Pixel Color-Magnitude Diagram Analysis of the Brightest Cluster Galaxies
  in Dynamically Young and Old Clusters, Abell 1139 and Abell 2589
  As a case study to understand the coevolution of Brightest Cluster Galaxies
(BCGs) and their host clusters, we investigate the BCGs in dynamically young
and old clusters, Abell 1139 (A1139) and Abell 2589 (A2589). We analyze the
pixel color-magnitude diagrams (pCMDs) using deep g- and r-band images,
obtained from the Canada-France-Hawaii Telescope observations. After masking
foreground/background objects and smoothing pixels in consideration of the
observational seeing size, detailed pCMD features are compared between the two
BCGs. (1) While the overall shapes of the pCMDs are similar to those of typical
early-type galaxies, the A2589-BCG tends to have redder mean pixel color and
smaller pixel color deviation at given surface brightness than the A1139-BCG.
(2) The mean pixel color distribution as a function of pixel surface brightness
(pCMD backbone) indicates that the A2589-BCG formed a larger central body (~
2.0 kpc in radius) by major dry mergers at an early epoch than the A1139-BCG (a
central body ~ 1.3 kpc in radius), while they have grown commonly by subsequent
minor mergers. (3) The spatial distributions of the pCMD outliers reveal that
the A1139-BCG experienced considerable tidal events more recently than the
A2589-BCG, whereas the A2589-BCG has an asymmetric compact core possibly
resulting from major dry merger at an early epoch. (4) The A2589-BCG shows a
very large faint-to-bright pixel number ratio compared to early-type non-BCGs,
whereas the ratio for the A1139-BCG is not distinctively large. These results
are consistent with the idea that the BCG in the dynamically older cluster
(A2589) formed earlier and is relaxed better.
