On the efficiency and reliability of cluster mass estimates based on member galaxies

Aims. We study the efficiency and reliability of cluster mass estimators that are based on the projected phase-space distribution of galaxies in a cluster region. Methods. We analyse a data-set of 62 clusters extracted from a concordance Lambda CDM cosmological hydrodynamical simulation. We consider both dark matter (DM) particles and simulated galaxies as tracers of the clusters gravitational potential. Two cluster mass estimators are considered: the virial mass estimator, corrected for the surface-pressure term, and a mass estimator (that we call M-sigma) based entirely on the velocity dispersion estimate of the cluster. In order to simulate observations, galaxies (or DM particles) are first selected in cylinders of given radius (from 0.5 to 1.5h(-1) Mpc) and similar or equal to 200h(-1) Mpc length. Cluster members are then identified by applying a suitable interloper removal algorithm. Results. The virial mass estimator overestimates the true mass by similar or equal to 10% on average, for sample sizes of greater than or similar to 60 cluster members. For similar sample sizes, M-sigma underestimates the true mass by similar or equal to 15%, on average. For smaller sample sizes, the bias of the virial mass estimator substantially increases, while the M-sigma estimator becomes essentially unbiased. The dispersion of both mass estimates increases by a factor similar to 2 as the number of cluster members decreases from similar to 400 to similar to 20. It is possible to reduce the bias in the virial mass estimates either by removing clusters with significant evidence for subclustering or by selecting early-type galaxies, which substantially reduces the interloper contamination. Early-type galaxies cannot however be used to improve the M-sigma estimates since their intrinsic velocity distribution is slightly biased relative to that of the DM particles. Radially-dependent incompleteness can drastically affect the virial mass estimates, but leaves the M-sigma estimates almost unaffected. Other observational effects, like centering and velocity errors and different observational apertures, have little effect on the mass estimates.