Optical confirmation and weak lensing mass constraints for distant Sunyaev-Zel'dovich-detected galaxy clusters

Abstract

Galaxy clusters reside in the densest regions of the large-scale matter distribution of the Universe. Therefore, their number density as a function of mass and redshift is a powerful tool for cosmological studies aiming, for example, to uncover the nature of dark matter and dark energy. A detection of galaxy clusters through the Sunyaev-Zel'dovich (SZ) effect, a spectral distortion of the cosmic microwave background (CMB) due to inverse Compton scattering of CMB photons with free electrons in the hot intracluster medium, has proven to be very useful to assemble nearly mass-limited samples of clusters. Two ingredients are essential to make full use of these samples for cosmological studies: firstly, the selection function of the cluster sample needs to be well-understood, and secondly, the scaling relation between the observable mass proxy and the true underlying cluster mass has to be accurately calibrated over a wide redshift range. The research in this thesis addresses both of these prerequisites. <br /> The first part of this thesis contributes to an understanding of the selection function of the second Planck catalogue of Sunyaev-Zeldovich sources (PSZ2). We conduct an optical follow-up of a sample of 32 PSZ2 cluster candidates preselected at low SZ detection significance 3 ≲ S/N ≲ 6 and approximately at redshifts z ≳ 0.7 with ACAM on the 4.2-m William Herschel Telescope. A red-sequence analysis based on observations in the r, i, and z band provides photometric redshift and richness estimates. We obtain spectroscopic redshifts from additional long-slit observations for a subset of clusters. Comparing the measurements to a scaling relation calibrated at low redshifts reveals that the optical richness is in many cases smaller than expected from the SZ-based mass. Likely reasons include Eddington bias, projection effects, or noise-induced detections, which are more frequent at low signal-to-noise ratios. Still, for 18 (7) of the cluster candidates at redshift z > 0.5 (z > 0.8), we measured a richness that is at least half of the average value expected from the scaling relation, marking the threshold we regard for the confirmation of massive clusters. The complex selection function of the investigated cluster sample based on SZ and optical data inhibits its use for cosmological studies. However, the validation of massive cluster candidates provides a basis for further astrophysical investigations of individual targets. <br /> The second part of this thesis aims to calibrate the cluster mass scale of the SZ--mass scaling relation of galaxy clusters in the high-redshift regime. For this purpose, weak lensing masses of nine massive clusters from the South Pole Telescope SZ (SPT-SZ) Survey are obtained from galaxy shape measurements based on Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) imaging in the bands F606W and F814W. This sample comprises clusters with high SZ-detection significances &#958; > 6.0 and high redshifts of z ≳ 1.0 in the SPT-SZ Survey. The HST/ACS data are supplemented with observations from the Wide Field Camera 3 (WFC3) onboard the HST and the FORS2 imager at the Very Large Telescope (VLT) to enable a robust removal of cluster members while preferentially selecting background source galaxies at z ≳ 1.8, carrying most of the weak lensing signal. We estimate the source redshift distribution and average geometric lensing efficiency with the help of photometric redshift catalogues from the CANDELS/3D-HST fields with a revised calibration. Fitting the tangential reduced shear profiles with spherical NFW models assuming a fixed concentration-mass relation provides weak lensing mass measurements. Combining these with results at lower redshifts from earlier studies, we constrain the redshift evolution of the SZ-mass scaling relation of clusters in the SPT-SZ Survey for the first time out to the highest redshift z ~ 1.7 for a sample with a well-defined selection function. We find that the refined constraints are consistent with previous results. That is, a lower mass scale is preferred in the analysis including a weak lensing mass calibration than in an analysis based on a flat Planck &#957;&#923;CDM cosmology combined with the SPT-SZ cluster counts.