Disentangling the gamma-ray emission of NGC 1275 and that of the Perseus cluster

Context. The Gamma-ray emission from galaxy clusters hosting active galaxies is a complex combination of diffuse and point-like emission features with different spectral and spatial properties. <BR /> Aims We discuss in details the case of the Perseus cluster containing the radio-galaxy NGC 1275 that has been recently detected as a bright gamma-ray source by the Fermi-LAT experiment, in order to disentangle the sources of emission. <BR /> Methods We provide a detailed study of the gamma-ray emission coming from the core of Perseus by modeling the central AGN emission with a multiple plasma blob model, and the emission from the extended cluster atmosphere with both a warming ray (WR) model and dark matter (DM) neutralino annihilation models. We set constraints on both the central galaxy and cluster SED models by using both archival multi-frequency data and the recent very high energy observations obtained by Fermi and MAGIC. <BR /> Results We find that i) in all the viable models for the cluster gamma-ray emission, the emission detected recently by Fermi from the center of the Perseus cluster is dominated by the active galaxy NGC 1275, that is found in a high-emission state; ii) the diffuse gamma-ray emission of the cluster, in the WR model and in the DM models with the highest allowed normalization, could be detected by Fermi if the central emission from NGC 1275 is in a low-emission state; iii) Fermi can have the possibility to resolve and detect the diffuse gamma-ray flux (predicted by the WR model) coming from the outer corona of the Perseus cluster atmosphere at distances r ⪆ 800 kpc. These results are consistent with the evidence that in the other frequency bands, the diffuse cluster emission dominates on the central galaxy one at low radio frequencies with nu ⪅ 1 GHz and at X-ray energies of order of E~ keV. <BR /> Conclusions Our results show that a simultaneous study of the various emission mechanisms that produce diffuse gamma-rays from galaxy clusters and the study of the emission mechanisms that produce gamma-rays from active galaxies residing in the cluster atmospheres is absolutely crucial first to disentangle the spectral and spatial characteristics of the gamma-ray emission and secondly to assess the optimal observational strategy in the attempt to reveal the still elusive diffuse gamma-ray emission widely predicted for the atmospheres of large-scale structures.