Measurement of redshift-dependent cross-correlation of HSC clusters and Fermi γ-rays

The cross-correlation study of the unresolved γ-ray background (UGRB) with galaxy clusters has the potential to reveal the nature of the UGRB. In this paper, we perform a cross-correlation analysis between γ-ray data by the Fermi Large Area Telescope and a galaxy cluster catalogue from the Subaru Hyper Suprime-Cam (HSC) survey. The Subaru HSC cluster catalogue provides a wide and homogeneous large-scale structure distribution out to a high redshift at z = 1.1, which has not been accessible in previous cross-correlation studies. We conduct the cross-correlation analysis not only for clusters in the all-redshift range (0.1 < z < 1.1) of the survey, but also for subsamples of clusters divided into redshift bins, the low-redshift bin (0.1 < z < 0.6) and the high-redshift bin (0.6 < z < 1.1), to utilize the wide redshift coverage of the cluster catalogue. We find evidence of cross-correlation signals with the significance of 2.0σ-2.3σ for all-redshift and low-redshift cluster samples. On the other hand, for high-redshift clusters, we find a signal with a weaker significance level (1.6σ-1.9σ). We also compare the observed cross-correlation functions with predictions of a theoretical model in which the UGRB originates from γ-ray emitters such as blazars, star-forming galaxies, and radio galaxies. We find that the detected signal is consistent with the model prediction.