Coordinated Assembly of Brightest Cluster Galaxies

Abstract

Brightest Cluster Galaxies (BCGs) in massive dark matter halos are shaped by complex merging processes. We present a detailed stellar population analysis in the central region of Abell 3827 at z ∼ 0.1, including five-nucleus galaxies involved in a BCG assembly. Based on deep spectroscopy from the Multi Unit Spectroscopic Explorer, we fit the optical spectra of 13 early-type galaxies (ETGs) in the central 70 kpc of the cluster. The stellar populations in the central R = 1 kpc of these ETGs are old (>6 Gyr). Their [Fe/H] increases with σ ∗ and stellar mass. More importantly, [α/Fe] of galaxies close to the cluster center do not seem to depend on σ ∗ or stellar mass, indicating that the cluster center shapes the [α/Fe]-σ ∗ and [α/Fe]-M ∗ relations differently than other environments where [α/Fe] is observed to increase with increasing σ ∗ or M ∗. Our results reveal the coordinated assembly of BCGs: their building blocks are different from the general low-mass populations by their high [α/Fe] and old ages. Massive galaxies thus grow by accreting preferentially high [α/Fe] and old stellar systems. The radial profiles also bear the imprint of the coordinated assembly. Their declining [Fe/H] and flat [α/Fe] radial profiles confirm that the accreted systems have low metallicity and high [α/Fe] stellar contents.