Outer halo of bright ellipticals: stellar population

NGC 4489 in the Coma cluster

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), see Figure 1. 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 [α/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.

Two steps formation process. 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 as already in place.

Size evolution. Our measurements are also consistent with recent results on the size evolution of bright Early Type Galaxies (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.

Figure 1. Left panel, radial profile of Lick line strength indices in NGC 4889; right panel, single stellar population model from indices in left panel. Open symbols: combined datased from Corsini et al. 2008, Mehlert et al. 2000 and corresponding models. Filled circles: measurements from Coccato et al. 2010 and corresponding models. Stellar population gradients are shown. Yellow region represent the allowed range of values considering uncertainties in the sky subtraction.

Related publications:

Coccato, L., Arnaboldi, M., Gerhard, O., K. C. Freeman, G. Ventimiglia, & N. Yasuda 2010, A&A, 519, 95 Kinematics and line strength indices in the halos of the COMA Brightest Cluster Galaxies NGC 4874 and NGC 4889.

Coccato, L., Gerhard, O. & Arnaboldi, M., 2010, MNRAS, 407, L26. Distinct core and halo stellar populations and the formation history of the bright Coma cluster early-type galaxy NGC 4889.