Tracing molecular gas mass in extreme extragalactic environments: an
  observational study
  We present a new observational study of the CO(1-0) line emission as an H2
gas mass tracer under extreme conditions in extragalactic environments. Our
approach is to study the full neutral interstellar medium (H2, HI and dust) of
two galaxies whose bulk interstellar medium (ISM) resides in environments that
mark (and bracket) the excitation extremes of the ISM conditions found in
infrared luminous galaxies, the starburst NGC3310 and the quiescent spiral
NGC157. Our study maintains a robust statistical notion of the so-called X
factor (i.e. a large ensemble of clouds is involved) while exploring its
dependency on the very different average ISM conditions prevailing within these
two systems. These are constrained by fully-sampled CO(3-2) and CO(1-0)
observations, at a matched beam resolution of Half Power Beam Width 15",
obtained with the JCMT the Nobeyama 45-m telescope, combined with sensitive 850
and 450 micron dust emission and HI interferometric images which allow a
complete view of all the neutral ISM components. We found an X factor varying
by a factor of 5 within the spiral galaxy NGC157 and about 2 times lower than
the Galactic value in NGC3310. In addition, the dust emission spectrum in
NGC3310 shows a pronounced submm "excess". We explain and fit this by an
enhanced abundance of Very Small Grains (VSGs). The alternative approach to
interpret this excess as very cold dust would yield a cold dust temperature of
5-11 K with a correspondingly very low gas-to-dust mass ratio of 5-43, very
unlikely for this starburst galaxy with low metallicity.
