2016MNRAS.459.1440W

We observed the spiral galaxies M 51 and M 83 at 20 arscec spatial resolution with the bolometer array Aztronomical Thermal Emission Camera (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 10⁹ M_☉ and 7.2 x 10⁹ M_☉ for M 51 and M 83, 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 discs of both galaxies, the X-factor is about 1 x 10²⁰ cm^- 2^ (K km/s)^- 1^. In the outer parts, the CO-dark molecular gas becomes more important. While the spiral density wave in M 51 appears to influence the interstellar medium and stars in a similar way, the bar potential in M 83 influences the interstellar medium and the stars differently. We confirm the result of Foyle et al. that the arms merely heighten the star formation rate (SFR) 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 centres, 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 discs of spiral galaxies.