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 ofgalaxies in a cluster region.Methods. We analyse a data-set of 62 clusters extracted from a concordance ΛCDM cosmological hydrodynamical simulation. Weconsider both dark matter (DM) particles and simulated galaxies as tracers of the clusters gravitational potential. Two cluster massestimators are considered: the virial mass estimator, corrected for the surface-pressure term, and a mass estimator (that we call Mσ)based entirely on the velocity dispersion estimate of the cluster. In order to simulate observations, galaxies (or DM particles) arefirst selected in cylinders of given radius (from 0.5 to 1.5h−1 Mpc) and 200h−1 Mpc length. Cluster members are then identified byapplying a suitable interloper removal algorithm.Results. The virial mass estimator overestimates the true mass by 10% on average, for sample sizes of >∼60 cluster members.For similar sample sizes, Mσ underestimates the true mass by 15%, on average. For smaller sample sizes, the bias of the virialmass estimator substantially increases, while the Mσ estimator becomes essentially unbiased. The dispersion of both mass estimatesincreases by a factor ∼2 as the number of cluster members decreases from ∼400 to ∼20.It is possible to reduce the bias in the virial mass estimates either by removing clusters with significant evidence for subclusteringor by selecting early-type galaxies, which substantially reduces the interloper contamination. Early-type galaxies cannot however beused to improve the Mσ 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 Mσ estimates almost unaffected.Other observational effects, like centering and velocity errors and different observational apertures, have little effect on the massestimates.