Correlation of extragalactic γ rays with cosmic matter density distributions from weak gravitational lensing

Abstract

The extragalactic $\gamma$-ray background (EGB) arises from the accumulation of $\gamma$-ray emissions from resolved and unresolved extragalactic sources as well as diffuse processes. It is important to study the statistical properties of the EGB in the context of cosmological structure formation. Known astrophysical $\gamma$-ray sources such as blazars, star-forming galaxies, and radio galaxies are expected to trace the underlying cosmic matter density distribution. We explore the correlation of the EGB from Fermi-LAT data with the large-scale matter density distribution from the Subaru Hyper Suprime-Cam (HSC) SSP survey. We reconstruct an unbiased surface matter density distribution $\kappa$ at $z<1$ by applying weak-gravitational lensing analysis to the first-year HSC data. We then calculate the $\gamma - \kappa$ cross-correlation. Our measurements are consistent with a null detection, but a weak correlation is found at angular scales of 30-60 arcmin, especially when distant source galaxies at $z > 1$ are used for the lensing $\kappa$ reconstruction. The large-scale correlation suggests strong clustering of high-redshift $\gamma$-ray sources such as blazars. However, the inferred bias factor of $4-5$ is larger by about a factor of two than results from other clustering analyses. The final HSC data covering 1,400 squared degrees will play an essential role to determine accurately the blazar bias at $z > 0.5$.