Dynamical measurement of the stellar surface density of face-on galaxies
  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.
