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(disk)(z,R=0)) is related to the maximum rotation speed (V-max) as sigma(disk)(z,R=0) similar to 0.26V(max), consistent with previous measurements for edge-on disk galaxies and a mean stellar velocity ellipsoid axial ratio alpha 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%-30% of the dynamical mass within 2.2 disk scale lengths (h(R)), with percentages increasing systematically with luminosity, rotation speed, and redder color. These trends indicate that the mass ratio of disk-to-total matter remains at or below 50% at 2.2 h(R) even for the most extreme, fast-rotating disks (V-max >= 300 km s(-1)) of the reddest rest frame, face-on color (B - K similar to 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 three.