The surface brightness of dark matter: unique signatures of neutralino annihilation in the Galactic halo

TL;DR

The paper addresses whether neutralino annihilation in the Galactic halo can produce detectable gamma-ray signals. It combines analytic line-of-sight flux integrals $\phi(\psi)=\frac{K}{4\pi}\int\rho^2(l)\,dl$ with high-resolution CDM halos and detailed substructure modeling, including a Moore cusp and a subhalo mass function $dn(m)/dm\propto m^{-1.9}$ and $dn(v_c)/dv_c\propto v_c^{-3.8}$. The results show that substructure boosts the diffuse gamma-ray background by more than two orders of magnitude and that halo shape (prolate vs oblate) and subhalo clustering produce distinctive all-sky patterns, with a population of bright subhalo sources following $N\propto F^{-0.7}$. These signatures offer falsifiable gamma-ray indicators of neutralino annihilation, enabling halo-shape quantification and cross-section constraints with current data (e.g., EGRET) and upcoming surveys such as GLAST and VERITAS.

Abstract

We use high resolution numerical simulations of the formation of cold dark matter halos to simulate the background of decay products from neutralino annihilation, such as gamma-rays or neutrinos. Halos are non-spherical, have steep singular density profiles and contain many thousands of surviving dark matter substructure clumps. This leads to several unique signatures in the gamma-ray background that may be confirmed or rejected by the next generation of gamma-ray experiments. Most importantly, the diffuse background is enhanced by over two orders of magnitude due to annihilation within substructure halos. The largest dark substructures are easily visibly above the background and may account for the unidentified EGRET sources. A deep strip survey of the gamma-ray background would allow the shape of the Galactic halo to be quantified.

Figures (26)

• Figure 1: (a) The circular velocity curves $V_c(r) = \sqrt{GM(r)/r}$, and (b) density profiles are plotted as a function of the radius for each of the halo models considered in the text.
• Figure 2: The gamma ray flux from neutralino annihilation, $\phi(\psi)$, plotted as a function of the angular distance from the galactic center $\psi$. The curves show the results using the three different density profiles plotted in Fig. \ref{['f:001']} The flux at a given position is averaged over $4\pi$ steradians.
• Figure 3: All-sky maps of the gamma ray background constructed using a single high-resolution N-body simulation of a cold dark matter halo. The observer has been placed in the short (a) and long (b) axis of the simulated halo.
• Figure 4: The left panel shows a unit oblate ellipsoid and the right hand panel shows a unit prolate ellipsoid. The axial ratios for both are 2:1.
• Figure 5: The gamma ray flux, $\phi$, plotted as a function of angle $\psi$, for smooth halos of the same total mass using the density profile given in Eq. \ref{['eq:0']} for spherical, oblate and prolate halo geometries. The points are values of the flux measured directly from the N-body halo illustrated in Fig. \ref{['f:003']}.