Black Holes enhance Dark Matter Annihilations

Title: Effect of Black Holes in Local Dwarf Spheroidal Galaxies on Gamma-Ray Constraints on Dark Matter Annihilation
Author: Alma X. Gonzalez-Morales, Stefano Profumo, Farinaldo S. Queiroz
PublishedarXiv:1406.2424 [astro-ph.HE]
Upper bounds on dark matter annihilation from a combined analysis of 15 dwarf spheroidal galaxies for NFW (red) and Burkert (blue) DM density profiles.
Upper bounds on dark matter annihilation from a combined analysis of 15 dwarf spheroidal galaxies for NFW (red) and Burkert (blue) DM density profiles. Fig. 4 from arXiv:1406.2424.

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 (\sim 10^4 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, \sigma_*. As a benchmark, the authors use the Tremaine relation to determine the black hole mass as a function of the observed velocity dispersion,

Screen Shot 2014-06-11 at 6.51.24 PM

Here M_{\odot} is the mass of the sun. Given this mass and its effect on the dark matter density, they can then calculate the 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:

DM annihilation cross-section constraints for the b ̄b final state, for individual dSph, and for a combined analysis of 15 galaxies, assuming an initial NFW DM density distributio
DM annihilation cross-section constraints for annihilation into a pair of b quarks, from 1406.2424 Fig. 1. The shaded band is the target cross section to obtain the correct dark matter relic density through thermal freeze out, the red box is the target cross section for a dark matter interpretation of an excess in gamma rays in the galactic center.

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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Flip Tanedo

Assistant Professor at University of California, Riverside
Flip is an assistant professor in theoretical physics at the University of California, Riverside. He previousy completed a Bachelors of Science at Stanford, Masters degrees at Cambridge and the IPPP in Durham, and a Ph.D at Cornell. He has been supported by a Goldwater scholarship, a Marshall scholarship, an NSF Gradaute Research Fellowship, a Paul & Daisy Soros fellowship, and a UCI Chancellor's ADVANCE fellowship. He was a participant in the original Communicating Science 2013 workshop which led to the creation of ParticleBites. His research focuses on models and signatures of physics beyond the Standard Model, including dark matter, supersymmetry, and extra dimensions. Much of his creative thinking is done while swimming or driving along Southern California's freeways.

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