FERMI OBSERVATIONS OF CASSIOPEIA AND CEPHEUS: DIFFUSE GAMMA-RAY EMISSION IN THE OUTER GALAXY
2010
We present the analysis of the interstellar gamma-ray emission measured by the Fermi Large Area Telescope toward a region in the second
Galactic quadrantat 100 deg < l < 145 deg and -15 deg < b < +30 deg. This region encompasses the prominent Gould-Belt clouds of
Cassiopeia, Cepheus and the
Polarisflare, 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 mediumbeyond 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 factoris 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 cloudsunder 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.
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