Identification of Members in the Central and Outer Regions of Galaxy Clusters

Abstract

The caustic technique measures the mass of galaxy clusters in both their virial and infall regions and, as a byproduct, yields the list of cluster galaxy members. Here we use 100 galaxy clusters with mass M 200 ≥ 10 14 h −1 M ⊙ extracted from a cosmological N -body simulation of a ΛCDM universe to test the ability of the caustic technique to identify the cluster galaxy members. We identify the true threedimensional members as the gravitationally bound galaxies. The caustic technique uses the caustic location in the redshift diagram to separate the cluster members from the interlopers. We apply the technique to mock catalogs containing 1000 galaxies in the field of view of 12h −1 Mpc on a side at the cluster location. On average, this sample size roughly corresponds to 180 real galaxy members within 3r 200 , similar to recent redshift surveys of cluster regions. The caustic technique yields a completeness, the fraction of identified true members, f c = 0.95 ± 0.03 within 3r 200 . The contamination, the fraction of interlopers in the observed catalog of members, increases from f i = 0.020 +0.046 −0.015 at r 200 to f i = 0.08 +0.11 −0.05 at 3r 200 . No other technique for the identification of the members of a galaxy cluster provides such large completeness and small contamination at these large radii. The caustic technique assumes spherical symmetry and the asphericity of the cluster is responsible for most of the spread of the completeness and the contamination. By applying the technique to an approximately spherical system obtained by stacking the individual clusters, the spreads decrease by at least a factor of two. We finally estimate the cluster mass within 3r 200 after removing the interlopers: for individual clusters, the mass estimated with the virial theorem is unbiased and within 30% of the actual mass; this spread decreases to less than 10% for the spherically symmetric stacked cluster.

The dynamical origin of early-type dwarfs in galaxy clusters: a theoretical investigation

“…Deviations from spherical symmetry result in a 50% uncertainty in individual cluster profiles. Serra & Diaferio (2013), also using cosmological simulations, showed that this technique can identify cluster galaxies with a completeness fraction of 95%. Observationally, galaxies outside these caustics are identified as not being bound to the cluster.…”

Deep spectroscopy of nearby galaxy clusters – I. Spectroscopic luminosity function of Abell 85

“…As a byproducts, tho technique identifies the cluster members, namely those galaxies whose radial velocity is smaller than the escape velocity at a given clustercentric radius. Numerical simulations show that the cluster membership obtained using this technique has an interloper contamination of only 2% within R200 (Serra & Diaferio 2013).…”

The Velocity Dispersion Function of Very Massive Galaxy Clusters: Abell 2029 and Coma

“…Contamination by foreground and background objects is significantly reduced relative to samples based on photometric redshifts (e.g., Geller et al 2014;Hwang et al 2014). Applying the caustic technique (Diaferio & Geller 1997;Diaferio 1999;Serra & Diaferio 2013) to the spectroscopic sample (see Section 3) identifies cluster members; the completeness of membership determination from the caustic technique is 95% within 3R 200 based on numerical simulations (Serra & Diaferio 2013). For bright A2029 galaxies we compiled redshifts from the SDSS spectroscopic survey.…”

“…We use the caustic technique (Diaferio & Geller 1997;Diaferio 1999;Serra & Diaferio 2013) to analyze the data. Simulations by Serra & Diaferio (2013) show that the caustic technique recovers 95% of the clusters members within 3R 200 for mock catalogs with ∼1000 galaxies in the field of view and ∼180 members per cluster; these objects all appear within the caustics. The interloper contamination from non-member galaxies that appear within the caustics is~2% at R 200 and~8% at 3R 200 .…”