Could the Fermi Large Area Telescope detect γ-rays from dark matter annihilation in the dwarf galaxies of the Local Group?

¹ Consorzio Interuniversitario di Fisica Spaziale, Villa Gualino, Viale Settimio Severo, 63, 10133 Torino, Italy e-mail: lidia.pieri@oapd.inaf.it
² Istituto Nazionale di Fisica Nucleare - Sezione di Padova, via Marzolo 8, 35131 Padova, Italy
³ Dipartimento di Astronomia, Università di Padova, vicolo dell'Osservatorio 3, 35122 Padova, Italy

Received: 1 September 2008
Accepted: 27 November 2008

Abstract

Context. The detection of γ-rays from dark matter (DM) annihilation is among the scientific goals of the Fermi Large Area Telescope (formerly known as GLAST) and Cherenkov telescopes.

Aims. In this paper we investigate the chances of such a discovery, selecting some nearby dwarf spheroidal galaxies (dSph) as a target, and adopting the DM density profiles derived from both astronomical observations and N-body simulations. We also make use of recent studies about the presence of black holes and of a population of sub-subhalos inside the Local Group (LG) dwarfs to carry out boost factor studies.

Methods. We study the detectability with the Fermi-LAT of the γ-ray flux from DM annihilation in four of the nearest and highly DM-dominated dSph galaxies of the LG, namely Draco, Ursa Minor, Carina, and Sextans, for which state-of-art DM density profiles were available. We assume the DM is made of weakly interacting massive particles such as the lightest supersymmetric particle and compute the expected γ-ray flux for estimations of the unknown underlying particle physics parameters. We then compute the boost factors due to the presence of DM clumps and of a central supermassive black hole. Finally, we compare our predictions with the Fermi-LAT sensitivity maps.

Results. We find that the dSph galaxies shine above the Galactic smooth halo: e.g., the Galactic halo is brighter than the Draco dSph only for angles smaller than 2.3 degrees above the Galactic Center. We also find that the presence of a cusp or a constant density core in the DM mass density profile does not produce any relevant effects in the γ-ray flux due to the fortunate combination of the geometrical acceptance of the Fermi-LAT detector and the distance of the galaxies. Moreover, no significant enhancement is given by the presence of a central black hole or a population of sub-subhalos.

Conclusions. We conclude that, even for the most optimistic scenario of particle physics, the γ-ray flux from DM annihilation in the dSph galaxies of the LG would be too low to be detected with the Fermi-LAT.