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

Abstract

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.

Figures (11)

• Figure 1: The mass-redshift distribution of clusters in the CEREAL sample. Left panel: The grey stars are clusters selected via the Planck SZ survey 2016AA...594A..27P. Colored points are clusters that are included in the CEREAL sample, where the point color refers to whether or not new observations were obtained as part of this program. Right panel: Grey stars are Planck clusters and grey squares are SPT clusters 2015ApJS..216...27B, showing a representative distribution of SZ-selected clusters. In both panels, the dotted purple line represents the evolution of a cluster with a mass like that of Coma, while the dotted pink line corresponds to a cluster like Perseus. The solid pink line represents the cutoff above which clusters will be observed with at least 5000 counts in the eRosita All-Sky Survey (eRASS8), and the solid purple line represents the field of view of ACIS-I on Chandra. These lines motivate the mass and redshift range of the CEREAL sample.
• Figure 2: Chandra images of all high-mass CEREAL clusters. Clusters are shown on 550$\times$550 pixel grids and with the same rest-frame intensity coloring, with $(1+z)^{-4}$ surface brightness dimming divided out, meaning that clusters that appear fainter in this figure are intrinsically less luminous or lower surface brightness. Images are adaptively smoothed to improve visual clarity for a sample spanning a wide range in signal-to-noise. For clusters with a single observation, ACIS-I chip gaps and regions with low exposure ($<$50%) are shown in black. This is done for illustrative purposes only, and a more careful exposure correction is done prior to analysis. Clusters are labeled with their Planck names and redshifts as well as their centroid shift ($w$) and surface brightness concentration values ($c$; calculated using 40 kpc and 400 kpc apertures) as described in §\ref{['sec:methods']}.
• Figure 3: Similar to Figure \ref{['fig:highm_img']}, but for the low-mass CEREAL clusters. Both sets of images use the same scaling for comparative purposes.
• Figure 4: A comparison of the concentration values measured using physical apertures (40 kpc and 400 kpc) versus using fractions of the radius ($0.15R_{500}$ and $R_{500}$). Significant outliers have been cropped for visualization purposes. The dashed line represents the line of best fit. The dotted lines represent the cutoffs for moderate and strong cool cores.
• Figure 5: Left: The distributions of concentration values for the CEREAL sample, the MACS sample 2017MNRAS.468.1917R, the 400 SD Sample 2010AA...521A..64S, and the HIFLUGCS sample 2010AA...513A..37H, calculated using physical apertures of 40 kpc and 400 kpc. Right: The distributions of concentration values for the CEREAL mass subsamples and the CHEX-MATE subsamples, calculated using apertures that scale with radius. Each plot shows the p-value for the null hypothesis that the measured distributions are drawn from the same underlying distribution (the left plot compares all distributions to CEREAL, and the right plot compares all distributions to CEREAL high-m), calculated using a K-S test. There is a statistically significant difference between the CEREAL sample and the MACS, and HIFLUGCS samples, while no significant difference is found between the CEREAL and 400 SD samples. Similarly, the low-m and high-m CEREAL subsamples are consistent with each other, and the CEREAL high-m sample is consistent with the CHEX-MATE Tier 2 subsample but not Tier 1.