The Scale Dependence of the Molecular Gas Depletion Time in M33
Abstract
We study the Local Group spiral galaxy M33 to investigate how the observed scaling between the (kpc-averaged) surface density of molecular gas (H2) and recent star formation rate (SFR) relates to individual star-forming regions. To do this, we measure the ratio of CO emission to extinction-corrected Halpha emission in apertures of varying sizes centered both on peaks of CO and Halpha emission. We parameterize this ratio as a molecular gas (H2) depletion time (τdep). On large (kpc) scales, our results are consistent with a molecular star formation law (SigmaSFR SigmaH2b) with b 1.1 - 1.5 and a median τdep 1 Gyr, with no dependence on type of region targeted. Below these scales, τdep is a strong function of adopted angular scale and the type of region that is targeted. Small ( 300pc) apertures centered on CO peaks have very long τdep (i.e., high CO-to-Halpha flux ratio) and small apertures targeted toward Halpha peaks have very short τdep. This implies that the star formation law observed on kpc scales breaks down once one reaches aperture sizes of 300pc. For our smallest apertures (75pc), the difference in τdep between the two types of regions is more than one order of magnitude. This scale behavior emerges from averaging over star-forming regions with a wide range of CO-to-Halpha ratios with the natural consequence that the breakdown in the star formation law is a function of the surface density of the regions studied. We consider the evolution of individual regions the most likely driver for region-to-region differences in τdep (and thus the CO-to-Halpha ratio).
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