The Correlation between WISE 12 μm Emission and Molecular Gas Tracers on Subkiloparsec Scales in Nearby Star-forming Galaxies

Abstract

We complement the MALATANG sample of dense gas in nearby galaxies with archival observations of 12CO and its isotopologues to determine scaling relations between Wide-field Infrared Survey Explorer (WISE) 12 μm emission and molecular gas tracers at subkiloparsec scales. We find that 12 μm luminosity is more tightly correlated with 12CO than it is with 13CO or dense gas tracers. Residuals between predicted and observed 12CO are only weakly correlated with molecular gas mass surface density (Σmol) in regions where Σmol is very low (∼10 M ⊙ pc−2). Above this limit, the 12CO residuals show no correlations with physical conditions of molecular gas, while 13CO residuals depend on the gas optical depth and temperature. By analyzing differences from galaxy to galaxy, we confirm that the 12CO−12 μm relation is strong and statistically robust with respect to star-forming galaxies and active galactic nucleus hosts. These results suggest that WISE 12 μm emission can be used to trace total molecular gas instead of dense molecular gas, likely because polycyclic aromatic hydrocarbons (PAHs, a major contributor to WISE 12 μm emission) may be well mixed with the gas that is traced by 12CO. We propose that WISE 12 μm luminosity can be used to estimate molecular gas surface density for statistical analyses of the star formation process in galaxies.

Star Formation and Molecular Gas Diagnostics with Mid- and Far-infrared Emission

“…In previous and recent studies, (e.g., Jiang et al 2015;Gao et al 2019Gao et al , 2022Chown et al 2021;), WISE3 12 μm emission is found to trace CO emission better than Hα traces CO at sub-kiloparsec scales. Our synthetic WISE3 results show the hot dust continuum contribution is greater than the PAH contribution in star-forming regions.…”

“…The WISE 12 μm band captured the complex of PAH features between 11 and 12 μm and has been used to calibrate both molecular gas and SF tracers (Donoso et al 2012;Jarrett et al 2013;Gao et al 2019;Chown et al 2021;Gao et al 2022). The IRAC 8 μm band is usually dominated by the strongest emission feature at 7.7 μm from PAHs, which are likewise supposed to trace UV photons produced primarily by O stars in regions of ongoing SF.…”

“…In addition, high-sensitivity, high-angular resolution MIR observations of galaxies at a wide range of redshifts are now possible with JWST. Many studies have investigated MIR emission lines, polycyclic aromatic hydrocarbon (PAH) vibrational bands, and dust continuum as tracers of SF and molecular gas (e.g., Calzetti et al 2005;Wu et al 2005;Alonso-Herrero et al 2006;Murphy et al 2011;Jarrett et al 2013;Maragkoudakis et al 2018;Gao et al 2019;Lai et al 2020;Chown et al 2021;Gao et al 2022). These MIR tracers, however, will generally have sensitivity to both dust content, and by extension gas content, as well as the strength of the UV radiation field illuminating the interstellar medium (which is related to SF).…”

PHANGS–JWST First Results: Mid-infrared Emission Traces Both Gas Column Density and Heating at 100 pc Scales

“…Likely because of its sensitivity to both dust heating and ISM column density, mid-IR emission correlates very well with both recombination line emission (e.g., Calzetti et al 2007;Kennicutt et al 2007) and CO emission (e.g., Regan et al 2006;Leroy et al 2013;Chown et al 2021;Gao et al 2022; in observations that integrate over large regions or whole galaxies. Indeed, Whitcomb et al (2022) combined CO emission and Spitzer mid-IR spectroscopy to show that different mid-IR bands exhibit different correlation strengths with tracers of star formation and CO emission, providing strong evidence that both heating and column density together generate the observed mid-IR emission.…”

PHANGS–JWST First Results: Multiwavelength View of Feedback-driven Bubbles (the Phantom Voids) across NGC 628

“…Specifically, the wavelength coverage of the Mid-Infrared Instrument (MIRI) is perfectly suited to this task, as it is sensitive to several polycyclic aromatic hydrocarbon (PAH) emission features (e.g., at 7.7 μm; see Draine & Li 2007;Smith et al 2007;Li 2020). Emission from PAHs is particularly useful in tracing the shells of feedback-driven bubbles (e.g., Pineda et al 2022), due to (a) the increased gas densities found in swept-up shells (PAHs are generally well mixed with the gas and illuminated by the average interstellar radiation field such that they trace the gas column very sensitively; e.g., Regan et al 2006;Leroy et al 2013;Chown et al 2021;Gao et al 2022;Leroy et al 2023), (b) the high number of ionizing photons emitted by the OB association powering the bubbles, leading to PAHs being destroyed in the photoionized interiors of the bubbles (e.g., Galliano et al 2018;Chastenet et al 2023aChastenet et al , 2023bEgorov et al 2023), and (c) the low optical depth from the shell interior to the edge, which will allow far-UV photons to easily heat the small dust grains (e.g., Draine & Li 2007;Draine 2011;Hensley & Draine 2021). Together, these cause the edges of bubbles to appear with high contrast against their interior PAH emission (e.g., Churchwell et al 2006;Watson et al 2008).…”