Gamma Rays
from Local Group Galaxies

Normal galaxies, such as the Milky Way, generate diffuse gamma-ray emission as a result of interactions of cosmic rays with the interstellar medium. However, their gamma-ray emissivity is generally low, and the only normal galaxy other than the Milky Way which diffuse gamma-ray emission was detected by EGRET was the Large Magellanic Cloud.

The upcoming NASA gamma-ray telescope, GLAST, will have a mch improved sensitivity, and is expected to detect several normal nearby galaxies in gamma-rays. In this paper, we model the expected gamma-emissivity of Local Group galaxies in order to predict which ones are promising targets for GLAST.

Under the assumption that cosmic rays are accelerated in supernova remnants, we show that the expected gamma-ray emissivity of each galaxy scales with its spernova rate (which is proportional to the galaxy's cosmic ray flux) and its gas content (representing the available targets with which cosmic rays can collide to produce gamma-rays). We then use observational data on the supernova rates and the gas masses of each galaxy to assess its detectability by GLAST.

We find that the LMC, SMC and M31 will be strong detections by GLAST, while, depending on the actual effective area of GLAST, cumulative exposure time and exact values of its gas content and supernova rate, M33 might also be detectable. We also discuss the information that can be extracted from the measurements of gamma-ray flux and spectra of these galaxies once they become available.

Reference: Pavlidou, V. & Fields, B. D. 2001, ApJ, 558, 63
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