The Fermi-LAT view of cosmic rays and interstellar gas in the Cygnus region: a not so special spot of the Local Arm

摘要

Cygnus X is the most conspicuous star-forming region close to the Sun, embedded in a giant complex of molecular clouds in the Local Arm. We present an analysis of the Fermi-LAT observations of Cygnus intended to probe the cosmic-ray and interstellar-matter content of the region. From gamma-ray data we estimate a total of 8+5 .1 × 106M of interstellar gas in the complex at a distance of 1.4 kpc. The gamma-ray emission from the atomic gas supports the average HI spin temperature derived from radio absorption/emission pairs to estimate its column-densities. The XCO = N(H2)/WCO ratio derived in the massive Cygnus complex is consistent with other LAT estimates for clouds in the Local and Perseus arms. The mass of dark gas, escaping HI and CO observations but traced by dust and gamma rays, amounts to 10% of the total. We find an average gamma-ray emissivity per interstellar H atom in the 0.1–100 GeV energy band in good agreement with measurements in other segments of the Local Arm. We infer that the cosmic-ray population averaged over a few hundred parsecs is fairly uniform in density and spectrum along the Local Arm. Despite the presence of potential accelerators and much larger interstellar densities in Cygnus X compared to the solar neighborhood, their cosmic-ray populations are similar on such a scale.