Distinct core and halo stellar populations and the formation history of the bright Coma cluster early-type galaxy NGC 4889

TL;DR

This study analyzes the stellar populations of NGC 4889 in the Coma cluster, revealing distinct formation histories for its core and halo, and providing insights into galaxy size evolution.

Contribution

It presents the first detailed radial profiles of stellar populations in NGC 4889's halo, highlighting different formation processes for core and halo regions.

Findings

Inner regions show steep metallicity gradient and high alpha/Fe ratios.

Halo has near-solar metallicity with a shallow gradient and decreasing alpha/Fe.

Evidence supports early rapid formation for the core and later accretion for the halo.

Abstract

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 8.2m telescope. We fit single stellar population models to the measured line-strength (Lick) indices (Hbeta, Mgb, [MgFe]' and <Fe>). Combining with literature data, we construct radial profiles of metallicity, [alpha/Fe] element abundance ratio and age for NGC 4889, from the center out to ~60 kpc (~4Re). We find evidence for different chemical and star formation histories for stars inside and outside 1.2Re = 18 kpc radius. The inner regions are characterized by a steep [Z/H] gradient and high [alpha/Fe] at ~2.5 times solar value. In the halo, between 18 and 60 kpc, the [Z/H] is near-solar with a shallow gradient, while [alpha/Fe] shows a strong negative gradient, reaching solar values at ~60 kpc. 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 ~6 kpc, 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.