Continuum Observations of M51 and M83 at 1.1 mm with AzTEC

TL;DR

This study used 1.1 mm continuum observations with AzTEC on JCMT to analyze the molecular gas distribution, star formation, and interstellar medium properties in the spiral galaxies M51 and M83, revealing variations in gas-to-CO ratios and star formation thresholds.

Contribution

First detailed 1.1 mm continuum maps of M51 and M83 at 20" resolution, analyzing gas and star formation properties across their disks.

Findings

Extended emission detected out to >12 kpc in both galaxies.

X-factor varies between arm and interarm regions, with outer regions showing more CO-dark gas.

Molecular gas depletion time increases from ~1 Gyr in centers to 10-20 Gyr at larger radii.

Abstract

We observed the spiral galaxies M51 and M83 at 20" spatial resolution with the bolometer array AzTEC on the JCMT in the 1.1$\,$mm continuum, recovering the extended emission out to galactocentric radii of more than 12 kpc in both galaxies. The 1.1 mm-continuum fluxes are 5.6+/-0.7 and 9.9+/-1.4 Jy, with associated gas masses estimated at 9.4 X 10^9 Mo and 7.2 X 10^9 Mo for M51 and M83, respectively. In the interarm regions of both galaxies the N(H2)/I(CO) (or X-factor) ratios exceed those in the arms by factors of ~1.5-2. In the inner disks of both galaxies, the X-factor is about 1 X 10^20 cm^-2 / (K km s^-1). In the outer parts, the CO-dark molecular gas becomes more important. While the spiral density wave in M51 appears to influence the interstellar medium and stars in a similar way, the bar potential in M83 influences the interstellar medium and the stars differently. We confirm the result of Foyle et al. (2010) that the arms merely heighten the star formation rate and the gas surface density in the same proportion. Our maps reveal a threshold gas surface density for an SFR increase by two or more orders of magnitude. In both galaxy centers, the molecular gas depletion time is about 1 Gyr climbing to 10-20 Gyr at radii of 6-8 kpc. This is consistent with an inside-out depletion of the molecular gas in the disks of spiral galaxies.