Although dark matter accounts for most of the matter in the Universe, its nature remains unknown. So far the presence of dark matter has only been inferred through its gravitational effects. However, if dark matter is made of neutralinos, a new particle predicted by Supersymmetry, it would also interact, although very weakly, with ordinary matter, and it might be detected soon in laboratories on Earth. In addition, neutralinos, being Majorana fermions, can self-annihilate to produce ordinary particles like positrons, neutrinos and gamma-ray photons. If these byproducts of the annihilation are copious enough, they could be detected by satellites such as FERMI, which has been mapping the gamma-ray sky since mid 2008.

This gamma-ray radiation is produced most abundantly in high density regions. Thus, it seems best to look for it in very dense nearby regions, such as the centre of our own Galaxy and/or the centres of its satellite galaxies. Actually, it turns out that the best prospects for the detection of gamma-rays from our Galaxy are obtained by looking slightly off-centre to avoid confusion of the signal with other sources of gamma rays residing at the Galactic centre (see Research Highlight December 2003).

However, outside of the Galactic halo, gamma-rays are also produced in large quantities by the annihilation of dark matter in all the many haloes and subhaloes within our past light-cone, contributing to the so-called extragalactic gamma-ray background (EGB) radiation. Although the EGB also receives contributions from other sources, such as blazars and cosmic rays accelerated at structure formation shocks, the energy spectrum and angular power spectrum of the annihilation radiation have distinctive features that may open up effective ways for disentangling the signal. Therefore, a detailed analysis of the EGB is a viable possibility for detecting dark matter.

In a new study, scientists at the MPA used the state-of-the-art Millennium-II simulation (an MPA project) to generate all-sky maps of the contribution of dark matter annihilation to the EGB radiation. A special map-making procedure was developed that re-creates the past light cone of a fiducial galactic observer, taking into account the gamma-ray luminosity of all numerically resolved haloes and their subhaloes. The method also includes corrections for unresolved components of the emission as well as an extrapolation to the minimum mass for bound neutralino dark matter haloes. The angular resolution of the created maps was chosen to be close to that of FERMI, approximately 0.115 degrees.

It was found that for most of the relevant energy range (0.1-30 GeV), the signal comes mainly from sources up to redshifts z~2. In the most optimistic scenario considered, the energy spectrum of the isotropic component of the background radiation lies approximately one order of magnitude below the values of the EGB measured by the telescope EGRET (predecessor of FERMI) in the energy range 1-20 GeV, where an apparent excess of gamma-rays had led to speculations of a possible origin in dark matter annihilations. The results found by the MPA-team indicate that if this excess is indeed confirmed by FERMI, then annihilation of neutralinos could only account for the signal if the annihilation process is somehow enhanced. Several mechanisms have been proposed that may indeed yield such an enhancement, such as the presence of highly dense “spikes” of dark matter formed around intermediate-mass black holes (with masses between a hundred and a million solar masses), or the so-called Sommerfeld enhancement, a quantum-mechanical focusing effect that increases the annihilation cross section.

The MPA-team also studied the anisotropic component of the EGB by computing the angular power spectrum of the simulated maps. This yielded specific predictions for the shape of the power spectrum, which can potentially be used to discriminate against other sources of gamma-rays because the annihilation signal depends in a specific and unique way on the large-scale distribution of haloes, on the distribution of subhaloes within haloes, and on the abundance and internal structure of haloes as a function of time. Also, the shape of the power spectrum was found to depend on the energy of the observations. Interestingly, these differences can be exploited to construct “color” maps that enhance the signal of nearby dark matter structures, akin to hardness ratio maps in X-ray observations. For example, the MPA scientists found that taking the ratio of the maps at energies of 0.1 GeV and 32 GeV greatly enhances the contrast of local dark matter structures, making them clearly visible in the gamma-ray sky. If strong spectral features in the rest-frame emission spectrum of the annihilation radiation are present, this could be especially powerful, perhaps even allowing tomographic observations of dark matter structures.

Jesus Zavala, Volker Springel and Michael Boylan-Kolchin

Publications

Jesus Zavala, Volker Springel, Michael Boylan-Kolchin, "Extragalactic gamma-ray background radiation from dark matter annihilation", 2009, submitted to MNRAS.
arXiv:0908.2428v2