GeV $\gamma$-ray emission from M33 and Arp~299
  Star-forming galaxies are huge reservoirs of cosmic rays (CRs) and these CRs
convert a significant fraction of their energy into $\gamma$-rays by colliding
with the interstellar medium (ISM). Several nearby star-forming galaxies have
been detected in GeV-TeV $\gamma$-rays. It is also found that the $\gamma$-ray
luminosities in 0.1-100 GeV correlate well with indicators of star formation
rates of the galaxies, such as the total infrared (IR) luminosity. In this
paper, we report a systematic search for possible $\gamma$-ray emission from
galaxies in the IRAS Revised Bright Galaxies Sample, using 11.4 years of
$\gamma$-ray data taken by the Fermi Large Area Telescope (LAT). Two new
galaxies, M33 and Arp 299, are detected significantly. The two galaxies are
consistent with the empirical correlation between the $\gamma$-ray luminosity
and total infrared luminosity, suggesting that their $\gamma$-ray emissions
should mainly originate from CRs interacting with ISM. Nevertheless, there is a
tentative evidence that the flux of the $\gamma$-ray emission from Arp~299 is
variable. If the variability is true, part of the emission from Arp 299 should
originate from the obscured AGN in this interacting galaxy system. In addition,
we find that the $\gamma$-ray excess from M33 is located at the northeast
region of the galaxy, where a supergiant H II region, NGC604, resides. This
indicates that some bright star-forming regions in spiral galaxies could play a
dominant role in the galaxy in producing $\gamma$-ray emission.
