[Paper Review] 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

F. Ruppin, M. McDonald+6 more|arXiv (Cornell University)|Dec 29, 2020

Galaxies: Formation, Evolution, Phenomena127 references30 citations

TL;DR

This study develops a joint X-ray/SZ analysis technique using Chandra and SPT data to measure intracluster medium (ICM) density profiles in 67 galaxy clusters at 0.3 < z < 1.3, progenitors of nearby clusters. By combining Chandra surface brightness and SPT Compton parameter, it reduces density profile uncertainty by a factor of ~5 compared to standard X-ray analysis, revealing that cool-core fractions and core densities remain stable over 9 Gyr despite a 4× mass growth, challenging current hydrodynamical simulations that predict increasing cool-core fractions with redshift.

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 $z > 1$.

Motivation & Objective

• To investigate the evolution of cool-core properties in galaxy clusters over cosmic time, focusing on the stability of intracluster medium (ICM) density profiles.
• To address the challenge of estimating ICM properties in low-mass, high-redshift clusters with shallow X-ray data (only ~150 counts) and high background contamination.
• To develop and validate a joint X-ray/SZ analysis method that reduces systematic uncertainties in ICM density profile estimation without relying on precise X-ray spectroscopy.
• To test whether cool-core fractions evolve with redshift in clusters growing from M500 ~2×10¹⁴ M⊙ at z~1.4 to M500 ~8×10¹⁴ M⊙ at z~0.
• To provide a legacy sample of 67 clusters along a common evolutionary track for future multi-wavelength studies.

Proposed method

• A joint deprojection method is applied to Chandra X-ray surface brightness profiles and SPT integrated Compton parameter (Y) to jointly constrain the ICM density profile.
• The analysis uses a forward modeling approach that combines X-ray surface brightness and SZ signal within a common physical model of the ICM, minimizing reliance on background-limited X-ray spectroscopy.
• The technique accounts for the X-ray point spread function and degrades the deprojection center to the X-ray centroid, which is more stable under low signal-to-noise conditions than the X-ray peak.
• The ICM density profile is modeled as a combination of a fixed, early-formed core (z > 1.3) and a self-similarly evolving non-cool core component.
• Systematic uncertainties from background-dominated X-ray spectra are reduced by leveraging the complementary constraints from the SZ signal, which is insensitive to background and sensitive to total electron pressure.
• The method is validated on a simulated sample and applied to the full sample of 67 clusters from the SPT and SPTpol catalogs.

Experimental results

Research questions

• RQ1 Does the fraction of cool-core clusters remain constant over 9 Gyr of cluster growth, despite a 4× increase in mass?
• RQ2 Can the ICM density profile in low-mass, high-redshift clusters (z > 1) be accurately measured with only ~150 X-ray counts using standard X-ray analysis?
• RQ3 How does the joint X-ray/SZ deprojection method reduce uncertainty in ICM density profile estimation compared to standard X-ray analysis?
• RQ4 Is the observed stability of cool-core properties consistent with current hydrodynamical simulations, which predict increasing cool-core fractions at higher redshift?
• RQ5 Can the progenitor sample of 67 clusters be used as a robust evolutionary sequence to trace ICM evolution across cosmic time?

Key findings

• The joint X-ray/SZ analysis reduces the relative uncertainty on the ICM density profile by a factor of approximately 5 compared to standard X-ray analysis, achieving ~20% uncertainty without requiring precise X-ray spectroscopy.
• The ICM density profile is well described by a fixed core formed at z > 1.3 combined with a self-similarly evolving non-cool core, indicating long-term stability of the core structure.
• The cool-core fraction remains constant at approximately 60% across the redshift range 0.3 < z < 1.3, despite a 4× increase in cluster mass over ~9 Gyr.
• The core density evolution is consistent with a constant value over the redshift range studied, indicating no significant evolution in the central ICM properties.
• The results contradict hydrodynamical simulations that predict an increasing cool-core fraction with redshift, suggesting that either cool cores form earlier than currently modeled or core disruption mechanisms are more frequent at high redshift than simulated.
• The progenitor sample of 67 clusters provides a unique, multi-wavelength resource for future studies with JWST, ALMA, Rubin Observatory, and future X-ray missions.