The shell elliptical NGC2865: evolutionary population synthesis of a kinematically distinct core

Abstract

We report on the discovery of a rapidly co-rotating stellar and gas component in the nucleus of the shell elliptical NGC2865. The stellar component extends ~ 0.51/h100 kpc along the major axis, and shows depressed velocity dispersion and absorption line profiles skewed in the opposite sense to the mean velocity. Associated with it is a young stellar population with enhanced \hbeta, lowered Mg and same Fe indices relative to the underlying elliptical. Its recent star formation history is constrained by considering ``bulge+burst'' models under 4 physically motivated scenarios, using evolutionary population synthesis. Scenarios in which the nuclear component is formed over a Hubble time or recently from continuous gas inflow are ruled out. Our results argue for a gas-rich accretion or merger origin for the shells and kinematic subcomponent in NGC2865. Arguments based on stellar populations and gas dynamics suggest that one of the progenitors is likely a Sb or Sc spiral. We demonstrate that despite the age and metallicity degeneracy of the underlying elliptical, the age and metallicity of the kinematic subcomponent can be constrained. This work strengthens the link between KDCs and shells, and demonstrates that a KDC can be formed from a late merger.