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Register a new userDynamics and Shape of Brightest Cluster Galaxies
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H. Andernach
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We identified Brightest Cluster Members (BCM) on DSS images of 1083 Abell clusters, derived their individual and host cluster redshifts from literature and determined the BCM ellipticity. Half the BCMs move at a speed higher than 37 % of the cluster velocity dispersion sigma_{cl}, suggesting that most BCMs are part of substructures falling into the main cluster. Both, the BCMs velocity offset in units of sigma_{cl}, and BCM ellipticity, weakly decrease with cluster richness.
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The K-band Hubble diagram of Brightest Cluster Galaxies (BCGs) is presented for a large, X-ray selected cluster sample extending out to z = 0.8. The controversy over the degree of BCG evolution is shown to be due to sample selection, since the BCG luminosity depends upon the cluster environment. Selecting only the most X-ray luminous clusters produces a BCG sample which shows, under the assumption of an Einstein-de Sitter cosmology, significantly less mass growth than that predicted by current semi-analytic galaxy formation models, and significant evidence of any growth only if the dominant stellar population of the BCGs formed relatively recently (z <= 2.6).
Observations of 170 local ($zlesssim0.08$) galaxy clusters in the northern hemisphere have been obtained with the Wendelstein Telescope Wide Field Imager (WWFI). We correct for systematic effects such as point-spread function broadening, foreground star contamination, relative bias offsets, and charge persistence. Background inhomogeneities induced by scattered light are reduced down to $Delta {rm SB} > 31~g$ mag arcsec$^{-2}$ by large dithering and subtraction of night-sky flats. Residual background inhomogeneities brighter than ${rm SB}_{sigma}< 27.6~g$ mag arcsec$^{-2}$ caused by galactic cirrus are detected in front of 23% of the clusters. However, the large field of view allows discrimination between accretion signatures and galactic cirrus. We detect accretion signatures in the form of tidal streams in 22%, shells in 9.4%, and multiple nuclei in 47% of the Brightest Cluster Galaxies (BCGs) and find two BCGs in 7% of the clusters. We measure semimajor-axis surface brightness profiles of the BCGs and their surrounding Intracluster Light (ICL) down to a limiting surface brightness of ${rm SB} = 30~g$ mag arcsec$^{-2}$. The spatial resolution in the inner regions is increased by combining the WWFI light profiles with those that we measured from archival textit{Hubble Space Telescope} images or deconvolved WWFI images. We find that 71% of the BCG+ICL systems have surface brightness (SB) profiles that are well described by a single Sersic (SS) function, whereas 29% require a double Sersic (DS) function to obtain a good fit. We find that BCGs have scaling relations that differ markedly from those of normal ellipticals, likely due to their indistinguishable embedding in the ICL.
We present kinematic parameters and absorption line strengths for three brightest cluster galaxies, NGC 6166, NGC 6173 and NGC 6086. We find that NGC 6166 has a velocity dispersion profile which rises beyond 20 arcsec from the nucleus, with a halo velocity dispersion in excess of 400 km/s. All three galaxies show a positive and constant h4 Hermite moment. The rising velocity dispersion profile in NGC 6166 thus indicates an increasing mass-to-light ratio. Rotation is low in all three galaxies, and NGC 6173 and NGC 6086 show possible kinematically decoupled cores. All three galaxies have Mg2 gradients similar to those found in normal bright ellipticals, which are not steep enough to support simple dissipative collapse models, but these could be accompanied by dissipationless mergers which would tend to dilute the abundance gradients. The [Mg/Fe] ratios in NGC 6166 and NGC 6086 are higher than that in NGC 6173, and if NGC 6173 is typical of normal bright ellipticals, this suggests that cDs cannot form from late mergers of normal galaxies.
We study the luminosity gap, dm12, between the first and second ranked galaxies in a sample of 59 massive galaxy clusters, using data from the Hale Telescope, HST, Chandra, and Spitzer. We find that the dm12 distribution, p(dm12), is a declining function of dm12, to which we fitted a straight line: p(dm12) propto -(0.13+/-0.02)dm12. The fraction of clusters with large luminosity gaps is p(dm12>=1)=0.37+/-0.08, which represents a 3sigma excess over that obtained from Monte Carlo simulations of a Schechter function that matches the mean cluster galaxy luminosity function. We also identify four clusters with extreme luminosity gaps, dm12>=2, giving a fraction of p(dm12>=2)=0.07+0.05-0.03. More generally, large luminosity gap clusters are relatively homogeneous, with elliptical/disky brightest cluster galaxies (BCGs), cuspy gas density profiles (i.e. strong cool cores), high concentrations, and low substructure fractions. In contrast, small luminosity gap clusters are heterogeneous, spanning the full range of boxy/elliptical/disky BCG morphologies, the full range of cool core strengths and dark matter concentrations, and have large substructure fractions. Taken together, these results imply that the amplitude of the luminosity gap is a function of both the formation epoch, and the recent infall history of the cluster. BCG dominance is therefore a phase that a cluster may evolve through, and is not an evolutionary cul-de-sac. We also compare our results with semi-analytic model predictions based on the Millennium Simulation. None of the models are able to reproduce all of the observational results, underlining the inability of current models to match the empirical properties of BCGs. We identify the strength of AGN feedback and the efficiency with which cluster galaxies are replenished after they merge with the BCG in each model as possible causes of these discrepancies. [Abridged]
We present new VLBI observations of Brightest Cluster Galaxies in eight nearby Abell clusters. These data show a possible difference between Brightest Cluster Galaxies in cool core clusters (two-sided pc scale jets) and in non cool core clusters (one-sided pc scale jets). We suggest that this difference could be due to the jet interaction with the surrounding medium. More data are necessary to discuss if pc-scale properties of Brightest Cluster Galaxies are influenced by their peculiar morphology and position in the center of rich clusters of galaxies.
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