Is the radio emission in the Bullet cluster due to dark matter annihilation?

Abstract

We study the complex structure of the Bullet cluster radio halo to determine the Dark Matter (DM) contribution to the emission observed in the different subhalos corresponding to the DM and baryonic dominated regions. We use different non-thermal models to study the different regions, and we compare our results with the available observations in the radio, X-ray and gamma-ray bands, and the Sunyaev-Zel'dovich (SZ) effect data. We find that the radio emission coming from the main DM subhalo can be produced by secondary electrons produced by DM annihilations. In this scenario there are however some open issues, like the difficulty to explain the observed flux at 8.8 GHz, the high value of the required annihilation cross section, and the lack of observed emission coming from the minor DM subhalo. We also find that part of the radio emission originated by DM annihilation could be associated with a slightly extended radio source present near the main DM subhalo. Regarding the baryonic subhalos, the radio measurements do not allow to discriminate between a primary or secondary origin of the electrons, while the SZ effect data point towards a primary origin for the non-thermal electrons in the Main Subcluster. We conclude that in order to better constrain the properties of the DM subhalos, it is important to perform detailed measurements of the radio emission in the regions where the DM halos have their peaks, and that the separation of the complex radio halo in different subhalos is a promising technique to understand the properties of each specific subhalo.

The role of Dark Matter sub-halos in the non-thermal emission of galaxy clusters

“…In the hard X-ray and soft gamma-ray bands the emission coming from DM-produced electrons is well below the sensitivity of the next generation and/or proposed instruments like Astro-H2 or AstroMeV and ASTROGAM (Colafrancesco & Marchegiani, in preparation): however, a possible detection of galaxy clusters in these bands can be used to rule out DM models. The same is true for the microwave band, where DM models do not provide a strong non-thermal Sunyaev-Zel'dovich effect (SZE) signal because they produce a very low amount of electrons at low energies (see, e.g., [34]); therefore, a possible detection of the non-thermal SZE in Coma would allow to exclude a DM origin for the non-thermal electrons.…”

Possible Dark Matter annihilation in the galaxy cluster A520

“…It is anyway possible that the DM can give a contribution in some regions of the cluster, as suggested in the case of the Bullet cluster [5]. In the next Section we explore this possibility.…”

“…However, other mechanisms originated by ordinary matter (baryonic or leptonic) can produce non-thermal emission in galaxy clusters [3]. In clusters where the DM density spatial distribution, as derived from gravitational lensing measures, is different from the Intra Cluster Medium (ICM) one, as derived from X-rays, like for example in the Bullet cluster [4], the detection of DM-induced signals can be easier in the cluster regions where the DM is supposed to be dominant [5,6].…”

Probing the low-energy spectrum of non-thermal electrons in galaxy clusters with soft gamma ray observations

“…Colafrancesco, Marchegiani & Palladino, 2003). A non-thermal contribution to the total SZE has been possibly detected in the Bullet cluster (Colafrancesco, Marchegiani & Buonanno, 2011;Marchegiani & Colafrancesco, 2015), suggesting the possibility of a very small value of the minimum energy of the electrons (γ min ∼ 1.4). Also, a statistical study of the SZE in 23 radio halo clusters (Colafrancesco et al, 2014) suggested an average value of the ratio between non-thermal and thermal electrons pressures of the order of X ≡ P non−th /P th ≈ 0.55.…”