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The decomposition of the 21 cm rotation curve of galaxies into contribution from the disc and dark halo remains uncertain and depends on the adopted mass to light ratio (M/L) of the disc. Given the vertical velocity dispersion of stars in the disc and its scale height, the disc surface density and hence the M/L can be estimated. Earlier works, such as the DiskMass Survey, have used this method to conclude that galaxy discs are submaximal. However, there is an important conceptual problem that has previously not been included in the velocity dispersion estimates. Measuring the surface density of the disc requires a velocity dispersion and a disc scale height but they must be for the same population of the tracers. Discs of spirals contain stars of all ages. The younger stars (ages < 3 Gyr) have a relatively small scale height and velocity dispersion, compared to the older, kinematically hotter stars. Since it is not possible to measure the scale height directly in face-on discs, we need to estimate it statistically using I-band and near-IR photometric data for edge-on galaxies; these estimates are weighted towards the scale height of the old disc. The spectra of the integrated light of the disc, which we use to measure the vertical velocity dispersion, come from the luminosity-weighted stellar population of the disc and contain a considerable contribution from the kinematically colder, younger disc population. Failing to separate out this younger component, will lead to underestimating the velocity dispersion. The surface density of the disc is therefore underestimated, and a maximal disc will appear to be submaximal. I will present an analysis of a sample of nearby spiral galaxies that demonstrate the presence of a young, kinematically colder population of stars and its effect on the disc-halo decomposition in these galaxies. |
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