The thermal Sunyaev-Zel'dovich effect as a probe of cluster physics and cosmology.

Abstract

The universe is a complex environment playing host to a plethora of macroscopic and microscopic processes. Understanding the interplay and evolution of such processes will help to shed light on the properties and evolution of the universe. The juxtaposition is that in order to study small scale effects one needs to observe large scale structure as the latter objects trace the history of our universe. Galaxy groups and clusters are the largest known objects in the universe and thus provide a means to probe the evolution of structure formation in the universe as well as the underlying cosmology. In this thesis we investigate how clusters observed through the Sunyaev-Zel’dovich (SZ) effect can be used to constrain cosmological models. In addition, we present the first results of the Atacama Cosmology Telescope (ACT), a mm-wave telescope measuring the small-scale microwave background anisotropy, and conclude with preliminary SZ cluster detection performed on the latest ACT sky maps. In the first part of this thesis we investigate the ability of high resolution cosmic microwave background (CMB) experiments to detect hot gas in the outer regions of nearby group halos. We construct two hot gas models for the halos; a simpler adiabatic formalism with the gas described by a polytropic equation of state, and a more general gas description which incorporates feedback effects in line with constraints from X-ray observations. We calculate the thermal Sunyaev- Zel’dovich (tSZ) signal in these halos and compare it to the sensitivities of upcoming and current tSZ survey experiments such as ACT, PLANCK and the South Pole Telescope (SPT). Through the application of a multi-frequency Wiener filter, we derive mass and redshift based tSZ detectability limits for the various experiments, incorporating effects of galactic and extragalactic foregrounds as well as the CMB. In this study we find that galaxy group halos with virial masses below 1014M. can be detected at z ~< 0.05 with the mass limit dropping to 3 − 4 × 1013M. at z ~< 0.01. Probing such halos with the tSZ effect allows one to map the hot gas in the outer regions, providing a means to constrain gas processes, such as feedback, as well as the distribution of baryons in the local universe.

In the fourth chapter, we extend this analysis and determine the ability of ACT to constrain galactic feedback and star formation in clusters and groups using the tSZ effect. We present a new microwave deblender, which provides a means of extracting accurate halo fluxes and radial profiles from maps of the tSZ effect. Considering various surveys that could be performed by ACT, we use multi-frequency filtering on simulated sky maps to predict how well such surveys will constrain gas properties using a Fisher matrix analysis. We find that the current ACT survey will be unable to constrain any gas parameters. However, if ACT were to survey a smaller area then we will be able to constrain feedback. Furthermore, with greater sensitivity, we will be able to place interesting constraints on the gas feedback, and baryon and stellar fractions. The fifth chapter in this thesis concerns itself with the first results of the Atacama Cosmology Telescope Project. In this section we discuss the map-making method as well as telescope beam characterisation, an understanding of which is important in any subsequent map analyses. In addition, we present maps of eight clusters observed at 148 GHz via the SZ effect, and provide flux and signal to noise estimates of the clusters. In the final chapter we present a preliminary analysis of the latest 148 GHz ACT maps from the 2008 observing season. We study the sky maps using single frequency wiener filtering, allowing for CMB, dust and correlated noise contamination. To substantiate our results, we compare the number counts, recovered fluxes and sample purity from simulated sky maps. The compounding effects of CMB and correlated noise result in high contamination levels below a signal to noise ratio of 6, however our investigation shows that above 8¾ our cluster sample is ¼ 80% pure. A cluster list containing 44 detections, of which 8 are previously known, is also presented, along with a Table listing the candidate cluster positions and fluxes. The candidate cluster catalogue will be used for follow-up studies using optical and X-ray observations.