NoSOCS in SDSS - I. Sample definition and comparison of mass estimates

Abstract

We use Sloan Digital Sky Survey (SDSS) data to investigate galaxy cluster properties of the systems first detected within Digitized Second Palomar Observatory Sky Survey. With the highquality photometry of SDSS, we derived new photometric redshifts and estimated richness and optical luminosity. For a subset of low-redshift (z ≤ 0.1) clusters, we have used SDSS spectroscopic data to identify groups in redshift space in the region of each cluster, complemented with massive systems from the literature to assure the continuous mass sampling. A method to remove interlopers is applied, and a virial analysis is performed resulting in the estimates of velocity dispersion, mass and a physical radius for each low-z system. We discuss the choice of maximum radius and luminosity range in the dynamical analysis, showing that a spectroscopic survey must be complete to at least M * + 1 if one wishes to obtain accurate and unbiased estimates of velocity dispersion and mass. We have measured X-ray luminosity for all clusters using archival data from ROSAT All Sky Survey. For a smaller subset (21 clusters), we selected temperature measures from the literature and estimated mass from the M−T X relation, finding that they show good agreement with the virial estimate. However, these two mass estimates tend to disagree with the caustic results. We measured the presence of substructure in all clusters of the sample and found that clusters with substructure have virial masses higher than those derived from T X . This trend is not seen when comparing the caustic and X-ray masses. That happens because the caustic mass is estimated directly from the mass profile, so it is less affected by substructure.

X-Ray-Selected Galaxy Groups in Boötes

“…Scaling relations for clusters and groups including Xray luminosity vs. mass, X-ray luminosity vs. velocity dispersion and X-ray luminosity vs. temperature, have been investigated extensively (Finoguenov et al 2001;Reiprich & Böhringer 2002;Popesso et al 2006;Rykoff et al 2008;Lopes et al 2009b;Vikhlinin et al 2009;Ettori et al 2010;Leauthaud et al 2010). However the question of whether the relations determined for clusters also hold true for poor clusters and groups still remains unsettled.…”

Galaxy distribution and evolution around a sample of 2dF groups

“…The main difference from the study of Lopes et al (2009) is that here we consider all galaxies within 10 times the virial radius (as listed in Tago et al 2006). In Lopes et al (2009), the interloper removal procedure was applied to galaxies within a maximum radius of 2.5 h −1 Mpc. Next, we estimate the velocity dispersions (σ) and physical radius (R 200 ) of each group.…”

“…Finally, a virial analysis is perfomed for mass estimation (M 200 ). Further details regarding the interloper removal and estimation of global properties (σ, physical radius and mass) are found in Lopes et al (2009).…”

“…To determine the group redshift, only galaxies within 0.50 h −1 Mpc are considered. Details about this procedure are found in Lopes et al (2009). With the new redshift and velocity limits, we apply an algorithm for interloper rejection to define the final list of group members.…”

“…This subset corresponds to those groups located in areas of at least 80% redshift coverage out to 10 times the virial radius roughly estimated from the projected harmonic mean. Group members and interlopers were redefined after the identification of gaps in the redshift distribution according to the technique described by Lopes et al (2009). Before selecting group members and rejecting interlopers we first refine the spectroscopic redshift of each group and identify its velocity limits.…”