Probing the "Sea" of Galactic Cosmic Rays with Fermi-LAT

Felix Aharonian, Giada Peron, Ruizhi Yang, Sabrina Casanova, Roberta Zanin

Published 2018-11-29Version 1

High energy $\gamma$ rays from Giant Molecular Clouds (GMCs) carry direct information about spatial and energy distributions of Galactic Cosmic Rays (CRs). The recently released catalogues of GMCs contain sufficiently massive clouds to be used as barometers for probing, through their $\gamma$-ray emission, the density of CRs throughout the Galactic Disk. Based on the data of Fermi-LAT, we report the discovery of $\gamma$-ray signals from fifteen GMCs located at distances from 0.6 kpc to 12.5 kpc. The Galactocentric radial distribution of the CR density derived from the $\gamma$-ray and CO observations of these objects, as well as from some nearby clouds that belong to the Gould Belt complex, unveil a homogeneous "sea" of CRs with a constant density and spectral shape close to the flux of directly (locally) measured CRs. The only exception we found in some locations characterized by enhanced CR density in the 4-6 kpc ring. We argue that the latter is not related to a global variation of the CR sea but has a local character caused by the presence of CR accelerators in the densely populated star-forming regions within the 4-6 kpc ring.