The merger history of clusters and its effect on the X-ray properties of the intracluster medium

We investigate the growth over time of 20 massive (>3 keV) clusters in a hydrodynamical simulation of the cold dark matter cosmology with radiative cooling. The clusters show a variety of formation histories: some accrete most of their mass in major mergers; others, more gradually. During major mergers, the long-term (temporally smoothed) luminosity increases such that the cluster moves approximately along the LX–T X relation; between times, it slowly decreases, tracking the drift of the LX–T X relation. We identify several different kinds of short-term luminosity and temperature fluctuations associated with major mergers including double-peaked mergers, in which the global intracluster medium merges first (LX and T X increase together) and then the cluster cores merge (LX increases and T X decreases). At both luminosity peaks, clusters tend to appear to be spherical and relaxed, which may lead to biases in high-redshift, flux-limited samples. There is no simple relationship between scatter in the LX–T X relation nor in the recent or overall merger activity or cluster formation time. The scatter in the LX–M and T X–M relations is reduced if properties are measured within R500 rather than Rvir.