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Abstract

High energy $\gamma$ 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 $\gamma$-ray
emission, the density of CRs throughout the Galactic Disk. Based on the data of
\fermi{}, we report the discovery of $\gamma$-ray signals from nineteen GMCs
located at distances up to 12.5 kpc. The galactocentric radial distribution of
the CR density derived from the $\gamma$-ray and CO observations of these
objects, as well as from some nearby clouds that belong to the Gould Belt
complex, unveil a homogeneous \textquotedblleft sea" of CRs with a constant
density and spectral shape close to the flux of directly (locally) measured
CRs. We found noticeable deviations from the \textquotedblleft sea level" only
in some locations characterized by enhanced CR density in the galactocentric
4--6 kpc ring. Furthermore, we found a hint for fluctuations of the CR density
in different locations within the same 4--6 kpc ring. The confirmation of this
result with the next-generation $\gamma$-ray detectors based on the higher
quality data and denser coverage of galactocentric distances, would have
dramatic implications for the understanding of the origin of Galactic CRs.