Characterising the optical properties of galaxy clusters with GMPhoRCC
Hood, Ross John
MetadataShow full item record
The properties of galaxy clusters, such as abundance and mass to light ratios, have long been used to investigate and constrain cosmology. With vast numbers of newly detected clusters, such as from the Planck mission (Planck Collaboration et al., 2013), full determination of cluster properties, particularly mass, can be hugely expensive and time consuming. Optical characterisation o ers a cheap solution, using optical data alone to estimate cluster properties such as redshift. With the abundance of current optical data, such as from the Sloan Digital Sky Survey (SDSS), (Ahn et al., 2012) and upcoming all sky surveys, such as the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) 3 survey (Magnier et al., 2013), optical characterisation will play a key role in the investigation of the latest clusters. Presented in this thesis is the Gaussian Mixture full Photometric Red sequence Cluster Characteriser (GMPhoRCC), which aims to provide such an analysis, o ering substantial advantages over existing algorithms. GMPhoRCC identi es and models the red sequence, early-type galaxies which dominate the cluster, and uses the properties of this to estimate cluster redshift and richness, an optical mass proxy based on the number of cluster members. The main features include, full treatment of multi-modal distributions by modelling properties with error-corrected Gaussian Mixtures, model independence by using empirical photometric redshifts rather than assumed colour-redshift relations and quality control used to identify probable catastrophic failures in order to clean the characterisations. Using a sample of 5500 clusters taken from the GMBCG (Hao et al., 2010), NORAS (Bohringer et al., 2000), REFLEX (Bohringer et al., 2004) and XCS (Mehrtens et al., 2012) catalogues, GMPhoRCC redshift estimates are compared to spectra showing low scatter ( σ∆z~ 0:042) around the actual value. In addition applying the quality control to produce a clean subset removes most outliers (|zGMPhoRCC-zspec| > 0:03) gives a much tighter agreement, σ∆z~ 0:018 showing signi cant improvement over maxBCG, σ∆z~ 0:025, and XCS, σ∆z~ 0:050. In addition to comparisons with real clusters, an extensive evaluation of the GMPhoRCC selection function is presented using mock clusters. These mocks are produced by stacking red sequence galaxies from existing clusters, analysed using the SDSS DR9, in redshift-richness bins from which new sequences are resampled. This extends the similar approach of maxBCG and GMBCG where only rich clusters are used as seeds to generate mocks with a range of properties. Comparisons with mocks agree well with real clusters attaining low redshift scatters ( σ∆z~ 0:01) with the clean subset removing the majority of outliers. In addition, with a de nitive mock value, richness comparisons are possible and although show a larger fractional scatter (σ∆z n200 ~ 0:12) are centred on the mock value. Richness estimates are shown to be more sensitive to discrepancies in redshift, background uctuations and poor modelling of the red sequence than redshift. Completeness is estimated by considering the fraction of clusters found with characterisations within given bounds. First incomplete photometry, simulated by an i-band < 21 cut, is shown to remove members for clusters with z > 0:45. Redshift completeness, the fraction of clusters within 0:03 of the mock value, is not immediately hindered by the photometry, attaining 93% for 0:05 < z < 0:62 for clusters with a richness greater than 20, showing improvement over maxBCG (with 90% for 0:1 < z < 0:3) and a larger range than GMBCG (with 96% for 0:1 < z < 0:46). Similar to results from GMBCG, richness attains lower completeness rates due to discrepancies introduced by projection e ects, background uctuations, and redshift errors. The fraction of clusters within 25% of the mock value, de ning completeness, is measured as 91% for 0:07 < z < 0:45 for clusters with richness greater than 20, 78% for those with richness between 10 and 20, and 64% for those with richnesses less than 10. The application of GMPhoRCC follows, where characterisations are found for new XCS X-ray extended sources (Lloyd-Davies et al., 2011). Applying GMPhoRCC to these preliminary DR2 candidates ( 10 times larger than the current catalogue) using the VLT Survey Telescope (VST) ATLAS catalogue (Shanks & Metcalfe, 2012) and the much deeper Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS) (Heymans et al., 2012) provides characterisations beyond the SDSS footprint. Of the 13; 956 candidates, 6124 have optical coverage, 5580 in the SDSS, 523 in ATLAS and 819 in CFHTLenS with some overlap. Overall characterisations are found for 4365 candidates, 1893 of which have an associated spectroscopic redshift. The clean subset comprises 1203 candidates, 904 with spectra. Considering XCS DR1, Mehrtens et al. (2012) presented 503 optically con rmed X-ray clusters of which 258 have spectroscopic redshifts and 108 have SDSS characterisations. GMPhoRCC provides characterisations for 360, 232 of which have spectroscopic redshifts. Overall GMPhoRCC provides 260 (149 of which are clean) new SDSS characterisations and 91 (61 of which are clean) new spectroscopic redshifts. Finally this thesis concludes with a discussion of future research, focusing mainly on a preliminary analysis of a clean spectroscopic subset of XCS DR1 in order to illustrate the potential to constrain X-ray scaling relations with the upcoming XCS DR2. Additionally, potential research into the e ect of environment on the red sequence is illustrated using the dependence of the CMR slope on X-ray temperature. While a slight dependence is found, the cluster sample is insu cient to contradict the independence on environment found by Hogg et al. (2004) and Hao et al. (2009).