Percolation Galaxy Groups and Clusters in the SDSS Redshift Survey: Identification, Catalogs, and the Multiplicity Function

Abstract

We identify galaxy groups and clusters in volume-limited samples of the Sloan Digital Sky Survey (SDSS) redshift survey, using a redshift-space friends-of-friends algorithm. We optimize the friends-of-friends linking lengths to recover galaxy systems that occupy the same dark matter halos, using a set of mock catalogs created by populating halos of N-body simulations with galaxies. Extensive tests with these mock catalogs show that no combination of perpendicular and line-of-sight linking lengths is able to yield groups and clusters that simultaneously recover the true halo multiplicity function, projected size distribution, and velocity dispersion. We adopt a linking length combination that yields, for galaxy groups with 10 or more members: a group multiplicity function that is unbiased with respect to the true halo multiplicity function; an unbiased median relation between the multiplicities of groups and their associated halos; a spurious group fraction of less than $1%; a halo completeness of more than $97%; the correct projected size distribution as a function of multiplicity; and a velocity dispersion distribution that is $20% too low at all multiplicities. These results hold over a range of mock catalogs that use different input recipes of populating halos with galaxies. We apply our group-finding algorithm to the SDSS data and obtain three group and cluster catalogs for three volume-limited samples that cover 3495.1 deg 2 on the sky, go out to redshifts of 0.1, 0.068, and 0.045, and contain 57,138, 37,820, and 18,895 galaxies, respectively. We correct for incompleteness caused by fiber collisions and survey edges and obtain measurements of the group multiplicity function, with errors calculated from realistic mock catalogs. These multiplicity function measurements provide a key constraint on the relation between galaxy populations and dark matter halos.

Flux- and volume-limited groups/clusters for the SDSS galaxies: catalogues and mass estimation

“…The value b com is given in units of the mean pairwise separation of galaxies in the survey. Eke et al (2004) is redshift dependent (because of the constant LL in physical units) and is slightly lower (up to 0.12) than that used by Eke et al (2004) and Berlind et al (2006). The ratio of the radial-totransversal LLs in our catalogues is slightly lower than in Eke et al (2004) and higher than in Berlind et al (2006).…”

“…Eke et al (2004) is redshift dependent (because of the constant LL in physical units) and is slightly lower (up to 0.12) than that used by Eke et al (2004) and Berlind et al (2006). The ratio of the radial-totransversal LLs in our catalogues is slightly lower than in Eke et al (2004) and higher than in Berlind et al (2006). Compared with our previous volume-limited catalogues based on SDSS DR7 (Tago et al 2010), the LL values used in this paper are higher by up to 50%, depending on the volume-limited sample and distance.…”

“…In several papers (e.g. Eke et al 2004;Berlind et al 2006;Yang et al 2007;Robotham et al 2011) the mock catalogues were used to find the "best" values for FoF parameters. However, no universally good value exists.…”

“…For example, Berlind et al (2006) used the value b com = 0.14 and the ratio of the radial-to-transversal LL was taken to be 5.4. The value b com is given in units of the mean pairwise separation of galaxies in the survey.…”

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.…”