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We observed the spiral galaxies M51 and M83 at 20" spatial resolution withthe bolometer array AzTEC on the JCMT in the 1.1$\,$mm continuum, recoveringthe extended emission out to galactocentric radii of more than 12 kpc in bothgalaxies. The 1.1 mm-continuum fluxes are 5.6+/-0.7 and 9.9+/-1.4 Jy, withassociated gas masses estimated at 9.4 X 10^9 Mo and 7.2 X 10^9 Mo for M51 andM83, respectively. In the interarm regions of both galaxies the N(H2)/I(CO) (orX-factor) ratios exceed those in the arms by factors of ~1.5-2. In the innerdisks of both galaxies, the X-factor is about 1 X 10^20 cm^-2 / (K km s^-1). Inthe outer parts, the CO-dark molecular gas becomes more important.  While the spiral density wave in M51 appears to influence the interstellarmedium and stars in a similar way, the bar potential in M83 influences theinterstellar medium and the stars differently. We confirm the result of Foyleet al. (2010) that the arms merely heighten the star formation rate and the gassurface density in the same proportion. Our maps reveal a threshold gas surfacedensity for an SFR increase by two or more orders of magnitude. In both galaxycenters, 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 themolecular gas in the disks of spiral galaxies.

Continuum Observations of M51 and M83 at 1.1 mm with AzTEC