The effects of stellar populations on galaxy scaling relations in the 6dF Galaxy Survey

We present an analysis of the stellar populations as a function of mass in a sample of ∼7000 galaxies of all morphological and emission types from the 6dF Galaxy Survey (6dFGS). We measure velocity dispersions and Lick indices from the spectra of the central regions of these galaxies, deriving ages and metallicities from the Lick indices using stellar population models. We also derive dynamical masses and dynamical mass-to-light ratios for these galaxies by combining the central velocity dispersions with global photometry in the B, R and K bands from SuperCOSMOS and 2MASS. Together, these data allow us to reduce the degeneracies between age, metallicity and star formation burst-strength that have limited previous studies. We find that old galaxies exhibit a mass-metallicity relation with logarithmic 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, M/L ∝ M. As we have accounted for stellar population effects, this remaining variation in the mass-to-light with mass (the residual ‘tilt’ of the Fundamental Plane) must have a dynamical origin. However, we demonstrate that any simple trend between massto-light-ratio and mass or luminosity is inconsistent with the observations, and that a more complex relationship must exist. We find that the central regions of galaxies of all masses often exhibit young stellar populations. However it is only in the lowest-mass galaxies studied (∼ 10 M⊙) 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 ∼ 2 × 10 M⊙. 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 starforming events in massive galaxies. These results have important ramifications for mass-to-light ratios estimated from photometric observations.