LX‐TRelation and Related Properties of Galaxy Clusters

Abstract

An observational approach is presented to constrain the global structure and evolution of the intracluster medium using the ROSAT and ASCA distant cluster sample. From statistical analysis of the gas density profile and the connection to the LX-T relation under the β-model, the scaled gas profile is found to be nearly universal for the outer region, and LX(>0.2r500) is tightly related to the temperature through T~3 rather than T2. On the other hand, a large density scatter exists in the core region, and there is clearly a deviation from the self-similar scaling for clusters with a small core. A link between the core size and the radiative cooling timescale, tcool, and the analysis of X-ray fundamental plane suggest that tcool is a parameter controlling the gas structure and that the appearance of small cores in regular clusters and may be much connected with the thermal evolution. We derive the luminosity-ambient temperature (T') relation, assuming the universal temperature profile for the clusters with short tcool, and find that the dispersion around the relation significantly decreases and the slope becomes marginally less steep. We further examined the LX-Tβ relation and showed a trend that merging clusters segregate from the regular clusters on the plane. Considering a correlation between tcool and the X-ray morphology, the observational results lead us to draw a phenomenological picture: after a cluster collapses and tcool falls below the age of the universe, the core cools radiatively with quasi-hydrostatic balancing in the gravitational potential, and the central density gradually becomes higher to evolve from an outer-core-dominant cluster, which follows the self-similarity, to an inner-core-dominant cluster.