Cross-correlating the gamma-ray sky with catalogs of galaxy clusters

TL;DR

This study detects a gamma-ray cross-correlation with galaxy cluster catalogs, indicating contributions from AGNs and possibly intra-cluster medium, revealing large-scale structure and cluster-related gamma-ray emissions.

Contribution

First detection of gamma-ray cross-correlation with multiple galaxy cluster catalogs, linking gamma-ray signals to large-scale structures and cluster emissions.

Findings

Positive correlation between gamma-ray maps and cluster catalogs

Detection of large-scale gamma-ray emission extending around 1 degree

Evidence for a compact gamma-ray component within clusters

Abstract

We report the detection of a cross-correlation signal between {\it Fermi} Large Area Telescope diffuse gamma-ray maps and catalogs of clusters. In our analysis, we considered three different catalogs: WHL12, redMaPPer and PlanckSZ. They all show a positive correlation with different amplitudes, related to the average mass of the objects in each catalog, which also sets the catalog bias. The signal detection is confirmed by the results of a stacking analysis. The cross-correlation signal extends to rather large angular scales, around 1 degree, that correspond, at the typical redshift of the clusters in these catalogs, to a few to tens of Mpc, i.e. the typical scale-length of the large scale structures in the Universe. Most likely this signal is contributed by the cumulative emission from AGNs associated to the filamentary structures that converge toward the high peaks of the matter density field in which galaxy clusters reside. In addition, our analysis reveals the presence of a second component, more compact in size and compatible with a point-like emission from within individual clusters. At present, we cannot distinguish between the two most likely interpretations for such a signal, i.e. whether it is produced by AGNs inside clusters or if it is a diffuse gamma-ray emission from the intra-cluster medium. We argue that this latter, intriguing, hypothesis might be tested by applying this technique to a low redshift large mass cluster sample.