Kavli Affiliate: Michael McDonald
| Summary:
With rapid improvements in the assembly of large samples of galaxy clusters, we are approaching the ability to study clusters at $zgtrsim2$. Evolutionary studies comparing these distant clusters to the clusters in our local universe depend heavily on the reliability of low-redshift cluster samples, most of which are subject to X-ray selection effects, biasing them to relaxed, cool core clusters. Here, we introduce the Cluster Evolutionary Reference Ensemble At Low-$z$ (CEREAL) sample, composed of Chandra X-ray observations of 169 galaxy clusters that have been selected from the Planck Sunyaev-Zel’dovich catalog. CEREAL has a simple and well-understood selection function, spans an order of magnitude in mass at $zsim0.15$, and has uniform, high-resolution X-ray follow-up. We present the full sample and provide results based on X-ray surface brightness properties, finding significantly more non-cool core systems than in X-ray-selected samples. We use surface brightness concentration (c$_mathrmSB$) as a proxy for cool core strength and centroid shift ($w$) to measure dynamical state. Over the full sample, we find a cool core (c$_mathrmSB > 0.075$) fraction of $0.39_-0.04^+0.04$, a strong cool core (c$_mathrmSB > 0.155$) fraction of $0.13_-0.03^+0.03$, and a dynamically relaxed ($w<0.01$) fraction of $0.42_-0.04^+0.04$. We find no mass dependence in the fraction of clusters that appear relaxed or have cool cores. We quantify the rarity of X-ray-bright central point sources (L$_mathrmnuc,~2-10~keV > 10^43$ erg s$^-1$), finding them to be intrinsically rare ($0.7_-0.5^+1.2$% of massive, low-z clusters) with a notable increase in occurrence rate at the centers of cool cores.
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