was based on
    the notion that the black hole interactions are taking place in three dimensions, as expected in the majority of stellar systems considered
    so far.

    "But then we started thinking about what would happen if the black hole interactions were instead to take place in a flat disk, which is closer
    to a two-dimensional environment. Surprisingly, we found in this limit
    that the probability of forming an eccentric merger increases by as much
    as a 100 times, which leads to about half of all black hole mergers in
    such disks possibly being eccentric," says Johan Samsing and continues:
    "And that discovery fits incredibly well with the observation in
    2019, which all in all now points in the direction that the otherwise spectacular properties of this source are not so strange again, if it
    was created in a flat gas disk surrounding a super massive black hole in
    a galactic nucleus." This possible solution also adds to a century-old
    problem in mechanics, "The interaction between 3 objects is one of the
    oldest problems in physics, which both Newton, myself, and others have intensely studied. That this now seems to play a crucial role in how
    black holes merge in some of the most extreme places of our Universe
    is incredibly fascinating ," says co-author Nathan W. Leigh, Prof. at Universidad de Concepcio'n, Chile.

    Black Holes in Gaseous Disks The theory of the gas disk also fits with
    other researchers' explanations of the other two puzzling properties
    of GW190521. The large masses of the black hole have been reached by
    successive mergers inside the disk, while the emission of light could
    originate from the ambient gas.

    "We have now shown that there can be a huge difference in the signals
    emitted from black holes that merge in flat, two-dimensional disks, versus those we often consider in three-dimensional stellar systems, which tells
    us that we now have an extra tool that we can use to learn about how
    black holes are created and merge in our Universe, " says Johan Samsing.

    But this study is only the beginning, "People have been working on understanding the structure of such gas disks for many years, but the
    problem is difficult. Our results are sensitive to how flat the disk is,
    and how the black holes move around in it. Time will tell whether we will
    learn more about these disks, once we have a larger population of black
    hole mergers, including more unusual cases similar to GW190521. To enable
    this, we must build on our now published discovery, and see where it leads
    us in this new and exciting field" concludes co-author Zoltan Haiman.


    ========================================================================== Story Source: Materials provided by University_of_Copenhagen_-_Faculty_of_Science. Note: Content may be
    edited for style and length.


    ========================================================================== Related Multimedia:
    * Swarm_of_smaller_black_holes_in_a_gas_disk ========================================================================== Journal Reference:
    1. J. Samsing, I. Bartos, D. J. D'Orazio, Z. Haiman, B. Kocsis,
    N. W. C.

    Leigh, B. Liu, M. E. Pessah, H. Tagawa. AGN as potential factories
    for eccentric black hole mergers. Nature, 2022; 603 (7900): 237 DOI:
    10.1038/ s41586-021-04333-1 ==========================================================================

    Link to news story: https://www.sciencedaily.com/releases/2022/03/220309131909.htm

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