On the relationship between age and dynamics in elliptical galaxies

Galaxy age estimates (mostly from spectroscopy of the central regions) are now available for many early-type galaxies. In a previous paper we showed that the offset of galaxies from the fundamental plane depends on galaxy age. Here, using the same sample of 88 galaxies, we examine the scatter about the Faber–Jackson (FJ) relation, and find that the position of a galaxy relative to this relation depends on its age. In particular, younger ellipticals are systematically brighter in MB and/or have a lower central velocity dispersion (σ0). The mean relation corresponds to galaxies that are ~10 Gyr old. We attempt to reproduce the observed trend of the FJ 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. We assume that this starburst was instantaneous, centrally located and involved 10 per cent of the galaxy by mass. The fading of this burst changes the MB component of the FJ residuals, with time. Such a model was very successful at reproducing the B−V and Mg2 evolution reported in our previous paper, but is unable to reproduce the strength of the FJ trend. A second model is required to describe age-correlated changes in galaxy dynamics. Following expectations from cosmological simulations, we assume that σ0, 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 do old ellipticals. We find that a combination of these two models provides a good match to the change in FJ residuals with galaxy age. This suggests that young ellipticals will have subtly different dynamical properties from old ellipticals. We also find that there is not a strong relationship between the age of a galaxy and its luminosity for our sample. This suggests that the tilt of the fundamental plane is not totally driven by age.