Dark matter black holes

Question

Provided that dark matter coaleces in halos around and within a galaxy and gravity seems to affect dark matter in a similar way as regular matter, would it be possible for dark matter to coalece into a black hole and would not the mass of a black hole include regular matter and dark matter.

Answer 1

It may be possible. They are trying to see if the data may fit. I think it is unlikely, but here are pros and cons.

In http://www.nasa.gov/feature/goddard/2016/nasa-scientist-suggests-possible-link-between-primordial-black-holes-and-dark-matter they are trying the hypothesis that dark matter is black holes formed in the first second after the Big Bang. They've calculated that if the masses are typically close to that of the black holes detected merging by LIGO in 2015, about 30 solar masses, it reproduces the astrophysical data seen for cosmic X Ray and IR radiation. I have not followed their calculations.

There are significant constraints on how many black holes may exist at different mass sizes, and the larger ones are not favored by those constraints. The constraints are from the CMB,gravitational lensing, etc. The sizes possible depends on how late they were formed, the later the bigger. Still, people are looking for more data. The Fermi Gamma Ray Space Telescope is looking for X rays from smaller black holes that would be finishing to evaporate through Hawking radiation about now. See https://en.m.wikipedia.org/wiki/Primordial_black_hole.

I like @Crowell's argument that it would not be easy for dark matter to dissipate energy in forming primordial black holes. Tends to say that black holes are not likely made of dark matter. However, see below, if the dark matter is cold (i.e., slow) it might be possible. Still, primordial black holes I refer to in the searches above are normal matter, not formed by stellar collapse, but by enough compression from radiation pressure bubbles that get small enough in a big enough mass to collapse directly. That is what the NASA reference on the primordial black hole search is looking for.

On the negative side I have seen reviews elsewhere that described the limits on MACHOS, as the sources of the dark matter, with black holes including primordial but not too small being the sources. At https://en.m.wikipedia.org/wiki/Dark_matter_halo the dark matter limits on MACHOS is described and shows that they are mostly ruled out. The current preferred hypothesis is that it is likely WIMPS. See also a summary at http://curious.astro.cornell.edu/about-us/88-the-universe/black-holes-and-quasars/observation-of-black-holes/421-could-the-universe-s-dark-matter-be-made-up-of-black-holes-advanced. The WIMPS are presumed to be cold dark matter, but smaller massive particles as opposed to larger bodies.

Answer 2

A black hole in a cloud of dark matter can absorb these particles just as with any other form of matter. However, it is unlikely dark matter can form black holes unless these particles interact by other unmeasured forces that dissipate energy. If they are purely gravitational in nature, then a cloud of dark matter is described by the Virial theorem with kinetic energy magnitude half the potential energy magnitude.

A black hole forms because matter dissipates a lot of energy. A star exhausts its nuclear fusion source at the core and gravitation implodes it. Stars form to begin with because of dissipative interactions. A lot of heat is lost by the system to set up the core implosion. If dark matter is purely gravitational, or maybe weakly interacting, then not enough energy can be dissipated to force this matter into a collapsed configuration, first a star and then ultimately the implosion of the stellar core.