| Résumé | 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 (σdisk z, R = 0) is related to the maximum rotation speed (V max) as σdisk z, R = 0 ~ 0.26V 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 (hR ), 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 hR 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 (MK < –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. |
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