IAU Symposium Abstract S186-041P

The recent star formation history of a shell elliptical with a Kinematically Distinct Core

G.K.T. Hau1, D. Carter2, M. Balcells3, R.C. Thomson4
1 Institute of Astronomy, Cambridge, UK < gkth@ast.cam.ac.uk >
2 Liverpool John Moores University, Liverpool, UK < dxc@ast.cam.ac.uk >
3 Instituto de Astrof'{\i}sica de Canarias, Tenerife, Spain < balcells@ll.iac.es >
4 Univ. of Hertfordshire, Hatfield, UK < rct@star.herts.ac.uk >

We report on the discovery of a rapidly rotating stellar nuclear component in the shell elliptical NGC 2865, with associated enhanced H${\beta}$ equivalent widths, lowered Mg line-indices, constant Fe index relative to the underlying elliptical, and a co-rotating disk of ionized gas. We set constraints on its recent star formation history by considering ``bulge+burst'' models using evolutionary multi-metallicity population synthesis. Scenarios where the nuclear component is formed over a Hubble time or recently out of a continuous gas inflow over a timescale of ~ 1 Gyr are ruled out. A recent short burst with a range of masses can reproduce the change in line-indices, if the metallicity and/or [Fe/Mg] of the burst is higher than that of the underlying elliptical. We show that when the Main Sequence turn-off of a starburst is between F0 & F5, the Mg lines may be diluted more than the Fe lines. Our results support a gas-rich accretion or merger origin for the Kinematically Distinct Core (KDC) in NGC 2865, and strengthens the link between KDCs & shells. Despite the age & metallicity degeneracy of the underlying bulge, physical parameters such as metallicity, mass & age of the KDC relative to the bulge can be constrained.