On the Assembly Bias of Cool Core Clusters Traced by Hα Nebulae

Abstract

Do cool-core (CC) and noncool-core (NCC) clusters live in different environments? We make novel use of Hα emission lines in the central galaxies of redMaPPer clusters as proxies to construct large (1,000's) samples of CC and NCC clusters, and measure their relative assembly bias using both clustering and weak lensing. We increase the statistical significance of the bias measurements from clustering by cross-correlating the clusters with an external galaxy redshift catalog from the Sloan Digital Sky Survey III, the LOWZ sample. Our cross-correlations can constrain assembly bias up to a statistical uncertainty of 6%. Given our Hα criteria for CC and NCC, we find no significant differences in their clustering amplitude. Interpreting this difference as the absence of halo assembly bias, our results rule out the possibility of having different large-scale (tens of Mpc) environments as the source of diversity observed in cluster cores. Combined with recent observations of the overall mild evolution of CC and NCC properties, such as central density and CC fraction, this would suggest that either the cooling properties of the cluster core are determined early on solely by the local (< 200 kpc) gas properties at formation or that local merging leads to stochastic CC relaxation and disruption in a periodic way, preserving the average population properties over time. Studying the small-scale clustering in clusters at high redshift would help shed light on the exact scenario.

The evolving cluster cores: Putting together the pieces of the puzzle

“…A similar result has been found for metal abundance profiles, see Ghizzardi et al (2021). Moreover, in a recent study Medezinski et al (2017Medezinski et al ( , 2019, measuring the assembly bias using both clustering and weak lensing, find no difference between CC and NCC systems. All these results disfavor "ab initio" models.…”

“…It should be pointed out that different authors use different criteria to divide CC from NCC systems: Ghirardini et al (2019) used the central entropy value (and a threshold value of 30 keV cm 2 ) as measured by Chandra and reported by Cavagnolo et al (2009);McDonald et al (2017) use the central density, avoiding the weak cool core regime as defined by Hudson et al (2010) and therefore applying the NCC recipe for n e,0 < 0.5 • 10 −2 cm −3 and CC for n e,0 > 1.5 • 10 −2 cm −3 ; Medezinski et al (2019) use the presence of strong H α nebular luminosity. These definitions lead to consistent classifications in the case of objects with marked CC or NCC traits, conversely intermediate or transitional objects can sometime end up being classified differently by different authors.…”