Recent detections of the starburst galaxies M82 and NGC 253 by gamma-ray telescopes suggest that galaxies rapidly forming massive stars are more luminous at gamma-ray energies compared to their quiescent relatives .
Building upon those results , we examine a sample of 69 dwarf , spiral , and luminous and ultraluminous infrared galaxies at photon energies 0.1–100 GeV using 3 years of data collected by the Large Area Telescope ( LAT ) on the Fermi Gamma-ray Space Telescope ( Fermi ) .
Measured fluxes from significantly detected sources and flux upper limits for the remaining galaxies are used to explore the physics of cosmic rays in galaxies .
We find further evidence for quasi-linear scaling relations between gamma-ray luminosity and both radio continuum luminosity and total infrared luminosity which apply both to quiescent galaxies of the Local Group and low-redshift starburst galaxies ( conservative P -values \lesssim 0.05 accounting for statistical and systematic uncertainties ) .
The normalizations of these scaling relations correspond to luminosity ratios of \log ( L _ { 0.1 - 100 GeV } / L _ { 1.4 GHz } ) = 1.7 \pm 0.1 _ { ( statistical ) } \pm 0 % .2 _ { ( dispersion ) } and \log ( L _ { 0.1 - 100 GeV } / L _ { 8 - 1000 \mum } ) = -4.3 \pm 0.1 _ { ( statistical% ) } \pm 0.2 _ { ( dispersion ) } for a galaxy with a star formation rate of 1 M _ { \odot } yr ^ { -1 } , assuming a Chabrier initial mass function .
Using the relationship between infrared luminosity and gamma-ray luminosity , the collective intensity of unresolved star-forming galaxies at redshifts 0 < z < 2.5 above 0.1 GeV is estimated to be 0.4–2.4 \times 10 ^ { -6 } ph cm ^ { -2 } s ^ { -1 } sr ^ { -1 } ( 4–23 % of the intensity of the isotropic diffuse component measured with the LAT ) .
We anticipate that \sim 10 galaxies could be detected by their cosmic-ray induced gamma-ray emission during a 10-year Fermi mission .