Molecular Cloud Formation and the Star Formation Efficiency in M~33
  Does star formation proceed in the same way in large spirals such as the
Milky Way and in smaller chemically younger galaxies? Earlier work suggests a
more rapid transformation of H$_2$ into stars in these objects but (1) a doubt
remains about the validity of the H$_2$ mass estimates and (2) there is
currently no explanation for why star formation should be more efficient. M~33,
a local group spiral with a mass $\sim 10$\% and a metallicity half that of the
Galaxy, represents a first step towards the metal poor Dwarf Galaxies. We have
searched for molecular clouds in the outer disk of M~33 and present here a set
of detections of both $^{12}$CO and $^{13}$CO, including the only detections
(for both lines) beyond the R$_{25}$ radius in a subsolar metallicity galaxy.
The spatial resolution enables mass estimates for the clouds and thus a measure
of the $N({\rm H}_2) / I_{\rm CO}$ ratio, which in turn enables a more reliable
calculation of the H$_2$ mass. Our estimate for the outer disk of M~33 is
$N({\rm H}_2) / I_{\rm CO(1-0)} \sim 5 \times 10^{20} \,{\rm cm^{-2}/(K{\rm \
km \ s^{-1}})}$ with an estimated uncertainty of a factor $\le 2$. While the
$^{12/13}$CO line ratios do not provide a reliable measure of $N({\rm H}_2) /
I_{\rm CO}$, the values we find are slightly greater than Galactic and
corroborate a somewhat higher $N({\rm H}_2) / I_{\rm CO}$ value. Comparing the
CO observations with other tracers of the interstellar medium, no reliable
means of predicting where CO would be detected was identified. In particular,
CO detections were often not directly on local HI or FIR or H$\alpha$ peaks,
although generally in regions with FIR emission and high HI column density. The
results presented here provide support for the quicker transformation of H$_2$
into stars in M~33 than in large local universe spirals.
