THE CO-TO-H{sub 2} CONVERSION FACTOR FROM INFRARED DUST EMISSION ACROSS THE LOCAL GROUP

Journal Article · Wed Aug 10 04:00:00 EDT 2011 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/737/1/12· OSTI ID:21579997

Leroy, Adam K ^[1]; Bolatto, Alberto ^[2]; Gordon, Karl ^[3]; Sandstrom, Karin ^[4]; Gratier, Pierre ^[5]; Rosolowsky, Erik ^[6]; Engelbracht, Charles W ^[7]; Mizuno, Norikazu ^[8]; Corbelli, Edvige ^[9]; Fukui, Yasuo; Kawamura, Akiko ^[10]

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1. National Radio Astronomy Observatory, Charlottesville, VA 22903 (United States)
2. Department of Astronomy, University of Maryland, College Park, MD 20742 (United States)
3. Space Telescope Science Institute, Baltimore, MD 21218 (United States)
4. Max-Planck-Institut fuer Astronomie, D-69117 Heidelberg (Germany)
5. Laboratoire d'Astrophysique de Bordeaux, Universite de Bordeaux, OASU, CNRS/INSU, 33271 Floirac (France)
6. University of British Columbia Okanagan, Kelowna, BC V1V 1V7 (Canada)
7. Steward Observatory, University of Arizona, Tucson, AZ 85721 (United States)
8. National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, 181-8588 Tokyo (Japan)
9. INAF-Osservatorio Astrofisico di Arcetri, 50125 Firenze (Italy)
10. Department of Astrophysics, Nagoya University, Furocho, Chikusaku, Nagoya 464-8602 (Japan)

We estimate the conversion factor relating CO emission to H{sub 2} mass, {alpha}{sub CO}, in five Local Group galaxies that span approximately an order of magnitude in metallicity-M 31, M 33, the Large Magellanic Cloud (LMC), NGC 6822, and the Small Magellanic Cloud (SMC). We model the dust mass along the line of sight from infrared (IR) emission and then solve for the {alpha}{sub CO} that best allows a single gas-to-dust ratio ({delta}{sub GDR}) to describe each system. This approach remains sensitive to CO-dark envelopes H{sub 2} surrounding molecular clouds. In M 31, M 33, and the LMC we find {alpha}{sub CO} {approx} 3-9 M{sub sun} pc{sup -2} (K km s{sup -1}){sup -1}, consistent with the Milky Way value within the uncertainties. The two lowest metallicity galaxies in our sample, NGC 6822 and the SMC (12 + log (O/H) {approx} 8.2 and 8.0), exhibit a much higher {alpha}{sub CO}. Our best estimates are {alpha}{sup NGC6822}{sub CO} {approx} 30 M{sub sun} pc{sup -2} (K km s{sup -1}){sup -1} and {alpha}{sup SMC}{sub CO} {approx} 70 M{sub sun} pc{sup -2} (K km s{sup -1}){sup -1}. These results are consistent with the conversion factor becoming a strong function of metallicity around 12 + log (O/H) {approx} 8.4-8.2. We favor an interpretation where decreased dust shielding leads to the dominance of CO-free envelopes around molecular clouds below this metallicity.

• OSTI ID: 21579997
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 737; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
CARBON COMPOUNDS
CARBON MONOXIDE
CARBON OXIDES
CHALCOGENIDES
CLOUDS
CONVERSION
DUSTS
ELECTROMAGNETIC RADIATION
ELEMENTS
EMISSION
GALAXIES
HYDROGEN
INFRARED RADIATION
NONMETALS
OXIDES
OXYGEN COMPOUNDS
RADIATIONS