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

TL;DR

This study analyzes stellar population parameters across the Fundamental Plane of galaxies, revealing new trends with radius and surface brightness, and linking galaxy properties to merger history.

Contribution

It uncovers stellar population variations across and through the Fundamental Plane, emphasizing the role of luminosity density and merger history, which was not highlighted in previous research.

Findings

Stellar populations vary with radius and surface brightness.

No variation of stellar populations along the plane ($v_2$ direction).

Luminosity density correlates with merger history.

Abstract

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 $v_2$.