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Title: Fermi observations of Cassiopeia and Cepheus: Diffuse gamma-ray emission in the outer galaxy

Abstract

Here, we present the analysis of the interstellar γ-ray emission measured by the Fermi Large Area Telescope toward a region in the second Galactic quadrant at 100° ≤ l ≤ 145° and –15° ≤ b ≤ +30°. 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 (CRs) and the interstellar medium beyond the solar circle. Furthermore, the γ-ray emissivity spectrum of the gas in the Gould Belt is consistent with expectations based on the locally measured CR spectra. The γ-ray emissivity decreases from the Gould Belt to the Perseus arm, but the measured gradient is flatter than expectations for CR sources peaking in the inner Galaxy as suggested by pulsars. The X CO = N(H 2)/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 tomore » (1.9 ± 0.2) × 10 20 cm –2 (K km s –1) –1 in the Perseus arm. We also 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 γ-ray emission: its mass amounts to ~50% of the CO-traced mass.« less

Authors:
 [1]
  1. Naval Research Lab. (NRL), Washington, DC (United States); National Academy of Sciences, Washington, DC (United States); et al.
Publication Date:
Research Org.:
SLAC National Accelerator Lab., Menlo Park, CA (United States)
Sponsoring Org.:
USDOE
Contributing Org.:
Fermi LAT Collaboration
OSTI Identifier:
1357581
Grant/Contract Number:  
AC02-76SF00515
Resource Type:
Accepted Manuscript
Journal Name:
Astrophysical Journal
Additional Journal Information:
Journal Volume: 710; Journal Issue: 1; Journal ID: ISSN 0004-637X
Publisher:
Institute of Physics (IOP)
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS; cosmic rays; gamma rays: diffuse background; ISM: clouds

Citation Formats

Abdo, A. A. Fermi observations of Cassiopeia and Cepheus: Diffuse gamma-ray emission in the outer galaxy. United States: N. p., 2010. Web. doi:10.1088/0004-637X/710/1/133.
Abdo, A. A. Fermi observations of Cassiopeia and Cepheus: Diffuse gamma-ray emission in the outer galaxy. United States. doi:10.1088/0004-637X/710/1/133.
Abdo, A. A. Fri . "Fermi observations of Cassiopeia and Cepheus: Diffuse gamma-ray emission in the outer galaxy". United States. doi:10.1088/0004-637X/710/1/133. https://www.osti.gov/servlets/purl/1357581.
@article{osti_1357581,
title = {Fermi observations of Cassiopeia and Cepheus: Diffuse gamma-ray emission in the outer galaxy},
author = {Abdo, A. A.},
abstractNote = {Here, we present the analysis of the interstellar γ-ray emission measured by the Fermi Large Area Telescope toward a region in the second Galactic quadrant at 100° ≤ l ≤ 145° and –15° ≤ b ≤ +30°. 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 (CRs) and the interstellar medium beyond the solar circle. Furthermore, the γ-ray emissivity spectrum of the gas in the Gould Belt is consistent with expectations based on the locally measured CR spectra. The γ-ray emissivity decreases from the Gould Belt to the Perseus arm, but the measured gradient is flatter than expectations for CR sources peaking in the inner Galaxy as suggested by pulsars. The X CO = N(H2)/W CO conversion factor is found to increase from (0.87 ± 0.05) × 1020 cm–2 (K km s–1)–1 in the Gould Belt to (1.9 ± 0.2) × 1020 cm–2 (K km s–1)–1 in the Perseus arm. We also 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 γ-ray emission: its mass amounts to ~50% of the CO-traced mass.},
doi = {10.1088/0004-637X/710/1/133},
journal = {Astrophysical Journal},
number = 1,
volume = 710,
place = {United States},
year = {2010},
month = {1}
}

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