On The GeV & TeV Detections of the Starburst Galaxies M82 & NGC 253

TL;DR

This paper analyzes gamma-ray data from starburst galaxies M82 and NGC 253, concluding that their gamma-ray emissions are mainly hadronic, with efficient cosmic ray proton losses, and discusses implications for cosmic ray physics and future observations.

Contribution

It provides new estimates of cosmic ray proton calorimetry in starburst galaxies and discusses the implications for gamma-ray and neutrino flux predictions, including testing calorimetric assumptions with ULIRGs.

Findings

Gamma-ray emission is predominantly hadronic in origin.

A significant fraction (20-40%) of cosmic ray energy is lost to pion production.

Non-synchrotron cooling processes are important for cosmic ray electrons.

Abstract

The GeV and TeV emission from M82 and NGC 253 observed by Fermi, HESS, and VERITAS constrains the physics of cosmic rays (CRs) in these dense starbursts. We argue that the gamma rays are predominantly hadronic in origin, as expected by previous studies. The measured fluxes imply that pionic losses are efficient for CR protons in both galaxies: we show that a fraction F_cal ~ 0.2 - 0.4 of the energy injected in high energy primary CR protons is lost to inelastic proton-proton collisions (pion production) before escape, producing gamma rays, neutrinos, and secondary electrons and positrons. We discuss the factor ~2 uncertainties in this estimate, including supernova rate and leptonic contributions to the GeV-TeV emission. We argue that gamma-ray data on ULIRGs like Arp 220 can test whether M82 and NGC 253 are truly calorimetric, and we present upper limits on Arp 220 from the Fermi data. We show that the observed ratio of the GeV to GHz fluxes of the starbursts suggests that non-synchrotron cooling processes are important for cooling the CR electron/positron population. We briefly reconsider previous predictions in light of the gamma-ray detections, including the starburst contribution to the gamma-ray background and CR energy densities. Finally, as a guide for future studies, we list the brightest star-forming galaxies on the sky and present updated predictions for their gamma-ray and neutrino fluxes.