Fermi observations of Cassiopeia and Cepheus: diffuse gamma-ray emission in the outer Galaxy

TL;DR

This study analyzes Fermi LAT gamma-ray data of the outer Galaxy, revealing how cosmic-ray interactions vary across different Galactic regions and identifying significant amounts of dark gas in molecular clouds.

Contribution

It provides new measurements of gamma-ray emissivity and the Xco conversion factor across Galactic regions, enhancing understanding of cosmic-ray distribution and dark gas presence.

Findings

Gamma-ray emissivity decreases from Gould Belt to Perseus arm.

Xco conversion factor increases from Gould Belt to Perseus arm.

Dark gas constitutes about 50% of CO-traced molecular mass.

Abstract

We present the analysis of the interstellar gamma-ray emission measured by the Fermi Large Area Telescope toward a region in the second Galactic quadrant at 100 deg < l < 145 deg and -15 deg < b < +30 deg. This region encompasses the prominent Gould-Belt clouds of Cassiopeia, Cepheus and the Polaris flare, as well as atomic and molecular complexes at larger distances, like that associated with NGC 7538 in the Perseus arm. The good kinematic separation in velocity between the local, Perseus, and outer arms, and the presence of massive complexes in each of them make this region well suited to probe cosmic rays and the interstellar medium beyond the solar circle. The gamma-ray emissivity spectrum of the gas in the Gould Belt is consistent with expectations based on the locally measured cosmic-ray spectra. The gamma-ray emissivity decreases from the Gould Belt to the Perseus arm, but the measured gradient is flatter than expectations for cosmic-ray sources peaking in the inner Galaxy as suggested by pulsars. The Xco=N(H2)/W(CO) conversion factor is found to increase from (0.87 +- 0.05) 10^20 cm^-2 (K km s^-1)^-1 in the Gould Belt to (1.9 +- 0.2) 10^20 cm^-2 (K km s^-1)^-1 in the Perseus arm. We derive masses for the molecular clouds under study. Dark gas, not properly traced by radio and microwave surveys, is detected in the Gould Belt through a correlated excess of dust and gamma-ray emission: its mass amounts to ~50% of the CO-traced mass.