2010MNRAS.407L..26C

We study the stellar population far into the halo of one of the two brightest galaxies in the Coma cluster, NGC 4889, based on deep medium-resolution spectroscopy with FOCAS at the Subaru 8.2-m telescope. We fit single stellar population models to the measured line-strength (Lick) indices (Hβ, Mgb, [MgFe]' and <Fe>). Combining with literature data, we construct radial profiles of metallicity, [α/Fe] element abundance ratio and age for NGC 4889, from the centre out to ∼ 60kpc (∼ 4 R_e). We find evidence for different chemical and star formation histories for stars inside and outside 1.2 R_e= 18 kpc radius. The inner regions are characterized by a steep [Z/H] gradient and high [α/Fe] at ∼ 2.5 solar value. In the halo, between 18 and 60kpc, the [Z/H] is near-solar with a shallow gradient, while [α/Fe] shows a strong negative gradient, reaching solar values at 60kpc. We interpret these data in terms of different formation histories for both components. The data for the inner galaxy are consistent with a rapid, quasi-monolithic, dissipative merger origin at early redshifts, followed by one or at most a few dry mergers. Those for the halo argue for later accretion of stars from old systems with more extended star formation histories. The half-light radius of the inner component alone is estimated as ∼ 6kpc, suggesting a significantly smaller size of this galaxy in the past. This may be the local stellar population signature of the size evolution found for early-type galaxies from high-redshift observations.