  We present an analysis of the stellar populations in a sample of ~7000
galaxies from the 6dF Galaxy Survey (6dFGS). We derive ages and metallicities
using stellar population models. We also derive dynamical masses and dynamical
mass-to-light ratios by combining central velocity dispersions with global
photometry in B, R and K bands. Together, these data allow to reduce the
degeneracies between age, metallicity and star formation burst-strength that
have limited previous studies. We find old galaxies exhibit a mass-metallicity
relation with slope d[Fe/H]/dlogM = 0.25, while young galaxies show slopes
consistent with zero. When we account for the effects of the mass-metallicity
relation, we obtain a single, consistent relation between mass-to-light ratio
and mass for old galaxies in all passbands. As we have accounted for stellar
population effects, this relation must have a dynamical origin. However, we
demonstrate that any simple trend between mass-to-light-ratio and mass or
luminosity is inconsistent with the observations, and that a more complex
relationship must exist. We find the central regions of galaxies often exhibit
young stellar populations. However it is only in the lowest-mass galaxies
(~10^{10} M$_{\odot}$) that these populations are evident in the global
photometry. In higher-mass galaxies, young central populations have decreasing
influence on the global photometry, with there being no discernible impact in
galaxies more massive than ~2x10^{11} M$_{\odot}$. We conclude that the young
stellar populations detected in spectroscopic studies are generally centrally
concentrated, and that there is an upper limit on the mass of star-forming
events in massive galaxies. These results have ramifications for mass-to-light
ratios estimated from photometric observations.
