Abstract
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.
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