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Register a new userThe ${it XMM}$ Cluster Survey: joint modelling of the $L_{rm X}-T$ scaling relation for clusters and groups of galaxies
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Pedro Teixeira Pereira Viana
Publication date
2018
fields
Physics
and research's language is
English
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We characterize the X-ray luminosity--temperature ($L_{rm X}-T$) relation using a sample of 353 clusters and groups of galaxies with temperatures in excess of 1 keV, spanning the redshift range $0.1 < z < 0.6$, the largest ever assembled for this purpose. All systems are part of the ${it XMM-Newton}$ Cluster Survey (XCS), and have also been independently identified in Sloan Digital Sky Survey (SDSS) data using the redMaPPer algorithm. We allow for redshift evolution of the normalisation and intrinsic scatter of the $L_{rm X}-T$ relation, as well as, for the first time, the possibility of a temperature-dependent change-point in the exponent of such relation. However, we do not find strong statistical support for deviations from the usual modelling of the $L_{rm X}-T$ relation as a single power-law, where the normalisation evolves self-similarly and the scatter remains constant with time. Nevertheless, assuming {it a priori} the existence of the type of deviations considered, then faster evolution than the self-similar expectation for the normalisation of the $L_{rm X}-T$ relation is favoured, as well as a decrease with redshift in the scatter about the $L_{rm X}-T$ relation. Further, the preferred location for a change-point is then close to 2 keV, possibly marking the transition between the group and cluster regimes. Our results also indicate an increase in the power-law exponent of the $L_{rm X}-T$ relation when moving from the group to the cluster regime, and faster evolution in the former with respect to the later, driving the temperature-dependent change-point towards higher values with redshift.
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The isotropy of the late Universe is an assumption greatly used in astronomy. However, many studies have recently reported deviations from isotropy with a definitive conclusion yet to be made. New, independent methods to robustly test the cosmic isotropy are of crucial importance. In this work, we investigate the directional behavior of the X-ray luminosity-temperature ($L_X-T$) relation of galaxy clusters. A tight correlation exists between the luminosity and temperature of the X-ray-emitting intracluster medium. While the measured luminosity depends on the underlying cosmology, the temperature can be determined without any cosmological assumptions. By exploiting this property one can effectively test the isotropy of cosmological parameters over the full extragalactic sky. Here, we used 313 homogeneously selected X-ray galaxy clusters from the MCXC catalog and obtained core-excised temperatures for all of them. We find that the behavior of the $L_X-T$ relation heavily depends on the direction of the sky. Strong anisotropies are detected at a $sim 4sigma$ level toward $(l,b)sim (280^{circ}, -20^{circ})$. Several X-ray and cluster-related effects that could potentially explain these anisotropies were examined, but none did so. Interestingly, two other available cluster samples appear to have a similar behavior throughout the sky, while being fully independent of each other and our sample. Performing a joint analysis of the three samples, the final anisotropy is further intensified ($sim 5sigma$), toward $(l,b)sim (303^{circ}, -27^{circ})$, which is in good agreement with other cosmological probes. This result demonstrates that X-ray galaxy cluster studies that assume perfect isotropy can produce strongly biased results whether the underlying reason is cosmological or related to X-rays. The identification of the exact nature of these anisotropies is therefore crucial.
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