Enhancement of CO(3-2)/CO(1-0) Ratios and Star Formation Efficiencies in Supergiant HII Regions

TL;DR

This study reveals that supergiant HII regions in galaxy M33 exhibit star formation efficiencies and molecular gas excitation ratios comparable to starburst galaxies, driven by a top-heavy IMF and dense gas fraction.

Contribution

It demonstrates that GHRs have star formation properties similar to starbursts, highlighting the roles of dense gas and IMF in this process.

Findings

GHRs have about 1 dex higher SFE than other disk regions.

CO(3-2)/CO(1-0) ratios are elevated in GHRs, akin to starburst galaxies.

High SFE in GHRs is linked to dense gas fraction and IMF, not conversion factor changes.

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.