We report a systematic multi-wavelength investigation of environments of the brightest cluster galaxies (BCGs), using the X-ray data from the Chandra archive, and optical images taken with 34x 27 field-of-view Subaru Suprime-Cam. Our goal is to help
understand the relationship between the BCGs and their host clusters, and between the BCGs and other galaxies, to eventually address a question of the formation and co-evolution of BCGs and the clusters. Our results include: 1) Morphological variety of BCGs, or the second or the third brightest galaxy (BCG2, BCG3), is comparable to that of other bright red sequence galaxies, suggesting that we have a continuous variation of morphology between BCGs, BCG2, and BCG3, rather than a sharp separation between the BCG and the rest of the bright galaxies. 2) The offset of the BCG position relative to the cluster centre is correlated to the degree of concentration of cluster X-ray morphology (Spearman rho = -0.79), consistent with an interpretation that BCGs tend to be off-centered inside dynamically unsettled clusters. 3) Morphologically disturbed clusters tend to harbour the brighter BCGs, implying that the early collapse may not be the only major mechanism to control the BCG formation and evolution.
We have discovered an X-ray selected galaxy cluster with a spectroscopic redshift of 1.753. The redshift is of the brightest cluster galaxy (BCG), which is coincident with the peak of the X-ray surface brightness. We also have concordant photometric
redshifts for seven additional candidate cluster members. The X-ray luminosity of the cluster is 3.68 +/- 0.70 x 10^43 erg s^-1 in the 0.1 - 2.4 keV band. The optical/IR properties of the BCG imply its formation redshift was ~5 if its stars formed in a short burst. This result continues the trend from lower redshift in which the observed properties of BCGs are most simply explained by a monolithic collapse at very high redshift instead of the theoretically preferred gradual hierarchical assembly at later times. However the models corresponding to different formation redshifts are more clearly separated as our observation epoch approaches the galaxy formation epoch. Although our infrared photometry is not deep enough to define a red sequence, we do identify a few galaxies at the cluster redshift that have the expected red sequence photometric properties.
The number density of galaxy clusters provides tight statistical constraints on the matter fluctuation power spectrum normalization, traditionally phrased in terms of sigma_8, the root mean square mass fluctuation in spheres with radius 8 h^-1 Mpc. W
e present constraints on sigma_8 and the total matter density Omega_m0 from local cluster counts as a function of X-ray temperature, taking care to incorporate and minimize systematic errors that plagued previous work with this method. In particular, we present new determinations of the cluster luminosity - temperature and mass - temperature relations, including their intrinsic scatter, and a determination of the Jenkins mass function parameters for the same mass definition as the mass - temperature calibration. Marginalizing over the 12 uninteresting parameters associated with this method, we find that the local cluster temperature function implies sigma_8 (Omega_m0/0.32)^alpha = 0.86+/-0.04 with alpha = 0.30 (0.41) for Omega_m0 < 0.32 (Omega_mo > 0.32) (68% confidence for two parameters). This result agrees with a wide range of recent independent determinations, and we find no evidence of any additional sources of systematic error for the X-ray cluster temperature function determination of the matter power spectrum normalization. The joint WMAP5 + cluster constraints are: Omega_m0 = 0.30+0.03/-0.02 and sigma_8 = 0.85+0.04/-0.02 (68% confidence for two parameters).
We investigated the influence of environment on cluster galaxies by examining the alignment of the brightest cluster galaxy (BCG) position angle with respect to the host cluster X-ray position angle. The cluster position angles were measured using hi
gh spatial resolution X-ray data taken from the Chandra ACIS archive, that significantly improved the determination of the cluster shape compared to the conventional method of using optical images. Meanwhile, those of the BCGs were measured using homogeneous dataset composed of high spatial resolution optical images taken with Suprime-Cam mounted on Subaru 8m telescope. We found a strong indication of an alignment between the cluster X-ray emission and optical light from BCGs, while we see no clear direct correlation between the degree of ellipticity of X-ray and optical BCG morphologies, despite the apparent alignment of two elliptical structures. We have also investigated possible dependence of the position angle alignment on the X-ray morphology of the clusters, and no clear trends are found. The fact that no trends are evident regarding frequency or degree of the alignment with respect to X-ray morphology may be consistent with an interpretation as a lack of dependence on the dynamical status of clusters.
