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 (σ disk z , R = 0 ) is related to the maximum rotation speed ( V max ) as σ disk z , R = 0 ∼ 0.26 V max , consistent with previous measurements for edge-on disk galaxies and a mean stellar velocity ellipsoid axial ratio α ≡ σ z /σ 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 ∼ 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.