Optical Spectroscopy and Velocity Dispersions of Galaxy Clusters From the SPT-Sz Survey

Abstract

We present optical spectroscopy of galaxies in clusters detected through the Sunyaev-Zel'dovich (SZ) effect with the South Pole Telescope (SPT). We report our own measurements of 61 spectroscopic cluster redshifts, and 48 velocity dispersions each calculated with more than 15 member galaxies. This catalog also includes 19 dispersions of SPT-observed clusters previously reported in the literature. The majority of the clusters in this paper are SPT-discovered; of these, most have been previously reported in other SPT cluster catalogs, and five are reported here as SPT discoveries for the first time. By performing a resampling analysis of galaxy velocities, we find that unbiased velocity dispersions can be obtained from a relatively small number of member galaxies ( 30), but with increased systematic scatter. We use this analysis to determine statistical confidence intervals that include the effect of membership selection. We fit scaling relations between the observed cluster velocity dispersions and mass estimates from SZ and X-ray observables. In both cases, the results are consistent with the scaling relation between velocity dispersion and mass expected from dark-matter simulations. We measure a ∼30% log-normal scatter in dispersion at fixed mass, and a ∼10% offset in the normalization of the dispersion-mass relation when compared to the expectation from simulations, which is within the expected level of systematic uncertainty.

STAR-FORMING BRIGHTEST CLUSTER GALAXIES AT 0.25 < z < 1.25: A TRANSITIONING FUEL SUPPLY

“…These spectra were initially obtained as part of a spectroscopic redshift campaign that targeted, primarily, red sequence galaxies. The full details of this spectroscopic follow-up are provided in Ruel et al (2014) and Bleem et al (2015). For each spectrum, we measure the [O II] and Hδ equivalent widths and the 4000 Å break strength (D 4000 ), via indices defined by Balogh et al (1999).…”

SPIDERS: the spectroscopic follow-up of X-ray-selected clusters of galaxies in SDSS-IV

“…We refer to Beers et al (1990) for details in their computation and the algorithm 4 used for these calculations. We refer the reader to Ruel et al (2014) for a discussion of measurements of velocity dispersions in the regime of low number of spectroscopic members, in the context of galaxy clusters selected by SunyaevZeldovich effect in the South Pole Telescope data. Both estimators are computed for each cluster, although it is clear that a high enough number of members must enter the derivation to ensure robust measurements.…”

CoMaLit – IV. Evolution and self-similarity of scaling relations with the galaxy cluster mass

“…For each cluster, high signal-to-noise spectra for ∼200 cluster members were acquired with MMT (Multi Mirror Telescope)/Hectospec. Ruel et al (2014) presented optical spectroscopy of galaxies in clusters detected through the SZ effect with the South Pole Telescope (SPT). They reported measurements of 61 spectroscopic cluster redshifts, and 48 velocity dispersions each calculated with more than 15 member galaxies.…”

“…To standardise the uncertainties, we followed Ruel et al (2014). They found that the uncertainty in the velocity dispersion σv is well described by…”

“…However, these statistical differences are very small. Ruel et al (2014) presented two independent measurements of σv, based either on the measured gapper scale or the biweight dispersion. The two estimates are very well consistent, with a distribution of relative differences whose centre deviates from zero by less than one per cent and whose scatter is less than 2 per cent.…”