Distinct core and halo stellar population in the bright coma cluster galaxy NGC 4889

Coccato, L., Gerhard, O., Arnaboldi, M. 2010,  MNRAS,  407,  L26 (to the paper)

We construct radial profiles of line strength indices along the major axis of NGC 4889 by combining literature data for the central regions and new deep spectroscopic data for the halo regions. We then derive age, metallicity and alpha-enhancement radial profiles and their gradients using Single Stellar Population models by Thomas et al. (2003).   This represents the most spatially extended dataset with both stellar kinematics and line strength indices for a brightest cluster galaxy.
We observe a different population content and gradient between the central regions of the galaxy (R<18 kpc) and the outer halo (R>18 kpc). The inner ~18 kpc (~1.2 Re) of NGC 4889 are characterized by a strong [Z/H] gradient and a nearly constant values of [alpha/Fe]. The outer regions (18 kpc < R < 60 kpc) are characterized by a constant metallicity content strong negative gradient in the abundance ratio and older ages.
These data indicate that the central parts of NGC 4889 and its halo have undergone different formation mechanisms. Data in the center indicate a short star formation timescale, where the stars formed outside-in, reminiscent of a quasi-monolithic dissipative collapse. On the contrary, the data in the halo suggest that it was accreted from shredded satellite galaxies, as suggested also by numerical simulations, over the central galaxy that was already in place.
Our measurements are also consistent with recent results on the size evolution of bright ETGs with redshift, i.e. at high redshifts ETGs are smaller and more compact than ETGs of similar mass at z = 0. Their effective radius evolves as Re ~ (1+z)-1.3 (van Dokkum et al. 2010). Scaling the present Re of NGC 4889 with this relation would predict Re = 6.2 kpc, at z = 1, which is consistent with the half light radius measured if considering the central regions of the galaxy only, on the assumption that outer regions of NGC 4889 were accreted later, at z < 1. Our finding for NGC 4889 suggests that we may have found local stellar population signatures of the observed ETG size evolution.

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