The Cluster Evolutionary Reference Ensemble at Low-$z$ (CEREAL) Sample of Galaxy Clusters I: X-ray Morphological Properties and Demographics

With rapid improvements in the assembly of large samples of galaxy clusters, we are approaching the ability to study clusters at $z\gtrsim2$. 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 $z\sim0.15$, 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$_\mathrm{SB}$) 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$_\mathrm{SB} > 0.075$) fraction of $0.39_{-0.04}^{+0.04}$, a strong cool core (c$_\mathrm{SB} > 0.155$) 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$_\mathrm{nuc,~2-10~keV} > 10^{43}$ 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.