The Clusters Hiding in Plain Sight (CHiPS) survey: CHIPS1911+4455, a Rapidly-Cooling Core in a Merging Cluster

TL;DR

This paper reports the discovery of a galaxy cluster with a cool core and high star formation rate, revealing complex dynamics and cooling processes during mergers, based on multi-wavelength observations.

Contribution

It presents the first detailed multi-wavelength analysis of CHIPS1911+4455, highlighting its unique combination of a cool core and asymmetric morphology indicating recent merger activity.

Findings

The cluster has a low core entropy of 17 keV cm^2, indicating a strong cool core.

The large-scale morphology is highly asymmetric, suggesting recent merger activity.

The brightest cluster galaxy exhibits an exceptionally high star formation rate of 140-190 Msun/yr.

Abstract

We present high-resolution optical images from the Hubble Space Telescope, X-ray images from the Chandra X-ray Observatory, and optical spectra from the Nordic Optical Telescope for a newly-discovered galaxy cluster, CHIPS1911+4455, at z=0.485+/-0.005. CHIPS1911+4455 was discovered in the Clusters Hiding in Plain Sight (CHiPS) survey, which sought to discover galaxy clusters with extreme central galaxies that were misidentified as isolated X-ray point sources in the ROSAT All-Sky Survey. With new Chandra X-ray observations, we find the core (r=10 kpc) entropy to be 17+2-9 keV cm^2, suggesting a strong cool core, which are typically found at the centers of relaxed clusters. However, the large-scale morphology of CHIPS1911+4455 is highly asymmetric, pointing to a more dynamically active and turbulent cluster. Furthermore, the Hubble images reveal a massive, filamentary starburst near the brightest cluster galaxy (BCG). We measure the star formation rate for the BCG to be 140--190 Msun/yr, which is one of the highest rates measured in a central cluster galaxy to date. One possible scenario for CHIPS1911+4455 is that the cool core was displaced during a major merger and rapidly cooled, with cool, star-forming gas raining back toward the core. This unique system is an excellent case study for high-redshift clusters, where such phenomena are proving to be more common. Further studies of such systems will drastically improve our understanding of the relation between cluster mergers and cooling, and how these fit in the bigger picture of active galactic nuclei (AGN) feedback.