Comparative study of gamma-ray emission from molecular clouds and
  star-forming galaxies
  Star-forming regions on different scales, such as giant molecular clouds in
our Galaxy and star-forming galaxies, emit GeV gamma-rays. These are thought to
originate from hadronic interactions of cosmic-ray (CR) nuclei with the
interstellar medium. It has recently been shown that the gamma-ray luminosity
($L_\gamma$) of star-forming galaxies is well correlated with their star
formation rates (SFR). We investigated \textsl{Fermi} data of eight Galactic
molecular clouds in the Gould belt and found that molecular clouds do not
follow the $L_\gamma-{\rm SFR}$ correlation of star-forming galaxies. We also
compared the scaling relations of gamma-ray luminosity, SFR, and the gas mass
for molecular clouds and star-forming galaxies. Using a multiple-variable
regression analysis, we found different dependences of gamma-ray emission on
SFR or mass for molecular clouds and star-forming galaxies. This suggests that
different mechanisms may govern the production of gamma-rays in these two types
of sources. Specifically, the strong dependence on mass supports that gamma-ray
emission of molecular clouds primarily comes from {\em \textup{passive}}
interaction by diffuse Galactic CRs, whereas the strong dependence on SFR
supports that gamma-ray emission of star-forming galaxies originates from CRs
that are accelerated by local {\em \textup{active}} sources.
