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The article alludes to an open question in astronomy known as the "last parsec problem," but doesn't quite state it outright. The issue is that we observe that
by antognini 11y ago
The article alludes to an open question in astronomy known as the "last parsec problem," but doesn't quite state it outright. The issue is that we observe that all galaxies have supermassive black holes at their centers and that galaxies merge. Due to a phenomenon called dynamical friction, as the supermassive black hole of one galaxy passes through the stellar field of the other galaxy, its velocity slows until it comes nearly to rest with respect to the common center of mass of the two galaxies. This leads both supermassive black holes to sink to the center of the newly merged galaxy and come into orbit with each other over a relatively short timescale (I want to say ~100 million years at most).
Over time, the orbit of these two supermassive black holes shrinks because stars wandering through the orbit get kicked out and take energy with them. However, once the orbit shrinks to about a parsec, this process stalls. All the stars that could wander through the orbit have already been kicked out and there's nothing left to shrink the orbit. Furthermore, gravitational wave radiation is still too weak to make much of a difference at that distance.
Now, supermassive black hole binaries are rare, so clearly they end up merging somehow. It's just not known how this happens. For a while it was thought that the last parsec problem could be solved by taking into account the triaxiality of the galactic potential [1] (and the references therein). The idea was that new stars from further out in the galaxy would wander in as quickly as the black holes could kick them out and the orbit would continue to shrink. However, new work seems to be casting doubt on this and whether or not triaxiality solves the final parsec problem is still being debated [2].
[1]: http://adsabs.harvard.edu/abs/2013ApJ...773..100K http://adsabs.harvard.edu/abs/2013ApJ...773..100K
[2]: http://adsabs.harvard.edu/abs/2014ApJ...785..163V http://adsabs.harvard.edu/abs/2014ApJ...785..163V
- AnimalMuppet 11y agoDynamic friction with dark matter? Interaction with the accretion disk?
- antognini 11y agoWith stars and dark matter in the galaxy. A picture explains it better than anything else: http://burro.astr.cwru.edu/JavaLab/cannibal/CannibalDynaFric.html http://burro.astr.cwru.edu/JavaLab/cannibal/CannibalDynaFric... or http://abyss.uoregon.edu/~js/lectures/cannibalism/cannibalism_7.html http://abyss.uoregon.edu/~js/lectures/cannibalism/cannibalis...
- AnimalMuppet 11y agoSo, if I understand correctly, if there's no stars in there, but there is a dark matter halo, the black holes could be slowed by dynamic friction with the black hole halo, correct? Could that solve the last parsec problem?
- antognini 11y agoYes, a dark matter halo can lead to dynamical friction, though I think that stars are more important through most of the process. But by the time you get to a ~few parsec orbit the orbital velocity is too fast and the dark matter density is too low to provide a substantial drag.
- darkmighty 11y agoAs an aside, is this dynamical friction completely analogous to friction in gases? Do stars reach "thermal equilibrium" in their velocities, and can be somehow talk about things like entropy w.r.t to stars instead of gases? Found it fascinating for some reason...
- antognini 11y agoIt's very similar, but not completely analogous to friction in gasses. But the long-term result is the same. An object moving through a gas will slow down until it comes to rest with respect to the gas. Similarly a massive object moving through a medium of stars will slow down until it comes to rest with respect to the stars. There are many parallels between thermodynamics and orbital mechanics. In fact, a collection of stars can be thought of as a self-gravitating gas where the particles don't collide (they're so far apart relative to their size that collisions are very, very rare). The analogous concept to thermal equilibrium in gravity is called "virial equilibrium." One of the very interesting things about a self-gravitating system in virial equilibrium (like a cluster of stars) is that it has negative specific heat! So if it collapses it loses energy, but the stars move faster so it heats up!
- ohazi 11y agoOh hey... Hi Joe! :-)
- antognini 11y agoHi Oren!
- mikeash 11y agoWhen you say that supermassive black hole binaries are rare, I understand this to mean that lots of galaxies have been surveyed and they almost all have a single supermassive black hole in the center, which indicates that something allows them to overcome this problem and merge. Is that right? How do you detect the existence of two supermassive black holes one parsec apart in a galaxy millions of light years away?
- antognini 11y agoThe main way is through the presence of active galactic nuclei (AGN). If the black hole accretes gas rapidly enough, it can become very luminous, sometimes brighter than all the stars of the galaxy combined. One thing astronomers have looked for has been two AGN in the center of a galaxy (known as dual AGN). I think the main way that they are detected is in the X-ray since the two AGN generally have different spectra, so it's possible to distinguish them. (I'm not an X-ray astronomer, though, so I may be way off base.) But these searches for dual AGN have found that they are very rare relative to single AGN.
- mikeash 11y agoVery interesting, thank you!