Probing the sea of galactic cosmic rays with Fermi-LAT

High energy $\ensuremath{\gamma}$-rays from giant molecular clouds (GMCs) carry direct information about the spatial and energy distributions of galactic cosmic rays (CRs). The recently released catalogs of GMCs contain sufficiently massive clouds to be used as barometers for probing, through their $\ensuremath{\gamma}$-ray emission, the density of CRs throughout the galactic disk. Based on the data of Fermi-LAT, we report the discovery of $\ensuremath{\gamma}$-ray signals from nineteen GMCs located at distances up to 12.5 kpc. The galactocentric radial distribution of the CR density derived from the $\ensuremath{\gamma}$-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. This concerns the galactocentric distances exceeding 8 kpc, as well as the Sagittarius B complex, in the region of the Galactic Center. On the other hand, for the galactocentric distances between 4 and 8 kpc, we found noticeable deviations from the CR sea level; in some locations, GMCs are characterized by enhanced CR density. This could be the result of a possible global increase of the level of the CR sea towards the Galactic Center and/or by the presence of recent CR accelerators close to some specific clouds.