Continuum observations of M51 and M83 at 1.1mm with AzTEC

We observed the spiral galaxies M51 and M83 at 20 arscec spatial resolution with thebolometer array Aztronomical Thermal Emission Camera (AzTEC) on the JCMT in the1.1mm continuum, recovering the extended emission out to galactocentric radii of more than12 kpc in both galaxies. The 1.1 mm-continuum fluxes are 5.6 ± 0.7 and 9.9 ± 1.4 Jy, withassociated gas masses estimated at 9.4 × 10⁹Mo˙ and 7.2 × 109Mo˙ for M51 and M83,respectively. In the interarm regions of both galaxies, the N(H2)/I(CO) (or X-factor) ratiosexceed those in the arms by factors of ~1.5-2. In the inner discs of both galaxies, the X-factoris about 1 × 10²⁰ cm^-2 (K km s^-1)^-1. In the outer parts, the CO-dark molecular gas becomesmore important. While the spiral density wave in M51 appears to influence the interstellarmedium and stars in a similar way, the bar potential inM83 influences the interstellar mediumand the stars differently.We confirm the result of Foyle et al. that the arms merely heighten thestar formation rate (SFR) and the gas surface density in the same proportion. Our maps reveala threshold gas surface density for an SFR increase by two or more orders of magnitude. Inboth galaxy centres, the molecular gas depletion time is about 1 Gyr climbing to 10-20 Gyrat radii of 6-8 kpc. This is consistent with an inside-out depletion of the molecular gas in thediscs of spiral galaxies.