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 $z\sim0.1$, including five-nucleus galaxies involved in a BCG assembly. Based on deep spectroscopy from Multi Unit Spectroscopic Explorer (MUSE), 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 (6-10 Gyr). Their [Fe/H] increases with $\sigma_{\star}$ and stellar mass. More importantly, [$\alpha$/Fe] of galaxies close to the cluster center do not seem to depend on $\sigma_{\star}$ or stellar mass, indicating that the cluster center shapes the [$\alpha$/Fe]-$\sigma_{\star}$ and [$\alpha$/Fe]-$M_{\star}$ relations differently than other environments where [$\alpha$/Fe] is observed to increase with increasing $\sigma_{\star}$ or $M_{\star}$. Our results reveal the coordinated assembly of BCGs: their building blocks are different from the general low mass populations by their high [$\alpha$/Fe]. Massive galaxies thus grow by accreting preferentially high [$\alpha$/Fe] systems. The radial profiles also bear the imprint of the coordinated assembly. Their declining [Fe/H] and flat [$\alpha$/Fe] radial profiles confirm that the accreted systems have low metallicity and high [$\alpha$/Fe] stellar contents.