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

TL;DR

This study uses Fermi-LAT gamma-ray data to analyze the cosmic-ray and interstellar gas content in the Cygnus region, revealing a fairly uniform cosmic-ray population and consistent interstellar gas properties with other parts of the Local Arm.

Contribution

It provides the first detailed gamma-ray based estimate of interstellar gas mass and cosmic-ray uniformity in the Cygnus region, challenging assumptions about local cosmic-ray acceleration.

Findings

Total interstellar gas mass estimated at ~8 million solar masses.

Cosmic-ray density and spectrum are similar to other Local Arm regions.

Dark gas constitutes about 10% of total interstellar matter.

Abstract

The Cygnus region hosts 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) x 10^6 solar masses 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 X(CO)=N(H2)/W(CO) 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 compared to the solar neighborhood, their cosmic-ray populations are similar on such a scale.