# Mapping extragalactic dark matter structures through gamma-rays 

J. Zavala, V. Springel and M. Boylan-Kolchin Max-Planck-Institut f¨ur Astrophysik, Karl-Schwarzschild-Straße 1, 85740 Garching bei M¨unchen, Germany 

If dark matter is composed of neutralinos, the gamma-ray radiation produced in their annihilation offers an attractive possibility for dark matter detection. This process may contribute significantly to the extragalactic gamma-ray background (EGB) radiation, which is being measured by the FERMI satellite with unprecedented sensitivity. Using the high-resolution Millennium-II simulation of cosmic structure formation we have produced the first full-sky maps of the expected contribution of dark matter annihilation to the EGB radiation. Our maps include a proper normalization of the signal according to a specific supersymmetric model based on minimal supergravity. The new simulated maps allow a study of the angular power spectrum of the gamma-ray background from dark matter annihilation, which has distinctive features associated with the nature of the annihilation process. Our results are in broad agreement with analytic models for the gamma-ray background, but they also include higher-order correlations not readily accessible in analytic calculations and, in addition, provide detailed spectral information for each pixel. In particular, we find that color maps combining different energies can reveal the cosmic large-scale structure at low and intermediate redshifts. 

## 1. Introduction 

Although dark matter accounts for most of the mat- ter in the Universe, its nature remains unknown and its presence has only been inferred through its grav- itational effects. However, if dark matter is made of neutralinos, a new particle predicted by Supersym- metry, it would also interact (although very weakly) with ordinary matter, and it might be detected soon in laboratories on Earth. 

In addition, neutralinos can self-annihilate to pro- duce ordinary particles like positrons Baltz and Edsj¨ o [1999], neutrinos Berezinsky et al. [1996] and gamma- ray photons. If these byproducts of the annihilation are copious enough, they could be detected soon. In the present work, we will focus on the gamma-ray pho- tons as a residual of the annihilation process. 

This gamma-ray radiation is expected to be pro- duced more abundantly in regions with high dark matter density. Thus, it seems best to look for it in very dense nearby regions, such as the cen- ter of our own Galaxy Bertone and Merritt [2005], Jacholkowska et al. [2006] and/or the centers of its satellite galaxies Wood et al. [2008]. Actually, it turns out that the best prospects for the detection of gamma-rays from our Galaxy are obtained by looking slightly off-center to avoid confusion of the signal with other sources of gamma rays residing at the Galactic center Stoehr et al. [2003], Springel et al. [2008]. 

Outside our Galactic halo, gamma-rays are also pro- duced in large quantities by the annihilation of dark matter in all the many halos and subhalos within our past light-cone, contributing to the so-called ex- tragalactic gamma-ray background (EGB) radiation. The EGB has been measured by different satellites, in particular in the energy range between 1 MeV and 30 GeV by COMPTEL and EGRET Strong et al. [2004]. Our understanding of the EGB is greatly im- proving by the Large Area Telescope aboard the re- cently launched FERMI satellite Atwood et al. [2009], 

which covers an energy range between 20 MeV and 300 GeV and features an improved sensitivity com- pared with its predecessor EGRET. 

Although the EGB also receives contributions from other sources, such as blazars Ando et al. [2007a] and cosmic rays accelerated at structure formation shocks Jubelgas et al. [2008], the energy spectrum and an- gular power spectrum of the annihilation radiation have distinctive features that may open up effective ways for disentangling the signal Ando and Komatsu [2006], Siegal-Gaskins and Pavlidou [2009]. There- fore, a detailed analysis of the EGB is a viable possi- bility for detecting dark matter. 

Previous studies have analyzed this possibility us- ing analytic approaches. However, it is not clear how accurately these methods capture the non-linear structures resolved in the newest generation of high- resolution cosmological simulations. Also, the previ- ous analysis have so far been restricted to statistical statements about the power spectrum of the EGB, or its mean flux. For a full characterization of the sig- nal, it would be useful to have accurate realizations of maps of the expected gamma-ray emission over the whole sky. 

In our study Zavala et al. [2009], we therefore focus on predicting the EGB radiation directly from cosmological N-body simulations. To this end, we use the Millennium-II simulation (MS-II) Boylan-Kolchin et al. [2009]. With 10 10 particles in a homogeneously sampled volume of (100 h − $^{1}$Mpc)$^{3}$, it is one of the best resolved structure formation simu- lations to date. We use a ray-tracing technique to ac- cumulate the signal from all halos and subhalos over the past light-cone of a fiducial observer, positioned at a plausible location of the Milky Way in the simu- lation box. 

The values we calculate for each pixel of such a sky map are that of the specific intensity, the energy of photons received per unit area, time, solid angle and 

