Galaxy Disks are Submaximal

TL;DR

This study measures the gravitational contribution of galaxy disks in face-on spirals, finding they are generally submaximal and contribute less than half of the total mass within 2.2 disk scale-lengths, affecting baryonic mass estimates.

Contribution

It provides new empirical evidence that galaxy disks are submaximal, challenging the maximal disk hypothesis and refining the understanding of mass distribution in spiral galaxies.

Findings

Galaxy disks contribute only 15-30% of the mass within 2.2 disk scale-lengths.

Disks are submaximal across a range of luminosities and rotation speeds.

Stellar mass-to-light ratios should be reduced by at least a factor of 3.

Abstract

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.