The 6dF Galaxy Survey: : Stellar Population Trends Across and Through the Fundamental Plane

Christopher M. Springob, Christina Magoulas, Rob Proctor, Matthew Colless, D. Heath Jones, Chiaki Kobayashi, Lachlan Campbell, John Lucey and Jeremy Mould, 'The 6dF Galaxy Survey: stellar population trends across and through the Fundamental Plane', Monthly Notices of the Royal Astronomical Society, Vol. 420, pp. 2773-2784, first published online 1 March 2012. The version of record is available online at doi: 10.1111/j.1365-2966.2011.19900.x. Published by Oxford University Press on behalf of the Royal Astronomical Society. © 2012 The Authors. Monthly Notices of the Royal Astronomical Society © 2012 RAS. A version of this paper with fully interactive 3D figures, viewable with Adobe Reader 8.0 or higher, is available as an ancillary file with this release, and can also be accessed from this link.We present results from an analysis of stellar population parameters for 7132 galaxies in the 6dFGS Fundamental Plane (FP) sample. We bin the galaxies along the axes, $v_1$, $v_2$, and $v_3$, of the tri-variate Gaussian to which we have fit the galaxy distribution in effective radius, surface brightness, and central velocity dispersion (FP space), and compute median values of stellar age, [Fe/H], [Z/H], and [$\alpha$/Fe]. We determine the directions of the vectors in FP space along which each of the binned stellar population parameters vary most strongly. In contrast to previous work, we find stellar population trends not just with velocity dispersion and FP residual, but with radius and surface brightness as well. The most remarkable finding is that the stellar population parameters vary through the plane ($v_1$ direction) and across the plane ($v_3$ direction), but show no variation at all along the plane ($v_2$ direction). The $v_2$ direction in FP space roughly corresponds to `luminosity density'. We interpret a galaxy's position along this vector as being closely tied to its merger history, such that early-type galaxies with lower luminosity density are more likely to have undergone major mergers. This conclusion is reinforced by an examination of the simulations of Kobayashi (2005), which show clear trends of merger history with $v_2$.Peer reviewe