A connection between star formation activity and cosmic rays in the starburst galaxy M 82

TL;DR

This study detects high-energy gamma rays from the starburst galaxy M 82, providing strong evidence that cosmic rays are primarily accelerated by supernovae and massive-star winds in regions of intense star formation.

Contribution

First direct measurement of cosmic-ray density in M 82's starburst core, linking cosmic-ray acceleration to star formation activity and supernovae.

Findings

Cosmic-ray density in M 82's core is about 500 times higher than in the Milky Way.

Detected gamma rays above 700 GeV from M 82.

Supports the theory that supernovae and massive-star winds accelerate cosmic rays.

Abstract

Although Galactic cosmic rays (protons and nuclei) are widely believed to be dominantly accelerated by the winds and supernovae of massive stars, definitive evidence of this origin remains elusive nearly a century after their discovery [1]. The active regions of starburst galaxies have exceptionally high rates of star formation, and their large size, more than 50 times the diameter of similar Galactic regions, uniquely enables reliable calorimetric measurements of their potentially high cosmic-ray density [2]. The cosmic rays produced in the formation, life, and death of their massive stars are expected to eventually produce diffuse gamma-ray emission via their interactions with interstellar gas and radiation. M 82, the prototype small starburst galaxy, is predicted to be the brightest starburst galaxy in gamma rays [3, 4]. Here we report the detection of >700 GeV gamma rays from M 82. From these data we determine a cosmic-ray density of 250 eV cm-3 in the starburst core of M 82, or about 500 times the average Galactic density. This result strongly supports that cosmic-ray acceleration is tied to star formation activity, and that supernovae and massive-star winds are the dominant accelerators.