The shell elliptical galaxy NGC 2865: evolutionary population synthesis ofa kinematically distinct core

We report on the discovery of a rapidly corotating stellar and gas componen t in the nucleus of the shell elliptical NGC 2865. The stellar component ex tends similar to 0.51 h(100)(-1) kpc along the major axis, and shows depres sed velocity dispersion and absorption-line profiles skewed in the opposite sense to the mean velocity. Associated with it is a young stellar populati on with enhanced H beta, lowered Mg, and the same Fe indices relative to th e underlying elliptical. Its recent star formation history is constrained b y considering 'bulge+burst' models under four physically motivated scenario s, using evolutionary population synthesis. Scenarios in which the nuclear component is formed over a Hubble time or recently from continuous gas infl ow are ruled out. A recent starburst can satisfy observational constraints only if its popula tion has metallicity 2.5-6.3 times that of the bulge, The nuclear iron-to-m agnesium index ratio can be explained by a temperature effect in the atmosp heres of stars at main-sequence turn-offs between A3 and F4, during which t he Fe indices of the burst population are high enough to compensate for dil ution effects. It is therefore possible to modify line-index ratios (and he nce the inferred abundance ratio) simply by the presence of a young populat ion with the same abundance. The high metallicity requirement suggests self -enrichment, and a burst duration longer than the SN II feedback time-scale . No solution exists for bursts longer than 0.4Gyr. Burst age estimates of 0.4-1.7Gyr are larger than that for the shells (0.24Gyr), assuming phase-wr apping. No starburst is required if the nuclear component is composed of stars with Fe abundance enhanced by similar to 0.08 dex relative to the underlying el liptical, which are accreted by an event which truncated the star formation . This relies on the abundance differences between giant ellipticals and sp irals. The age estimates of 0.1-0.4Gyr in this scenario are in closer agree ment with those for phase-wrapped shells. Our results argue for a gas-rich accretion;nn or merger. origin fnr the she lls and kinematic subcomponent in NGC 2865. Arguments based on stellar popu lations and gas dynamics suggest that one of the progenitors is probably an Sb or Sc spiral. We demonstrate that despite the age and metallicity degen eracy of the underlying elliptical, the age and metallicity of the kinemati c subcomponent can be constrained. This work strengthens the link between k inematically distinct cores (KDCs) and shells, and demonstrates that a KDC can be formed from a late merger.