Can Dark Matter Annihilation Dominate the Extragalactic Gamma-Ray Background?

Annihilating dark matter (DM) has been discussed as a possible source of gamma rays from the galactic center and as a contribution to the extragalactic gamma-ray background. Assuming universality of the density profile of DM halos, we show that it is quite unlikely that DM annihilation is a main constituent of extragalactic gamma-ray background, without exceeding the observed gamma-ray flux from the galactic center. This argument becomes stronger when we include enhancement of the density profiles by supermassive black holes or baryon cooling. The presence of a substructure may loosen the constraint, but only if a very large cross section as well as the rather flat profile are realized.

Figure 1

(a) Gamma-ray flux from the GC from annihilating DM, with mass 50 GeV or 2 TeV, evaluated with the NFW and M99 profiles. Data from EGRET [3] and CANGAROO-II [6] are also plotted; the HESS result [7] is shown as a solid line. (b) EGB intensity from DM annihilation. EGRET data points [13] are also plotted.

Figure 2

Boost factors, $b_{\mathrm{GC}}^{\max }$ and $b_{\mathrm{EGB}}^{\max }$, required to make DM annihilation a main component of the data, for the NFW and M99 profiles. Assumed masses are 10, 20, 50, 100, 200, 500, 1000, 2000, and 5000 GeV (from bottom to top). The solid and dotted lines represent $b_{\mathrm{EGB}}=b_{\mathrm{GC}}$, $10b_{\mathrm{GC}}$, $100b_{\mathrm{GC}}$ and $b_{\mathrm{GC}}=1$, $b_{\mathrm{EGB}}=1$, plotted for comparison.