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Study of the optical properties of X-ray selected galaxy clusters : multi-wavelengths analysis and implications for the future large surveys

Abstract : Addressing fundamental questions regarding our understanding of the Universe, such as the cause of its accelerated expansion or the nature of dark matter, requires to confront theories and observations. In this context, galaxy clusters can be used as powerful observational probes. However, their current utilisation is limited by uncertainties and systematic effects, notably affecting the measurement of their mass, which is presumably dominated by dark matter.Galaxy clusters can be studied at different wavelengths: the hot gas composing the Intra Cluster Medium (ICM) shines in X-ray and is observable at millimetre wavelengths via the Sunyaev-Zel’dovich (SZ) effect, whereas galaxies emit principally in the optical and infrared. Combining and comparing these observables allows us to reduce the uncertainties and systematics in the cosmological constraints obtained from clusters. In this context, this thesis aims at paving the way of future large surveys such as Euclid and the Large Synoptic Survey Telescope. It presents the multi-wavelengths analyses of a sample of clusters detected in X-ray in the XXL survey, spanning a wide range of masses and redshifts. The first part of the thesis introduces the cosmological context and presents the observational properties of galaxies and clusters, and the ingredients to build cosmological cluster samples. The second part concentrates on the optical characterisation of XXL clusters and the properties of their member galaxies. It starts by presenting XXL and the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS), an optical counterpart survey. Then, it focuses on the characterisation of the CFHTLS photometric redshifts quality and their use to construct the optical galaxy luminosity functions (LF) of XXL clusters. The LF of satellite galaxies is found to slightly depend on cluster richness, the main optical mass proxy, but no significant redshift evolution is observed. Then, the study of the colour and active galactic nuclei (AGN) fraction in XXL cluster galaxies is performed, finding that the mass plays a key role in shaping AGN and star formation activity in clusters. Finally, the WaZP optical cluster finder algorithm is used to investigate the optical counterparts of XXL clusters. The third part of this thesis is dedicated to the observational project dedicated to the mapping of the SZ signal in three distant XXL clusters, with the high angular resolution NIKA2 camera. The preparation of the project is discussed, making use of the X-ray and optical data to predict the expected SZ signal. Then, the observation procedure at the telescope and the data reduction, dedicated to produce calibrated SZ maps, are presented. The project is still ongoing and one cluster, XLSSC102, at z = 0.97, has been partially observed. The development of the blind detection of galaxies potentially contaminating the SZ signal is developed, allowing for the serendipitous discovery of dusty star forming galaxies in the field of XLSSC102. The morphology and dynamical state of XLSSC102 are then characterised using optical, SZ and X-ray data and the radial ICM thermodynamics and mass profiles are measured combining SZ and X-ray data. This allows us to show that XLSSC102 is a merging cluster with a mass ∼ 3 × 10 14 M, and is compatible with the standard evolution scenario of cluster formation.
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Marina Ricci. Study of the optical properties of X-ray selected galaxy clusters : multi-wavelengths analysis and implications for the future large surveys. Cosmology and Extra-Galactic Astrophysics [astro-ph.CO]. Université Côte d'Azur, 2018. English. ⟨NNT : 2018AZUR4070⟩. ⟨tel-02418619⟩

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