VERITAS observations of M 31 (The andromeda galaxy)

Abstract

Diffuse gamma rays are tracers of cosmic rays, providing information on their origin, interaction and diffusion through a galaxy. M 31 (the Andromeda Galaxy) is the closest spiral galaxy to the Milky Way (d = 780 kpc) and is very well studied at all wavelengths. Thus it is a prime target for the study of diffuse gamma-ray emission. The very-high-energy (VHE, E > 100 GeV) gamma-ray observatory VERITAS has conducted 54 hours of observations of M 31 and an upper limit on the VHE flux is presented along with an updated Fermi-LAT (0.1 < E < 300 GeV) analysis. These observations will be compared with predictions of the gamma-ray flux derived from models of the inelastic scattering of VHE cosmic rays of the interstellar medium (ISM) and the interstellar radiation field. M 31 provides an ideal opportunity to probe this mechanism. Its proximity and spatial extent, significantly larger than the VERITAS point spread function but smaller than the field-of-view, potentially enables the star-forming ring, 10 kpc from the galaxy core, with its dense ISM and numerous supernova remnants to be resolved.