(Tomonori Totani, Norita Kawanaka)
We present a new theoretical modeling to predict gamma-ray and neutrino luminosity and spectrum of a star-forming galaxy, from its star formation rate, gas mass, stellar mass, and size. This model reproduces the observed gamma-ray luminosities of nearby galaxies better than empirical power-law relations. Then this model is used to predict the cosmic background flux of gamma-ray and neutrino from star-forming galaxies, by using a semi-analytical model of cosmological galaxy formation. Calibration of the model using gamma-ray luminosities of nearby galaxies allows us to make a more reliable prediction than previous studies. In our baseline model star-forming galaxies produce about 20% of isotropic gamma-ray background unresolved by Fermi, and only 0.5% of IceCube neutrinos. Even with an extreme model assuming a hard injection cosmic-ray spectral index of 2.0 for all galaxies, at most 22% of IceCube neutrinos can be accounted for. These results indicate that it is difficult to explain most of IceCube neutrinos by star-forming galaxies, without violating the gamma-ray constraints from nearby galaxies.