Galaxy Disks are Submaximal
  We measure the contribution of galaxy disks to the overall gravitational
potential of 30 nearly face-on intermediate-to-late-type spirals from the
DiskMass Survey. The central vertical velocity dispersion of the disk stars,
sigma(z,R=0), is related to the maximum rotation speed (Vmax) as sigma(z,R=0) ~
0.26 Vmax, consistent with previous measurements for edge-on disk galaxies and
a mean stellar velocity ellipsoid axial ratio sigma(z) / sigma(R) = 0.6. For
reasonable values of disk oblateness, this relation implies these galaxy disks
are submaximal. We find disks in our sample contribute only 15% to 30% of the
dynamical mass within 2.2 disk scale-lengths (hR), with percentages increasing
systematically with luminosity, rotation speed and redder color. These trends
indicate the mass ratio of disk-to-total matter remains at or below 50% at 2.2
hR even for the most extreme, fast-rotating disks (Vmax > 300 km/s), of the
reddest rest-frame, face-on color (B-K ~ 4 mag), and highest luminosity
(M(K)<-26.5 mag). Therefore, spiral disks in general should be submaximal. Our
results imply that the stellar mass-to-light ratio and hence the accounting of
baryons in stars should be lowered by at least a factor of 3.
