The ${\hat{I}}^3$-ray Emission of Star-Forming Galaxies

Abstract

A majority of the ${\hat{I}}^3$-ray emission from star-forming galaxies is generated by the interaction of high-energy cosmic rays with the interstellar gas and radiation fields. Star-forming galaxies are expected to contribute to both the extragalactic ${\hat{I}}^3$-ray background and the IceCube astrophysical neutrino flux. Using roughly 10\,years of ${\hat{I}}^3$-ray data taken by the {\it Fermi} Large Area Telescope, in this study we constrain the ${\hat{I}}^3$-ray properties of star-forming galaxies. We report the detection of 11 bona-fide ${\hat{I}}^3$-ray emitting galaxies and 2 candidates. Moreover, we show that the cumulative ${\hat{I}}^3$-ray emission of below-threshold galaxies is also significantly detected at $\sim$5\,$\ddot{I}ƒ$ confidence. The ${\hat{I}}^3$-ray luminosity of resolved and unresolved galaxies is found to correlate with the total (8-1000\,$\hat{I}¼$m) infrared luminosity as previously determined. Above 1\,GeV, the spectral energy distribution of resolved and unresolved galaxies is found to be compatible with a power law with a photon index of $\approx 2.2-2.3$. Finally, we find that star-forming galaxies account for roughly 5\,\% and 3\,\% of the extragalactic ${\hat{I}}^3$-ray background and the IceCube neutrino flux, respectively.

Accepted for publication in The Astrophysical Journal