Coordinated Assembly of Brightest Cluster Galaxies
  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.
