The C4 Clustering Algorithm: Clusters of Galaxies in the Sloan Digital Sky Survey

Abstract

We present the C4 Cluster Catalog, a new sample of 748 clusters of galaxies identified in the spectroscopic sample of the Second Data Release (DR2) of the Sloan Digital Sky Survey (SDSS). The C4 cluster-finding algorithm identifies clusters as overdensities in a seven-dimensional position and color space, thus minimizing projection effects that have plagued previous optical cluster selection. The present C4 catalog covers $2600 deg 2 of sky and ranges in redshift from z ¼ 0:02 to 0.17. The mean cluster membership is 36 galaxies (with measured redshifts) brighter than r ¼ 17:7, but the catalog includes a range of systems, from groups containing 10 members to massive clusters with over 200 cluster members with measured redshifts. The catalog provides a large number of measured cluster properties including sky location, mean redshift, galaxy membership, summed r-band optical luminosity (L r ), and velocity dispersion, as well as quantitative measures of substructure and the surrounding large-scale environment. We use new, multicolor mock SDSS galaxy catalogs, empirically constructed from the ÃCDM Hubble Volume (HV) Sky Survey output, to investigate the sensitivity of the C4 catalog to the various algorithm parameters (detection threshold, choice of passbands, and search aperture), as well as to quantify the purity and completeness of the C4 cluster catalog. These mock catalogs indicate that the C4 catalog is '90% complete and 95% pure above M 200 ¼ 1 ; 10 14 h À1 M and within 0:03 z 0:12. Using the SDSS DR2 data, we show that the C4 algorithm finds 98% of X-ray-identified clusters and 90% of Abell clusters within 0:03 z 0:12. Using the mock galaxy catalogs and the full HV dark matter simulations, we show that the L r of a cluster is a more robust estimator of the halo mass (M 200 ) than the galaxy line-of-sight velocity dispersion or the richness of the cluster. However, if we exclude clusters embedded in complex large-scale environments, we find that the velocity dispersion of the remaining clusters is as good an estimator of M 200 as L r . The final C4 catalog will contain '2500 clusters using the full SDSS data set and will represent one of the largest and most homogeneous samples of local clusters.

Galaxy And Mass Assembly (GAMA): the environments of high- and low-excitation radio galaxies

“…The difference is a factor of ten for galaxies with stellar mass below 10 11 M ⊙ , but less than a factor of two for stellar masses of ∼ 5×10 11 M ⊙ . The Best et al analysis used groups with a median velocity dispersion of σgroup ≃ 400 km s −1 at z < 0.1 (Miller et al 2005;von der Linden et al 2007). By comparison the groups in our sample have a median dispersion of σgroup = 170 km s −1 .…”

Observational probes of cosmic acceleration

“…The former benefit from much finer spatial resolution along the line of sight. They tend to be shallow, with typical z 0.2 (Merchán and Zandivarez, 2002;Kochanek et al, 2003;Miller et al, 2005;Merchán and Zandivarez, 2005;Berlind et al, 2006;Yang et al, 2007;Li and Yee, 2008;Blackburne and Kochanek, 2012), though high redshift spectroscopic catalogs do exist (Gerke et al, 2005;Coil et al, 2006). Photometric cluster catalogs hail back as far as the original Abell (1958) catalog, which contained upwards of 2500 systems and served as the primary basis of cluster studies for decades.…”

The abundance and colours of galaxies in high-redshift clusters in the cold dark matter cosmology

“…Baldry et al 2004;Bell et al 2004;Brinchmann et al 2004), with many studies supporting a high formation redshift of the stellar population of z f 2 (e.g. Ellis et al 1997;Smail et al 1998;Stanford et al 1998;Ponman, Cannon & Navarro 1999;López-Cruz, Barkhouse & Yee 2004;Gladders & Yee 2005;Miller et al 2005;Voit 2005;Mei et al 2006aMei et al ,b, 2009Mei et al , 2012Koester et al 2007;Gilbank et al 2008Gilbank et al , 2011Lidman et al 2008;Wilson et al 2009;Lin et al 2012). There is, however, some observational evidence for ongoing star formation in clusters at z 1, (e.g.…”