NoSOCS in SDSS. I. Sample Definition and Comparison of Mass Estimates

TL;DR

This study utilizes SDSS data to refine galaxy cluster properties, compare different mass estimation methods, and analyze the impact of substructure on mass measurements, providing insights into cluster dynamics and measurement biases.

Contribution

It introduces a comprehensive comparison of virial, caustic, and X-ray mass estimates for SDSS-detected clusters, highlighting the effects of substructure and survey completeness.

Findings

Virial and X-ray masses agree well for relaxed clusters.

Substructure presence correlates with higher virial masses.

Caustic masses are less affected by substructure than virial masses.

Abstract

We use Sloan Digital Sky Survey (SDSS) data to investigate galaxy cluster properties of systems first detected within DPOSS. With the high quality photometry of SDSS we derived new photometric redshifts and estimated richness and optical luminosity. For a subset of low redshift ($z \le 0.1$) 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 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 RASS. For a smaller subset (twenty-one 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.