Imprint of galaxy clustering in the cosmic gamma-ray background

TL;DR

This paper predicts how galaxy clustering influences the cosmic gamma-ray background and suggests that Fermi observations can detect these signatures, helping to identify the sources of the background radiation.

Contribution

It provides the first detailed predictions of the angular auto-power and cross-power spectra of the CGB from star-forming galaxies, highlighting their potential detectability.

Findings

Auto-power spectrum amplitude is smaller than that of blazars and dark matter.

Cross-power spectrum with galaxy surveys shows larger amplitude.

Fermi can detect correlation signatures at 1-10 degrees after 5 years.

Abstract

Star-forming galaxies are predicted to contribute considerably to the cosmic gamma-ray background (CGB) as they are confirmed gamma-ray emitters and are the most numerous population of gamma-ray sources, although individually faint. Even though the Fermi Gamma-ray Space Telescope will be able to resolve few star-forming galaxies individually, their fractional contribution to the CGB should become far more significant than it was for past measurements of the CGB as many of the brighter, formerly unresolved sources will be resolved out. Thus, the clustering feature of galaxies imprinted on the CGB might be detectable by Fermi. In anticipation of such measurements, we calculate the predicted angular auto-power and cross-power spectra of the CGB from normal galaxies. We find that the amplitude of the auto-power spectrum is smaller than that for other sources such as blazars and dark-matter annihilation; the shape is also characteristic. We also show that the cross-power spectrum with galaxy surveys features larger amplitude. Fermi should be able to detect the correlation signature in both the auto-power and cross-power spectra at angular scales of ~1-10 deg after 5-yr of operation. Such a detection would be valuable in confirming the level of the star-forming galaxy contribution to the CGB, and more importantly, in serving as a tool in the effort to discriminate between possible origins of the CGB.