PhD Thesis: Observations of nearby Galaxy Clusters with the Fermi Large Area Telescope
Monday 23 November 2015
to 16:00 at
Stephan Zimmer (Stockholm University, Department of Physics)
Galaxy clusters are the most massive bound systems known in the Universe and are believed to have formed through
large scale structure formation. They host relativistic cosmic-ray (CR) populations and are gravitationally bound by large
amounts of Dark Matter (DM), both providing conditions in which high-energy gamma rays may be produced either via
CR interactions with the intracluster medium or through the annihilation or decay of DM particles.
Prior to the launch of the Fermi satellite, predictions were optimistic that these sources would be established as γ-raybright
objects by observations through its prime instrument, the Large Area Telescope (LAT). Yet, despite numerous efforts,
even a single firm cluster detection is still pending. This thesis presents a number of studies based on data taken by the
LAT over its now seven year mission aiming to discover these γ rays.
Using a joint likelihood technique, we study the γ-ray spectra of a sample of nearby clusters searching for a CR-induced
signal due to hadronic interactions in the intracluster medium. While we find excesses in some individual targets, we
attribute none to the cluster. Hence, we constrain the maximum injection efficiency of hadrons being accelerated in structure
formation shocks and the fraction of CR-to-thermal pressure. We also perform a refined search targeting the Coma cluster
specifically due to its large variety of existing observations in other wavebands. In the latter case we find weak indications
of an excess which however falls below the detection threshold.
Because the cluster emission we consider is inherently extended, we need to take into account the imperfect modeling
of the foreground emission, which may be particularly difficult such as is the case with the Virgo cluster. Here, we assess
the systematics associated with the foreground uncertainties and derive limits based on an improved background model
of the region. For the first time we derive limits on the γ-ray flux from CR and DM-interactions in which we take into
account the dynamical state of the system. For DM we also include the contribution from substructure. The DM domain is
further explored by searching for line-like features as they arise from the annihilation of DM into two photons in a large
sample of clusters, including Virgo and Coma. Finding no evidence for γ-ray lines, we derive limits on the DM annihilation
cross section that are roughly a factor 10 (100) above that derived from observations of the galactic center assuming an
optimistic (conservative) scenario regarding the boost due to DM substructure.