  The decomposition of the 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
and disc scale height, the disc surface mass density and hence the M/L can be
estimated. We address a conceptual problem with previous measurements of the
scale height and dispersion. When using this method, the dispersion and scale
height must refer to the same population of stars. The scale height is obtained
from near-IR studies of edge-on galaxies and is weighted towards older
kinematically hotter stars, whereas the dispersion obtained from integrated
light in the optical bands includes stars of all ages. We aim to extract the
dispersion for the hotter stars, so that it can then be used with the correct
scale height to obtain the disc surface mass density. We use a sample of
planetary nebulae (PNe) as dynamical tracers in the face-on galaxy NGC 628. We
extract two different dispersions from its velocity histogram -- representing
the older and younger PNe. We also present complementary stellar absorption
spectra in the inner regions of this galaxy and use a direct pixel fitting
technique to extract the two components. Our analysis concludes that previous
studies, which do not take account of the young disc, underestimate the disc
surface mass density by a factor of ~ 2. This is sufficient to make a maximal
disc for NGC 628 appear like a submaximal disc.
