We observed the spiral galaxies M 51 and M 83 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 \pm 0.7 and 9.9 \pm 1.4 Jy , with associated gas masses estimated at 9.4 \times 10 ^ { 9 } M _ { \odot } and 7.2 \times 10 ^ { 9 } M _ { \odot } for M 51 and M 83 , respectively .
In the interarm regions of both galaxies the N ( H _ { 2 } ) / I ( CO ) ( or X-factor ) ratios exceed those in the arms by factors of \sim 1.5 -2 .
In the inner disks of both galaxies , the X-factor is about 1 \times 10 ^ { 20 } cm ^ { -2 } \cdot ( K \cdot km \cdot s ^ { -1 } ) ^ { -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 .
( 36 ) 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 .