The JCMT Nearby Galaxies Legacy Survey IV. Velocity Dispersions in the
  Molecular Interstellar Medium in Spiral Galaxies
  An analysis of large-area CO J=3-2 maps from the James Clerk Maxwell
Telescope for 12 nearby spiral galaxies reveals low velocity dispersions in the
molecular component of the interstellar medium. The three lowest luminosity
galaxies show a relatively flat velocity dispersion as a function of radius
while the remaining nine galaxies show a central peak with a radial fall-off
within 0.2-0.4 r(25). Correcting for the average contribution due to the
internal velocitydispersions of a population of giant molecular clouds, the
average cloud-cloud velocity dispersion across the galactic disks is 6.1 +/-
1.0 km/s (standard deviation 2.9 km/s), in reasonable agreement with previous
measurements for the Galaxy andM33. The cloud-cloud velocity dispersion derived
from the CO data is on average two times smaller than the HI velocity
dispersion measured in the same galaxies. The low cloud-cloudvelocity
dispersion implies that the molecular gas is the critical component determining
the stability of the galactic disk against gravitational collapse, especially
in those regions of the disk which are H2 dominated. The cloud-cloud velocity
dispersion shows a significant positivecorrelation with both the far-infrared
luminosity, which traces the star formation activity, and the K-band absolute
magnitude, which traces the total stellar mass. For three galaxies in the Virgo
cluster, smoothing the data to a resolution of 4.5 kpc (to match the typical
resolution of high redshift CO observations) increases the measured velocity
dispersion by roughly a factor of two, comparable to the dispersion measured
recently in a normal galaxy at z=1. This comparison suggests that the mass and
star formation rate surface densities may be similar in galaxies from z=0-1 and
that the high star formation rates seen at z=1 may be partly due to the
presence of physically larger molecular gas disks.
