Enhancement of CO(3-2)/CO(1-0) Ratios and Star Formation Efficiencies in
  Supergiant HII Regions
  We present evidence that super giant HII regions (GHRs) and other disk
regions of the nearby spiral galaxy, M33, occupy distinct locations in the
correlation between molecular gas, $\Sigma_{\rm H_2}$, and the star formation
rate surface density, $\Sigma_{\rm SFR}$. This result is based on wide field
and high sensitivity CO(3-2) observations at 100 pc resolution. Star formation
efficiencies (SFE), defined as $\Sigma_{\rm SFR}$/$\Sigma_{\rm H_2}$, in GHRs
are found to be about 1 dex higher than in other disk regions. The
CO(3-2)/CO(1-0) integrated intensity ratio is also higher than the average over
the disk. Such high SFE and CO(3-2)/CO(1-0) can reach the values found in
starburst galaxies, which suggests that GHRs may be the elements building up a
larger scale starburst region. Three possible contributions to high SFEs in GHR
are investigated: (1) the $I_{CO}$-$N({\rm H_2})$ conversion factor, (2) the
dense gas fraction traced by CO(3-2)/CO(1-0), and (3) the initial mass function
(IMF). We conclude that these starburst-like properties in GHRs can be
interpreted by a combination of both a top-heavy IMF and a high dense gas
fraction, but not by changes in the $I_{CO}$-$N({\rm H_2})$ conversion factor.
