4 ms·

With stars and dark matter in the galaxy. A picture explains it better than anything else: http://burro.astr.cwru.edu/JavaLab/cannibal/CannibalDynaFric.html h

With 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 ago
So, 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 ago
  Yes, 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 ago
As 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 ago
  It'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!

AnimalMuppet 11y ago

So, 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 ago
Yes, 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 ago

As 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 ago
It'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!