Where do stars explode in the ISM? -- The distribution of dense gas around evolved massive stars in M33
The effect of supernovae (SNe) on star-formation in the interstellar medium (ISM) depends sensitively on where SNe explode with respect to ISM clouds. Observationally, SN ISM environments characterized by spatially-resolved gas maps can empirically guide the placement of SNe in subgrid models, but unfortunately such measurements remain scarce, as SNe are rare and often distant. Here we demonstrate a new approach -- mapping the ISM around evolved massive stars that are soon to explode. These provide a substantially larger sample of `explosion sites' (than just historical SNe) in nearby galaxies that have high-resolution atomic and molecular ISM maps from Jansky VLA and ALMA. We demonstrate this technique in the well-resolved Local Group spiral M33 by analyzing the 50 pc-scale projected ISM densities around red supergiants (RSGs, 8-30 M$_{\odot}$ stars) Wolf-Rayet stars (WRs, $>$30 M$_{\odot}$ stars), and supernova remnants (SNRs). We find a \emph{mass-dependent} correlation between stars and gas clouds, with atleast 45\% of WRs and upto 77\% of RSGs having no detectable H$_2$ at their pixel locations. In the sample with H$_2$ detections, we find that more massive younger progenitors are coincident with denser gas. We show that the density distributions for stars $>$15 M$_{\odot}$ are statistically distinct from random alignment of stars and gas in M33. Our work provides the first observationally-derived estimate of the fraction of the SN-producing stellar population correlated with ISM density peaks. We demonstrate how this can be compared with galaxy simulations, and advocate similar comparisons to the community for constraining sub-grid models.