Fraction Of Brightest Cluster Galaxies Research Articles

The regulation of star formation in cool-core clusters: imprints on the stellar populations of brightest cluster galaxies

A fraction of brightest cluster galaxies (BCGs) shows bright emission in the UV and the blue part of the optical spectrum, which has been interpreted as evidence of recent star formation. Most of these results are based on the analysis of broadband photometric data. Here, we study the optical spectra of a sample of 19 BCGs hosted by X-ray luminous galaxy clusters at 0.15 < z < 0.3, a subset from the Canadian Cluster Comparison Project (CCCP) sample. We identify plausible star formation histories of the galaxies by fitting Simple Stellar Populations (SSPs) as well as composite populations, consisting of a young stellar component superimposed on an intermediate/old stellar component, to accurately constrain their star formation histories. We detect prominent young (~200 Myr) stellar populations in 4 of the 19 galaxies. Of the four, the BCG in Abell 1835 shows remarkable A-type stellar features indicating a relatively large population of young stars, which is extremely unusual even amongst star forming BCGs. We constrain the mass contribution of these young components to the total stellar mass to be typically between 1% to 3%, but rising to 7% in Abell 1835. We find that the four of the BCGs with strong evidence for recent star formation (and only these four galaxies) are found within a projected distance of 5 kpc of their host cluster's X-ray peak, and the diffuse, X-ray gas surrounding the BCGs exhibit a ratio of the radiative cooling-to-free-fall time ($t_{c}/t_{ff}$) of < 10. These are also some of the clusters with the lowest central entropy. Our results are consistent with the predictions of the precipitation-driven star formation and AGN feedback model, in which the radiatively cooling diffuse gas is subject to local thermal instabilities once the instability parameter $t_{c}/t_{ff}$ falls below ~10, leading to the condensation and precipitation of cold gas. [Abridged]

Major dry mergers in early-type brightest cluster galaxies

We search for ongoing major dry-mergers in a well selected sample of local Brightest Cluster Galaxies (BCGs) from the C4 cluster catalogue. 18 out of 515 early-type BCGs with redshift between 0.03 and 0.12 are found to be in major dry-mergers, which are selected as pairs (or triples) with $r$-band magnitude difference $\dm<1.5$ and projected separation $\rp<30$ kpc, and showing signatures of interaction in the form of significant asymmetry in residual images. We find that the fraction of BCGs in major dry-mergers increases with the richness of the clusters, consistent with the fact that richer clusters usually have more massive (or luminous) BCGs. We estimate that present-day early-type BCGs may have experienced on average $\sim 0.6 (\tmerge/0.3\Gyr)^{-1}$ major dry-mergers and through this process increases their luminosity (mass) by $15% (\tmerge/0.3\Gyr)^{-1} (\fmass/0.5)$ on average since $z=0.7$, where $\tmerge$ is the merging timescale and $\fmass$ is the mean mass fraction of companion galaxies added to the central ones. We also find that major dry-mergers do not seem to elevate radio activities in BCGs. Our study shows that major dry-mergers involving BCGs in clusters of galaxies are not rare in the local Universe, and they are an important channel for the formation and evolution of BCGs.

The stellar kinematics of the giant halos of Brightest Cluster Galaxies

We have investigated the stellar kinematics of two brightest cluster galaxies (BCGs) to large distances from the cluster centre. In both galaxies, the velocity dispersion rises with radius. Orbital anisotropy is ruled out as the cause and we conclude that the kinematics in the outer parts of BCGs are dark matter dominated. This result contrasts with the current problem for intermediate luminosity ellipticals, which appear to have less dark matter than predicted by CDM. Combining the present results with our previous work, the fraction of BCGs with outwardly rising dispersions () from the inner parts of BCGs. We argue that this phenomenon may be more common than previously thought. The agreement with radial trends expected for a smooth galaxy/cluster transition, and the lack of dependence on cD/BCG dominance and cluster environment, suggest that the outwardly rising dispersion is a genuine response of the stars to the underlying cluster potential. There is evidence that BCG in Abell 2063 is not at the centre of the cluster potential.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html