FIR galaxies and the gamma-ray background

Abstract

Contribution to the gamma-ray background (GRB) by galaxies bright in the far-IR (FIR) is discussed. Using observational correlations between the FIR and synchrotron emission in radio wavelengths, it is shown that the concentration of cosmic ray electrons in FIR luminous galaxies is substantially higher than in the general population of galaxies. Microwave background and intrinsic galaxy radiation scattered via the inverse Compton (IC) effect by cosmic ray electrons into high energies makes the FIR galaxies relatively strong gamma-ray sources (in comparison with normal galaxies).x Using the FIR galaxy luminosity function and the relationship between FIR and radio luminosities, the contribution (due to the IC only) of FIR luminous galaxies to the GRB above ~ 1 MeV is estimated at 10 to 25 % depending on the rate of cosmic evolution of star burst galaxies. This is substantially larger than the normal galaxies contribution. To obtain more accurate assessment of relationship between the GRB and FIR galaxies, gamma-ray observations of individual galaxies bright in the FIR are required.