  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.
