The DiskMass survey recently provided measurements of the vertical velocity dispersions of disk stars in a sample of nearly face-on galaxies .
By setting the disk scale-heights to be equal to those of edge-on galaxies with similar scale-lengths , it was found that these disks must be sub-maximal , with surprisingly low K-band mass-to-light ratios of the order of M _ { \star } / L _ { K } \simeq 0.3 M _ { \odot } / L _ { \odot } .
This study made use of a simple relation between the disk surface density and the measured velocity dispersion and scale height of the disk , neglecting the shape of the rotation curve and the dark matter contribution to the vertical force , which can be especially important in the case of sub-maximal disks .
Here , we point out that these simplifying assumptions led to an overestimation of the stellar mass-to-light ratios .
Relaxing these assumptions , we compute even lower values than previously reported for the mass-to-light ratios , with a median M _ { \star } / L _ { K } \simeq 0.18 M _ { \odot } / L _ { \odot } , where 14 galaxies have M _ { \star } / L _ { K } < 0.11 .
Invoking prolate dark matter halos made only a small difference to the derived M _ { \star } / L _ { K } , although extreme prolate halos ( q > 1.5 for the axis ratios of the potential ) might help .
The cross-terms in the Jeans equation are also generally negligible .
These deduced K-band stellar mass-to-light ratios are even more difficult to reconcile with stellar population synthesis models than the previously reported ones .