Amy Lien, Brian D. Fields
The origin of the diffuse extragalactic gamma-ray background (EGB) has been
intensively studied but remains unsettled. Current popular source candidates
include unresolved star-forming galaxies, starburst galaxies, and blazars. In
this paper we calculate the EGB contribution from the interactions of cosmic
rays accelerated by Type Ia supernovae (SNe), extending earlier work which only
included core-collapse SNe. We consider Type Ia events in star-forming
galaxies, but also in quiescent galaxies that lack star formation. For
star-forming galaxies, consistently including Type Ia events makes little
change to the star-forming EGB prediction, so long as both SN types have the
same cosmic-ray acceleration efficiencies in star-forming galaxies. Thus, our
updated EGB estimate continues to show that star-forming galaxies can represent
a substantial portion of the signal measured by Fermi. For quiescent galaxies,
conversely, we find a wide range of possibilities for the EGB contribution. The
dominant uncertainty we investigated comes from the mass in hot gas, which
provides targets for cosmic rays; total gas masses are as yet poorly known,
particularly at larger radii. Additionally, the EGB estimation is very
sensitive to the cosmic-ray acceleration efficiency and confinement, especially
in quiescent galaxies. In the most optimistic allowed scenarios, quiescent
galaxies can be an important source of the EGB. In this case, star-forming
galaxies and quiescent galaxies together will dominate the EGB and leave little
room for other contributions. If other sources, such as blazars, are found to
have important contributions to the EGB, then either the gas mass or cosmic-ray
content of quiescent galaxies must be significantly lower than in their
star-forming counterparts. In any case, improved Fermi EGB measurements will
provide important constraints on hot gas and cosmic rays in quiescent galaxies.
View original:
http://arxiv.org/abs/1201.3447
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