Connecting optical and X-ray tracers of galaxy cluster relaxation

• Published in: Monthly notices of the Royal Astronomical Society Vol. 475; no. 4; pp. 4704 - 4716
• Main Authors: Roberts, Ian D, Parker, Laura C, Hlavacek-Larrondo, Julie
• Format: Journal Article
• Language: English
• Published: London Oxford University Press 21.04.2018
• Subjects: Asymmetry; Centroids; Correlation; Emission; Galactic clusters; Galaxy distribution; Luminosity; Morphology; Radial velocity; Red shift; Sky surveys (astronomy); Statistical analysis; Stellar mass; Tracers; Velocity; Velocity distribution; X-ray astronomy; XMM (spacecraft); galaxies: evolution; galaxies: statistics; galaxies: clusters: general
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/sty131

Abstract Substantial effort has been devoted in determining the ideal proxy for quantifying the morphology of the hot intracluster medium in clusters of galaxies. These proxies, based on X-ray emission, typically require expensive, high-quality X-ray observations making them difficult to apply to large surveys of groups and clusters. Here, we compare optical relaxation proxies with X-ray asymmetries and centroid shifts for a sample of Sloan Digital Sky Survey clusters with high-quality, archival X-ray data from Chandra and XMM–Newton. The three optical relaxation measures considered are the shape of the member-galaxy projected velocity distribution – measured by the Anderson–Darling (AD) statistic, the stellar mass gap between the most-massive and second-most-massive cluster galaxy, and the offset between the most-massive galaxy (MMG) position and the luminosity-weighted cluster centre. The AD statistic and stellar mass gap correlate significantly with X-ray relaxation proxies, with the AD statistic being the stronger correlator. Conversely, we find no evidence for a correlation between X-ray asymmetry or centroid shift and the MMG offset. High-mass clusters (Mhalo > 1014.5 M⊙) in this sample have X-ray asymmetries, centroid shifts, and Anderson–Darling statistics which are systematically larger than for low-mass systems. Finally, considering the dichotomy of Gaussian and non-Gaussian clusters (measured by the AD test), we show that the probability of being a non-Gaussian cluster correlates significantly with X-ray asymmetry but only shows a marginal correlation with centroid shift. These results confirm the shape of the radial velocity distribution as a useful proxy for cluster relaxation, which can then be applied to large redshift surveys lacking extensive X-ray coverage.