Dependence of the Fundamental Plane Scatter on Galaxy Age

Abstract

The fundamental plane (FP) has an intrinsic scatter that cannot be explained purely by observational errors. Using the recently available age estimates for nearby early-type galaxies, we show that a galaxy's position relative to the FP depends on its age. In particular, the mean FP corresponds to elliptical galaxies with an age of ~10 Gyr. Younger galaxies are systematically brighter, with a higher surface brightness relative to the mean relation. Old elliptical galaxies form an upper envelope to the FP. For our sample of mostly noncluster galaxies, age can account for almost half of the scatter in the B-band FP. Distance determinations based on the FP may have a systematic bias, if the mean age of the sample varies with redshift. We also show that fundamental plane residuals, B-V colors, and Mg2 line strength are consistent with an ageing central burst superposed on an old stellar population. This reinforces the view that these age estimates are tracing the last major episode of star formation induced by a gaseous merger event. We briefly discuss the empirical evolutionary tracks of merger remnants and young elliptical galaxies in terms of their key observational parameters.