Cosmic Gamma-Ray Background from Star-Forming Galaxies

TL;DR

This paper presents a new calculation showing that normal star-forming galaxies, rather than rare bright galaxies, likely make up most of the extragalactic gamma-ray background detected by Fermi, with implications for cosmology.

Contribution

The study introduces a novel model indicating that numerous faint normal galaxies dominate the gamma-ray background, challenging previous assumptions about the sources.

Findings

Normal galaxies may constitute the bulk of the gamma-ray background.

The gamma-ray background can be used to study cosmic star formation history.

Implications for detecting dark matter substructures through anisotropy.

Abstract

The origin of the extragalactic gamma-ray background is a pressing cosmological mystery. The Fermi Gamma-Ray Space Telescope has recently measured the intensity and spectrum of this background; both are substantially different from previous measurements. We present a novel calculation of the gamma-ray background from normal star-forming galaxies. Contrary to long-standing expectations, we find that numerous but individually faint normal galaxies may comprise the bulk of the Fermi signal, rather than rare but intrinsically bright active galaxies. This result has wide-ranging implications, including: the possibility to probe the cosmic star formation history with gamma rays; the ability to infer the cosmological evolution of cosmic rays and galactic magnetic fields; and an increased likelihood of identifying subdominant components from rare sources (e.g., dark matter clumps) through their large anisotropy.