Optical characterisation of the planck psz1 galaxy cluster cataloguebuilding a reference sample for cosmology

Resumen

This thesis has been dedicated to the study of Galaxy Clusters as Cosmological tools. The work is divided into two main parts; the first is purely observational, whereas the second is dedicated to the preparation of tools for cosmological analyses. The observational section has been developed within the frame of the optical validation program Sunyaev-Zeldovich (SZ) sources, observed by the Planck satellite in the northern hemisphere, and included in the PSZ1 catalogue. The 212 targets were observed during a two-year International Time Project (ITP) at the Roque de Los Muchachos Observatory (ORM) facilities on La Palma Island. During the observational programme, each target underwent a validation process in two phases: photometric and spectroscopic. In the first phase, we performed imaging in g0, r 0 and i0-bands at INT/WFC and WHT/ACAM. We obtained a deep photometry for the majority of the targets, reaching a magnitude in r 0-band of about 23.2 and 23.8 for WFC/INT and ACAM/WHT, respectively. This allowed us to estimate their photometric redshift up to zphot  0:8 cluster richness. In the second phase, we performed the spectroscopy of the photometrically-confirmed clusters. We used TNG/DOLORES and GTC/OSIRIS spectrographs in order to observe clusters at zphot  0:4 and zphot > 0:4, respectively. The aim of the spectroscopic follow-up is to confirm clusters and to characterise their physical properties, such as velocity dispersion and mass. We used the Multi-Object Spectroscopy (MOS) technique in order to observe as many cluster members as possible. Due to the large sample of SZ effect sources, we were able to use, on average, only one mask per cluster, so we retrieved a median number of cluster members of Ngal  14. Due to this low number of galaxy members we could not use sophisticated membership techniques. Therefore, we assigned the cluster membership according to the galaxy radial velocity and distance from the cluster centre. At the end of the follow-up programme we were able to validate for the first time a total of 88 new galaxy clusters. The second part of this thesis is focused in the cosmology, with the main aim of estimating the mass bias parameter (1􀀀b) through the characterisation of the scaling relation between the cluster masses calculated from dynamical and SZ proxies. The Planck Collaboration demonstrated that the mass bias is crucial to determine the cosmological parameters m and 8 by using the cluster number counts formalism. The Planck Collaboration showed that the latter are in tension with the parameters derived from CMB primary anisotropies, and that, by varying the mass bias parameter, this tension could be alleviated. Since the (1􀀀b) should be a measure of the bias of the SZ mass estimation, it is extremely important to understand the possible biases in the dynamical mass estimate. It is impossible to obtain an accurate dynamical mass estimate if the velocity dispersion is biased. The nature of the biases may be statistical or physical, and in this thesis we studied both kinds of biases by using hydrodynamic simulations. We tested three velocity dispersion estimators, namely biweight, gapper and standard deviation, and observed that they present a statistical bias in the low galaxy numbers regime. In order to correct this effect, we designed a receipt to obtain unbiased velocity dispersions in the whole Ngal regime. Physical biases are mainly related to the cluster members sampling. For instance, due to the velocity dispersion radial profile a 5% bias is introduced when clusters are sampled in their cores only. We also observed that the velocity dispersion estimate obtained by using only the most massive galaxy members is biased by about 2%. However, the most important source of bias is the interlopers contamination, which overestimates the velocity dispersion by about 10%. Furthermore, we demonstrated that even an unbiased velocity dispersion could lead to a biased estimate of the cluster mass. We defined new mass estimator, which takes into account this statistical effect. In order to perform the cosmological analysis, we selected all clusters within the ITP sample with reliable velocity dispersion estimation (more than 7 members and no multiple detections). On the other hand, in order to improve the statistical significance of our study, we applied the cluster identification procedure to the PSZ1 clusters within the SDSS footprint as well. This way, we built a sample of 207 galaxy clusters. This sample is the largest catalogue of clusters for which both the SZ and dynamic masses have been estimated. Based on it and after applying all corrections, we found that the mass bias parameters is (1 􀀀 B) = 0:78  0:02. The compatibility of this result with the one obtained by the Planck Collaboration ensures that we are not able to alleviate the tension on the cosmological parameters. However, we were able to reduce the uncertainty on the mass bias down to  2% for the first time, based on velocity dispersion mass estimators. Therefore, although the parameter estimates will not vary, our result will allow us to reduce the uncertainty on the combination of m and 8 by about a factor 2 with respect to previous studies. Other solutions are discussed.