The 6dF Galaxy Survey: Stellar Population Trends Across and Through the
  Fundamental Plane
  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 $v_2$.
