Fermi Gamma-ray Space Telescope

Measuring Cosmic Rays in the Halo of the Milky Way from Fermi-LAT Observations of Intermediate and High-Velocity Clouds

S. W. Digel
L. Tibaldo, Fermi-LAT Collaboration

Abstract:

We present the final results from a study of intermediate and high-velocity clouds in Fermi LAT data. These clouds of atomic hydrogen in the halo of the Milky Way have low column densities but subtend substantial solid angles and with the sensitivity provided by more than six years of LAT data they are useful targets for investigating the distribution of cosmic rays in the halo. Investigating gamma-ray emission from cosmic-ray interactions with these clouds, and carefully accounting for cosmic-ray interactions with foreground interstellar gas, including dark gas not traced by radio spectral lines, we find at the 97.5% confidence level that the densities of cosmic rays decrease with distance from the disk. The detections and upper limits suggest a steep decline of the intensity of GeV-TeV cosmic rays in the range 1-2 kpc above the plane.