First detection of GeV emission from an ultraluminous infrared galaxy: Arp 220 as seen with the Fermi Large Area Telescope

TL;DR

This paper reports the first detection of GeV gamma-ray emission from the ultra luminous infrared galaxy Arp 220, confirming it as a source of high-energy cosmic rays and providing insights into cosmic ray acceleration efficiency in extreme starburst environments.

Contribution

The study presents the first evidence of GeV gamma-ray emission from Arp 220, linking gamma-ray luminosity to cosmic ray interactions and estimating cosmic ray acceleration efficiency.

Findings

Detected gamma-ray emission above 200 MeV from Arp 220 with 6.3 sigma confidence.

Gamma-ray emission is steady and consistent with star-forming galaxy scaling relations.

Cosmic ray acceleration efficiency in Arp 220 is approximately 4.2% for energies above 1 GeV.

Abstract

Cosmic rays (CRs) in starburst galaxies produce high energy gamma-rays by colliding with the dense interstellar medium (ISM). Arp 220 is the nearest ultra luminous infrared galaxy (ULIRG) that has star-formation at extreme levels, so it has long been predicted to emit high-energy gamma-rays. However, no evidence of gamma-ray emission was found despite intense efforts of search. Here we report the discovery of high-energy gamma-ray emission above 200 MeV from Arp 220 at a confidence level of $\sim 6.3 \sigma $ using 7.5 years of \textsl {Fermi} Large Area Telescope observations. The gamma-ray emission shows no significant variability over the observation period and it is consistent with the quasi-linear scaling relation between the gamma-ray luminosity and total infrared luminosity for star-forming galaxies, suggesting that these gamma-rays arise from CR interactions. As the high density medium of Arp 220 makes it an ideal CR calorimeter, the gamma-ray luminosity can be used to measure the efficiency of powering CRs by supernova (SN) remnants given a known supernova rate in Arp 220. We find that this efficiency is about $4.2\pm2.6\%$ for CRs above 1 GeV.