1995A&A...298..405S

The Virgo elliptical galaxy NGC 4365 harbours a kinematically decoupled, counter-rotating core region. Its line-of-sight integrated velocity distributions are studied along 3 spectroscopic axes using Gauss-Hermite moments and double-gaussian line models. Stellar populations are investigated through high-resolution (B-R) colour photometry and spectroscopic absorbtion line indices Mg₂ , Hβ and <Fe> , which are compared to population models of Worthey (1992, PhD thesis, University of California, Santa Cruz). We find consistent evidences for a distinct core population C of stars with disk or bar-like structure, high rotation velocity and low kinematic temperature (v/σ=1.3), flattened density distribution (ε_C=0.39±0.07 versus ε_spheroid=0.21 ...0.245) and a mass content of at least (8.1±2.5) .10 9M_☉. The metallicity of this core population shows an excessively flattened distribution (relative to the isophotes) and reaches values of at least 2.3Z_☉ at a population age of about ≤(7 ±1.5)Gyr. We favour a formation scenario with gas included in early hierarchical or late major mergers, where the gas is subject to strong dissipation effects and chemical enrichment and produces a violent starburst of order 10 2M_☉/yr. Our line indices show the well-known enhanced [Mg/Fe] of massive Es, but furthermore excessive enhancement of magnesium relative to iron in the innermost core region. This is interpreted as confirming evidence for a violent gaseous merger which probably over-produced SNii versus SNia due to a top-heavy IMF or because of its burst-like star formation history.