Is the Kinematic Diversity in Ellipticals Reflected in Their Color Gradients?

Abstract

Recent observations of the kinematics of elliptical galaxies have shown a variety of outstanding features: a significant fraction of galaxies have kinematically-distinct cores, while some even appear to rotate around their long axes. These discoveries lead one to ask whether the stellar populations in ellipticals differ in any way that can be related to the dynamical properties of the galaxies. Such relationships could provide further clues about the formation process in early-type galaxies. Remarkably enough, the colors and color gradients of ellipticals are very regular. Even more surprising, the local color within an elliptical appears to be related to the local escape velocity. The most straightforward interpretation is that the mean metallicity is a function of the local escape velocity. Such a relation could result from the star formation and metallicity enrichment being regulated by interplay between the input of energy from supernovae, and the temperature and pressure of the hot X-ray gas in ellipticals. Since color gradients may not be strongly affected by subsequent merging and evolution, we may have a diagnostic of early events during formation.