Black Holes enhance Dark Matter Annihilations

In a previous ParticleBite we showed how dwarf spheroidal galaxies can tell us about dark matter interactions. As a short summary, these are dark matter-rich “satellite [sub-]galaxies” of the Milky Way that are ideal places to look for photons coming from dark matter annihilation into Standard Model particles. In this post we highlight a recent update to that analysis.

The rate at which a pair of dark matter particles annihilate in a galaxy is proportional to the square of the dark matter density. The authors point out that if the dwarf spheroidal galaxies contain intermediate mass black holes ( times the mass of the sun), then its possible that the dark matter in the dwarf is more densely packed near the black hole. The authors redo the FERMI analysis for DM annihilation in dwarf spheroidals with 4 years of data (see our previous ParticleBite) with the assumption that these dwarfs contain a black hole consistent with their observed properties.

While the dwarf galaxies have little stellar content, one can use the visible stars to measure the stellar velocity dispersion, . As a benchmark, the authors use the Tremaine relation to determine the black hole mass as a function of the observed velocity dispersion,

Here is the mass of the sun. Given this mass and its effect on the dark matter density, they can then calculate the J factor that encodes the `astrophysical’ line of slight integral of the squared dark matter density to observers on the Earth. Following the FERMI analysis, authors then set bounds on the dark matter annihilation cross section as a function of the dark matter mass for 15 dwarf spheroidals:

Observe that the bounds are significantly stronger than those in the original FERMI analysis. In particular, the strongest bounds thoroughly rule out the “40 GeV DM annihilating into a pair of b quarks” interpretation of a reported excess in gamma rays coming from the galactic center. These bounds, however, come with several caveats that are described in the paper. The largest caveat is that the existence of a black hole in any of these systems is only assumed. The authors note that numerical simulations suggest that there should be black holes in these systems, but to date there has been no verification of their existence.

Further Reading

• We refer to the previous ParticleBite for introductory material on indirect detection of dark matter.
• See this blog post at io9 for a public-level exposition and video of observational evidence for the supermassive black hole (much heavier than the intermediate mass black holes posited in the dwarf spheroidals) at the center of the Milky Way.
• See Ullio et al. (astro-ph/0101481) for an early paper describing the effect of black holes on the dark matter distribution.

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