A new estimator of resolved molecular gas in nearby galaxies

Abstract

A relationship between dust-reprocessed light from recent star formation and the amount of star-forming gas in a galaxy produces a correlation between WISE 12 $\mu$m emission and CO line emission. Here we explore this correlation on kiloparsec scales with CO(1-0) maps from EDGE-CALIFA matched in resolution to WISE 12 $\mu$m images. We find strong CO-12 $\mu$m correlations within each galaxy and we show that the scatter in the global CO-12 $\mu$m correlation is largely driven by differences from galaxy to galaxy. The correlation is stronger than that between star formation rate and H${}_2$ surface densities ($\Sigma ({\mathrm{H}}_2)$). We explore multi-variable regression to predict $\Sigma ({\mathrm{H}}_2)$ in star-forming pixels using the WISE 12 $\mu$m data combined with global and resolved galaxy properties, and provide the fit parameters for the best estimators. We find that $\Sigma ({\mathrm{H}}_2)$ estimators that include $\Sigma (12\mu \mathrm{m})$ are able to predict $\Sigma ({\mathrm{H}}_2)$ more accurately than estimators that include resolved optical properties instead of $\Sigma (12\mu \mathrm{m})$. These results suggest that 12 $\mu$m emission and H${}_2$ as traced by CO emission are physically connected at kiloparsec scales. This may be due to a connection between polycyclic aromatic hydrocarbon (PAH) emission and the presence of H${}_2$. The best single-property estimator is $\log \frac{\Sigma ({\mathrm{H}}_2)}{{\mathrm{M}}_{\odot }\mathrm{p}{\mathrm{c}}^{-2}}=(0.48\pm 0.01)+(0.71\pm 0.01)\log \frac{\Sigma (12\mu \mathrm{m})}{{\mathrm{L}}_{\odot }\mathrm{p}{\mathrm{c}}^{-2}}$. This correlation can be used to efficiently estimate $\Sigma ({\mathrm{H}}_2)$ down to at least $1M_{\odot }\mathrm{p}{\mathrm{c}}^{-2}$ in star-forming regions within nearby galaxies.

19 pages, 8 figures, 6 tables, accepted by MNRAS. Data tables and a script for making 2D cutouts for each galaxy are available as MNRAS supplementary material and at https://lig.astro.tsinghua.edu.cn/astrodata/publication/20/estimator-resolved-molecular/

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