The CO-to-H2 Conversion Factor
  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 = 2x10^{20} cm^-2/(K km/s)^-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.
