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Galaxy Clusters
Galaxy clusters are the largest gravitationally bound structures in the universe. Galaxy clusters comprise tens to thousands of galaxies plus an X-ray emitting intra-cluster medium (ICM) bound by a dark-matter-dominated gravitational potential well. My research involves the statistical analysis of galaxy clusters using optical and X-ray detection methods.
IMAGE CREDIT: ESA/XMM-Newton/SDSS/J. Sanders et al. 2019
Scaling relations
Assuming galaxy groups form solely through gravitational mergers it is expected that all clusters should be scaled versions of each other. The assumption of a self-similar model Kaiser (1986) predicts power-law scaling relations between cluster properties such as luminosity, temperature and mass. When measured observationally, these scaling relations are generally found to differ from self similar expectations. This is most prominent for low mass systems, while some studies of high mass clusters do find self-similar behaviour in some scaling relations. The departures from self-similarity are most noticeable for the relations which are dependant on density and distribution of ICM (e.g. luminosity, gas mass). These properties are found to show steeper correlation with mass and temperature than expected. This behaviour is broadly understood to be a result of non-gravitational processes that are ignored by the self-similar model. These processes (e.g., active galactic nuclei (AGN) feedback, cooling, star formation, supernovae) are expected to disproportionately affect the properties of the ICM in lower mass systems where the gravitational potential well is shallower.
Left: X-Ray Luminosity - Mass scaling relation and RIght: Best fit parameters for the slope (B) and normalisation of the LM relation found in Wood et al. (2024; in prep).
The XXL Survey LIV. X-ray luminosity function and luminosity–mass relation of optically selected galaxy groups
Wood et al (2025) Abstract: The overlap between the GAMA spectroscopic survey and the XXL X-ray survey was used to study the X-ray properties of optically selected groups of galaxies. Forced X-ray aperture photometry was applied to an optically selected sample of 235 groups (containing at least five member galaxies) to measure their X-ray luminosities in the regime of low signal-to-noise X-ray data. The sample encompasses X-ray luminosities over an order of magnitude fainter than typical X-ray selected samples, and avoids X-ray selection biases. This gives access to low mass groups where the effects of non-gravitational processes, such as AGN-feedback, should be most apparent and could inhibit their detection in an X-ray survey. We measured the X-ray luminosity function (XLF) of the sample, and found it to be consistent with the extrapolation of the XLF from X-ray selected samples at higher luminosities. The XLF was combined with a theoretical halo mass function to infer the form of the scaling relation between X-ray luminosity and mass (LM relation) for the GAMA groups. We found a slope of 1.87 ± 0.12, which is steeper than self similarity in this mass regime. When comparing with other measurements of the LM relation, we find evidence for a steepening of the slope in the low mass regime, likely due to the impact of non-gravitational processes. Our approach can be translated to eROSITA data using multiwavelength surveys to constrain the X-ray properties of galaxy groups in the limits of high redshift and low mass.
