On The Relationship Between Age and Dynamics in Elliptical Galaxies

Abstract

Here, using a sample of 88 galaxies, we examine the scatter about the Faber-Jackson (F-J) relation, and find that a galaxy's position relative to this relation depends on its age. We attempt to reproduce the observed trend of the F-J residuals with age using two simple models. The first assumes that galaxy age is tracing the last major star formation event in an elliptical galaxy. The fading of this burst changes the M_B component of the F-J residuals, with time. Such a model was very successful at reproducing the B-V and Mg_2 evolution reported in our previous paper, but is unable to reproduce the strength of the F-J trend. A second model is required to describe age-correlated changes in galaxy dynamics. Following expectations from cosmological simulations, we assume that the velocity dispersion, for a galaxy of a given mass, scales with the epoch of galaxy formation, i.e. with the mean density of the Universe. Hence recently formed ellipticals have systematically lower velocity dispersions than old ellipticals. We find that a combination of these two models provides a good match to the change in F-J residuals with galaxy age. This suggests that young ellipticals will have subtly different dynamical properties to old ellipticals. We also find that there is not a strong relationship between a galaxy's age and its luminosity for our sample. This suggests that the tilt of the fundamental plane is not totally driven by age.