Imprint of galaxy clustering in the cosmic gamma-ray background
  Star-forming galaxies are predicted to contribute considerably to the cosmic
gamma-ray background (CGB) as they are confirmed gamma-ray emitters and are the
most numerous population of gamma-ray sources, although individually faint.
Even though the Fermi Gamma-ray Space Telescope will be able to resolve few
star-forming galaxies individually, their fractional contribution to the CGB
should become far more significant than it was for past measurements of the CGB
as many of the brighter, formerly unresolved sources will be resolved out.
Thus, the clustering feature of galaxies imprinted on the CGB might be
detectable by Fermi. In anticipation of such measurements, we calculate the
predicted angular auto-power and cross-power spectra of the CGB from normal
galaxies. We find that the amplitude of the auto-power spectrum is smaller than
that for other sources such as blazars and dark-matter annihilation; the shape
is also characteristic. We also show that the cross-power spectrum with galaxy
surveys features larger amplitude. Fermi should be able to detect the
correlation signature in both the auto-power and cross-power spectra at angular
scales of ~1-10 deg after 5-yr of operation. Such a detection would be valuable
in confirming the level of the star-forming galaxy contribution to the CGB, and
more importantly, in serving as a tool in the effort to discriminate between
possible origins of the CGB.
