High-energy gamma-ray and neutrino production in star-forming galaxies across cosmic time: Difficulties in explaining the IceCube data

TL;DR

This study models gamma-ray and neutrino emissions from star-forming galaxies, finding they cannot fully explain IceCube neutrino observations without conflicting with gamma-ray data.

Contribution

It introduces a new comprehensive model linking galaxy properties to gamma-ray and neutrino emissions, calibrated with observed galaxy data, to better predict cosmic backgrounds.

Findings

Star-forming galaxies produce about 20% of unresolved gamma-ray background.

They account for only 0.5% of IceCube neutrinos.

Even extreme models only explain up to 22% of IceCube neutrinos.

Abstract

We present a new theoretical modeling to predict luminosity and spectrum of gamma-ray and neutrino emission of a star-forming galaxy, from star formation rate ($\psi$), gas mass ($M_{\rm gas}$), stellar mass, and disk size, taking into account production, propagation and interactions of cosmic rays. The model reproduces the observed gamma-ray luminosities of nearby galaxies detected by {\it Fermi} better than the simple power-law models as a function of $\psi$ or $\psi M_{\rm gas}$. Then this model is used to predict the cosmic background flux of gamma-ray and neutrinos from star-forming galaxies, by using a semi-analytical model of cosmological galaxy formation that reproduces many observed quantities of local and high-redshift galaxies. 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 {\it 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.