Stability of Cool Cores During Galaxy Cluster Growth: A Joint $Chandra$/SPT Analysis of 67 Galaxy Clusters Along a Common Evolutionary Track Spanning 9 Gyr

TL;DR

This study analyzes 67 galaxy clusters over 9 billion years using joint Chandra X-ray and SPT SZ data, revealing stable cool-core fractions despite significant mass growth, and introduces a new method to improve ICM property estimates in low-mass, high-redshift clusters.

Contribution

Developed a joint X-ray and SZ analysis technique that reduces uncertainties in ICM density profiles, enabling detailed study of cluster evolution over cosmic time.

Findings

ICM core density evolution is well modeled by a fixed core plus a self-similar profile.

The fraction of cool-core clusters remains stable across redshifts up to 1.3.

Mass of clusters has increased by a factor of about 4 over 9 Gyr.

Abstract

We present the results of a joint analysis of $Chandra$ X-ray and South Pole Telescope (SPT) SZ observations targeting the first sample of galaxy clusters at $0.3 < z < 1.3$, selected to be the progenitors of well-studied nearby clusters based on their expected accretion rate. We develop a new procedure in order to tackle the analysis challenge that is estimating the intracluster medium (ICM) properties of low-mass and high-redshift clusters with ${\sim}150$ X-ray counts. One of the dominant sources of uncertainty on the ICM density profile estimated with a standard X-ray analysis with such shallow X-ray data is due to the systematic uncertainty associated with the ICM temperature obtained through the analysis of the background-dominated X-ray spectrum. We show that we can decrease the uncertainty on the density profile by a factor ${\sim}5$ with a joint deprojection of the X-ray surface brightness profile measured by $Chandra$ and the SZ integrated Compton parameter available in the SPT cluster catalog. We apply this technique to the whole sample of 67 clusters in order to track the evolution of the ICM core density during cluster growth. We confirm that the evolution of the gas density profile is well modeled by the combination of a fixed core and a self-similarly evolving non-cool core profile. We show that the fraction of cool-cores in this sample is remarkably stable with redshift although clusters have gained a factor ${\sim}4$ in total mass over the past ${\sim}9$ Gyr. This new sample combined with our new X-ray/SZ analysis procedure and extensive multi-wavelength data will allow us to address fundamental shortcomings in our current understanding of cluster formation and evolution at $z > 1$.