Where do stars explode in the ISM? -- The distribution of dense gas around evolved massive stars in M33

Abstract

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.

Figures (14)

• Figure 1: The spatial distribution of red supergiants (RSGs), Wolf-Rayet stars (WRs) and supernova remnants (SNRs) with respect to the cold (H$_2$ + H1) ISM in the M33 ACA survey region (shown in the inset plot). Orange circles represent RSGs with log $(L/L_{\odot})\geq 4.7$ (roughly corresponding to $\gtrsim$12M$_{\odot}$). Bluish circles show WRs and green stars show SNRs. H1 is shown in grey-scale with the density range shown in the bottom colorbar. Contours of CO (2-1) emission (converted to $H_2$ surface densities) are also shown at levels of 1.9, 7.6, 15.3 and 30.6 M$_{\odot}$ pc$^{-2}$. The prominent star-forming regions in the ACA footprint -- NGC 604, IC 142 and NGC 595 -- are specifically labelled (showing high WR concentrations). The black box shows a region containing a WR and SNR with higher-resolution ALMA data in Figure 6. The map shows the 50 pc-scale cold ISM environments where core-collapse SNe will occur in the future (or have occured in the case of SNRs). Statistical analysis of these environments and their implications are discussed in Section \ref{['sec:results']} and \ref{['sec:discussion']}.
• Figure 2: Cumulative distribution functions showing H$_2$, H1, and total (H$_2$ + H1) surface densities. Distributions are shown for the locations of WRs (blue lines), SNRs (green lines) and RSGs (red lines), while grey histograms denote the distribution of densities of the full M33 region, representing what would be expected for a random distribution. Vertical dashed lines denote the completeness limit of each dataset. Colored circles show the median of the distribution above the completeness limit. The distributions show a higher probability of more massive stars (e.g. the WRs) to evolve in denser gas.
• Figure 3: Cumulative distribution functions showing H$_2$, H1 and total gas surface densities (as in Figure \ref{['fig:3paneldensities']}). Here only the RSGs are plotted, dividing them into 3 luminosity bins, roughly tracing mass ranges of 8-12 M$_{\odot}$ (peach lines), 12-15 M$_{\odot}$ (orange lines) and $>$15 M$_{\odot}$ (red lines). Grey histograms denote the distributions for the full M33 region. Colored circles show the median of the distribution above the completeness limit.
• Figure 4: Distribution of mock stars (purple) randomly drawn from different stellar population tracers -- uniform random (top left), 3.4 $\mu$m near-IR tracing bulk stellar mass (top right), the 24 $\mu$m-corrected far-ultraviolet (FUV) tracing the $<100$ Myr star-formation (bottom left) and 24 $\mu$m-corrected H$\alpha$ tracing the $<10$ Myr star-formation (bottom right). Each map shows a population of $N=500$ randomly-drawn stars. The maps are themselves shown in grey-scale. Orange contours denote H$_2$ clouds with densities $>$4.3 M$_{\odot}$ pc$^{-2}$ from the ACA survey. The figure visualizes where stars from different progenitor populations occur, with their corresponding densities shown in Figure \ref{['fig:mocks']}.
• Figure 5: Cumulative density histograms of the observed and modeled populations of RSGs, WRs, and SNRs from Section \ref{['sec:sfrtracers']}. Solid grey curves in each panel denote the bulk H$_2$ cumulative distribution in M33 within the ACA area. Solid colored lines denote the H$_2$ surface density distribution of the star category. Colored shaded region shows the 5th-95th percentile region of the mock populations drawn according to Section \ref{['sec:sfrtracers']}.