Copyright © 2013 by Annual Reviews. All rights reserved
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| Annu. Rev. Astron. Astrophys. 2013. 51:
207-268 Copyright © 2013 by Annual Reviews. All rights reserved |
Reprinted with kind permission from Annual Reviews, 4139 El Camino Way, Palo Alto, California, USA
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Abstract:
CO line emission represents the most accessible and widely used tracer
of the molecular interstellar medium. This renders the translation of
observed CO intensity into total H2 gas mass critical to
understand star formation and the interstellar medium in our Galaxy
and beyond. We review the theoretical underpinning, techniques, and
results of efforts to estimate this CO-to-H2 "conversion
factor," XCO, in different environments. In the Milky
Way disk, we recommend a conversion factor XCO = 2 ×
1020 cm-2(K km s-1)-1
with ± 30% uncertainty. Studies of other "normal galaxies"
return similar values in Milky Way-like disks, but with greater
scatter and systematic uncertainty. Departures from this Galactic
conversion factor are both observed and expected. Dust-based
determinations, theoretical arguments, and scaling relations all
suggest that XCO increases with decreasing
metallicity, turning up sharply below metallicity
1/3 - 1/2 solar in a
manner consistent with model predictions that identify shielding as a
key parameter. Based on spectral line modeling and dust
observations, XCO appears to drop in the central,
bright regions of some but not all galaxies, often coincident with
regions of bright CO emission and high stellar surface density. This lower
XCO is also
present in the overwhelmingly molecular interstellar medium of
starburst galaxies, where several lines of evidence point to a lower
CO-to-H2 conversion factor. At high redshift, direct evidence
regarding the conversion factor remains scarce; we review what is
known based on dynamical modeling and other arguments.
Keywords: ISM: general — ISM: molecules — galaxies: ISM — radio lines: ISM
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