  Based on sensitive CO measurements from HERACLES and HI data from THINGS, we
show that the azimuthally averaged radial distribution of the neutral gas
surface density (Sigma_HI + Sigma_H2) in 33 nearby spiral galaxies exhibits a
well-constrained universal exponential distribution beyond 0.2*r25 (inside of
which the scatter is large) with less than a factor of two scatter out to two
optical radii r25. Scaling the radius to r25 and the total gas surface density
to the surface density at the transition radius, i.e., where Sigma_HI and
Sigma_H2 are equal, as well as removing galaxies that are interacting with
their environment, yields a tightly constrained exponential fit with average
scale length 0.61+-0.06 r25. In this case, the scatter reduces to less than 40%
across the optical disks (and remains below a factor of two at larger radii).
We show that the tight exponential distribution of neutral gas implies that the
total neutral gas mass of nearby disk galaxies depends primarily on the size of
the stellar disk (influenced to some degree by the great variability of
Sigma_H2 inside 0.2*r25). The derived prescription predicts the total gas mass
in our sub-sample of 17 non-interacting disk galaxies to within a factor of
two. Given the short timescale over which star formation depletes the H2
content of these galaxies and the large range of r25 in our sample, there
appears to be some mechanism leading to these largely self-similar radial gas
distributions in nearby disk galaxies.
