Detection of Gamma Rays From a Starburst Galaxy

TL;DR

This paper reports the detection of gamma rays from the starburst galaxy NGC 253, indicating a significantly higher cosmic-ray density and energy conversion efficiency than in the Milky Way, using the H.E.S.S. telescopes.

Contribution

First detection of gamma rays from a starburst galaxy, revealing elevated cosmic-ray densities and energy transfer efficiencies in such environments.

Findings

Gamma-ray flux above 220 GeV measured as (5.5 +/- 1.0stat +/- 2.8sys) x 10^-13 ph. s-1 cm-2.

Cosmic-ray density in NGC 253 is about 1000 times higher than in the Milky Way.

Energy fraction of cosmic rays converted to gamma rays is five times larger than in our Galaxy.

Abstract

Starburst galaxies exhibit in their central regions a highly increased rate of supernovae, the remnants of which are thought to accelerate energetic cosmic rays up to energies of ~ 10^15 eV. We report the detection of gamma rays -- tracers of such cosmic rays -- from the starburst galaxy NGC 253 using the H.E.S.S. array of imaging atmospheric Cherenkov telescopes. The gamma-ray flux above 220 GeV is F = (5.5 +/- 1.0stat +/- 2.8sys) x 10^-13 ph. s-1 cm-2, implying a cosmic-ray density about three orders of magnitude larger than that in the center of the Milky Way. The fraction of cosmic-ray energy channeled into gamma rays in this starburst environment is 5 times larger than that in our Galaxy.