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Register a new userMulticolor surface photometry of brightest cluster galaxies: II. Enlarging the sample and improving the analysis
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Intrinsic color profiles of a sample of nine Brightest Cluster Galaxies (BCGs) are recovered from the observed color profiles by evaluating spurious gradients introduced by errors in the determination of the sky levels and by different seeing conditions between the observations. Isophote shapes and surface brightness profiles are presented for the four newly observed BCGs. Three out of nine BCGs show color gradients of the order of 0.10 mag per decade in radius. Five BCGs do not possess any color gradient larger than 0.01 mag per decade in radius. We do not see any correlation between the presence (or the sign) of a color gradient and the BCG morphology (slope and shape of its surface brightness profile), or the cluster richness. This argues against a strong and recent influence of the environment on the BCG morphology. The slope of the BCG surface brightness profile is correlated to the cluster richness, posing a constraint on the initial conditions, or on the physical mechanism that is responsible for the present BCG morphology. Finally, only one BCG can be a cD despite visual classification as such of all studied BCGs.
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We study the dwarf galaxy population in the central ~700 arcmin^2 of the Coma cluster, the majority of which are early-type dwarf elliptical (dE) galaxies. Analysis of the statistically-decontaminated dE galaxy sequence in the color-magnitude diagram reveals a highly significant trend of color with magnitude (Delta (B-R)/Delta R = -0.056pm0.002 mag), in the sense that fainter dEs are bluer and thus presumably more metal-poor. The mean color of the faintest dEs in our sample is (B-R)~1.15 mag, consistent with a color measurement of the diffuse intracluster light in the Coma core. This intracluster light could then have originated from the tidal disruption of faint dEs in the cluster core. The total galaxy luminosity function (LF) is well modeled as the sum of a log-normal distribution for the giant galaxies, and a Schechter function for the dE galaxies with a faint-end slope alpha = -1.41pm0.05. This value of alpha is consistent with those measured for the Virgo and Fornax clusters. The spatial distribution of the faint dE galaxies (19.0 < R le 22.5 mag) has R_c = 22.15 arcmin (~0.46h^{-1} Mpc), significantly larger than the R_c = 13.71 arcmin (~0.29h^{-1} Mpc) found for the cluster giants and the brighter dEs (R le 19.0 mag), consistent with tidal disruption of the faint dEs. Finally, we find that most dEs belong to the general Coma cluster potential rather than as satellites of individual giant galaxies: An analysis of the number counts around 10 cluster giants reveals that they each have on average 4pm 1 dE companions within a projected radius of 13.9h^{-1} kpc. (Abridged)
We have carried out surface photometry and an isophotal analysis for a sample of 25 early-type dwarf (dE and dS0) galaxies in the Virgo cluster based on CCD images taken at the VLT with FORS1 and FORS2. For each galaxy we present $B$ and $R$-band surface brightness profiles, as well as the radial colour ($B-R$) profile. We give total apparent $BR$ magnitudes, effective radii, effective surface brightnesses and total colour indices. The light profiles have been fitted with Sersic models and the corresponding parameters are compared to the ones for other classes of objects. The observed profiles of the brightest cluster dwarfs show significant deviations from a simple Sersic model, indicating that there is more inner structure than just a nucleus. In addition, we find a relation between the effective surface brightness, at a given luminosity, and the strength of the offset of the galaxys nucleus with respect to the center of the isophotes. Dwarfs with large nuclear offsets also tend to have stronger isophotal twists. In sum, our findings suggest the presence of substructure in most, and preferentially in the less compact, bright early-type dwarfs. The physical (dynamical) meaning of this has yet to be explored. (abridged)
The dust destruction timescales in the cores of clusters of galaxies are relatively short given their high central gas densities. However, substantial mid-infrared and sub-mm emission has been detected in many brightest cluster galaxies. In this letter we present Herschel PACS and SPIRE photometry of the brightest cluster galaxy in three strong cooling flow clusters, A1068, A2597 and Zw3146. This photometry indicates that a substantial mass of cold dust is present (>3 x 10^7 Mo) at temperatures significantly lower (20-28K) than previously thought based on limited MIR and/or sub-mm results. The mass and temperature of the dust appear to match those of the cold gas traced by CO with a gas-to-dust ratio of 80-120.
An optical photometric observation with the Beijing-Arizona-Taiwan-Connecticut (BATC) multicolor system is carried out for A98 (z=0.104), a galaxy cluster with two large enhancements in X-ray surface brightness. The spectral energy distributions (SEDs) covering 15 intermediate bands are obtained for all sources detected down to V ~ 20 mag in a field of $58 times 58$. After the star-galaxy separation by the color-color diagrams, a photometric redshift technique is applied to the galaxy sample for further membership determination. The color-magnitude relation is taken as a further restriction of the early-type cluster galaxies. As a result, a list of 198 faint member galaxies is achieved. Based on newly generated sample of member galaxies, the dynamical substructures, A98N, A98S, and A98W, are investigated in detail. A separate galaxy group, A98X, is also found to the south of main concentration of A98, which is gravitationally unbound to A98. For 74 spectroscopically confirmed member galaxies, the environmental effect on the star formation histories is found. The bright galaxies in the core region are found to have shorter time scales of star formation, longer mean stellar ages, and higher metallicities of interstellar medium, which can be interpreted in the context of hierarchical cosmological scenario.
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).
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