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Title: Galaxy clusters as astrophysical laboratories and probes of cosmology
Author: Le Brun, A. M. C.
ISNI:       0000 0004 5992 8954
Awarding Body: Liverpool John Moores University
Current Institution: Liverpool John Moores University
Date of Award: 2014
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Galaxy clusters are the most recent of cosmological structures to have formed by the present time in the currently favoured hierarchical scenario of structure formation and are widely regarded as powerful probes of cosmology and galaxy formation physics alike. Over the past few years, it became increasingly clear that precision cluster cosmology requires the development of detailed, realistic theoretical models of galaxy clusters and the confrontation of synthetic surveys generated using these models with observations. This motivates a campaign of large cosmological hydrodynamical simulations, with plausible 'sub-grid' prescriptions for the relevant galaxy formation physics. This thesis presents a new suite of large-volume cosmological hydrodynamical simulations called cosmo-OWLS. They form an extension to the Overwhelmingly Large Simulations (OWLS) project, and have been designed to help improve our understanding of cluster astrophysics and the non-linear structure formation, which are now the limiting systematic errors when using clusters as cosmological probes. Starting from identical initial conditions in either the Planck or WMAP7 cosmologies, the most important 'sub-grid' physics, including feedback from supernovae and active galactic nuclei (AGN) has been systematically varied. Via the production of synthetic surveys of the simulations and comparisons with observations, the realism of these state-of-the-art models was explored. At the same time, the simulations were shown to providea valuable tool for interpreting the observational data, as well as powerful means for testing commonly-employed methods for estimating, for example, cluster masses and determining survey selection functions, which are crucial for cluster cosmology. The properties of the simulated galaxy groups and clusters were first compared to a wide range of observational data,such as x-ray luminosity and temperature, gas mass fractions, entropy and density profiles, Sunyaev-Zel'dovich flux, I-band mass-to-light ratio, dominance of the brightest cluster galaxy, and central massive black hole (BH) masses, by producing synthetic observations and mimicking observational analysis techniques. These comparisons demonstrated that some AGN feedback models can produce a realistic population of galaxy groups and clusters, broadly reproducing both the median trend and, for the first time, the scatter in physical properties over approximately two decades in mass (¹³M⊙≲500≲10¹⁵M⊙) and 1.5 decades in radius (0.05≲500≲1.5). However, in other models, the AGN feedback is too violent (even though they reproduce the observed BH scaling relations), implying calibration of the models is required. The production of realistic populations of simulated groups and clusters, as well as models that bracket the observations, opens the door to the creation of synthetic surveys for assisting the astrophysical and cosmological interpretation of cluster surveys, as well as quantifying the impact of selection effects. A study of the scatter and evolution of the hot gas properties of the populations of galaxy groups and clusters, such as X-ray luminosity and temperature, gas mass and Sunyaev-Zel'dovich flux, as a function of the important non-gravitational physics of galaxy formation was then conducted. The median relations and the scatter about them are reasonably well-modelled by evolving broken power-laws. The non-radiative model and the model that neglects AGN feedback are consistent with having selfsimilar mass slopes, whereas the mass slopes of the AGN feedback models deviate significantly from the self-similar expectation. Self-similar evolution, which is widely adopted in current cosmological studies, was also found to break down when efficient feedback is included. The log-normal scatter varies mildly with mass, is relatively insensitive to non-gravitational physics, but shows a moderately strong decreasing trend with increasing redshift. The X-ray luminosity has a significantly larger scatter than all the other hot gas proxies examined. It is thus the poorest one, while the 'best' one is the mean X-ray temperature. Synthetic Sunyaev-Zel'dovich observations, generated using a 'multi-purpose' light cone software package developed during the thesis, were used to check the veracity of some of the results reported by the Planck collaboration at the end of 2012. Taken at face value, their results seem to favour a close to self-similar scaling relation between the Sunyaev-Zel'dovich flux and total mass all the way down to individual galaxy haloes, which is in contradiction with X-ray and absorption lines observation. The matched filter used by the Planck collaboration recovers fluxes which are biased increasingly high as feedback intensity increases. Two likely causes for the bias, i.e. confusion and deviations from the universal pressure profiles were investigated. Confusion was found to have a negligible effect when the signal is averaged over a large number of systems. Instead a shape mismatch (in terms of pressure profiles) was identified as being mostly responsible for the bias. Finally, synthetic X-ray observations, generated using a combination of the developed light cone software and of the XMM-Newton simulator and processed with the detection pipeline of the XXL survey, were used to start quantifying the selection function of the XXL survey. Preliminary results suggest that: (i) XXL is only able to find a very small fraction of the galaxy group population, (ii) the survey is best at finding lowmass clusters (14.0≲log₁₀[M₅₀₀(M⊙)]≲14.5) at z ≲ 0:75, and (iii) the detection pipeline misses a few very massive, very extended, nearby systems.
Supervisor: McCarthy, Ian ; Collins, Chris Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: QB Astronomy