Star Formation Histories and Stellar Dynamics in the Central Galaxies of RX J0820.9+0752, A1835, and PKS 0745-191

TL;DR

This study uses Keck observations to analyze star formation and stellar dynamics in central galaxies of cooling flow clusters, revealing complex environments with recent star formation, outflows, and old stellar populations.

Contribution

It provides detailed insights into the star formation histories and stellar dynamics in three galaxy centers, highlighting the role of gaseous outflows and feedback in these environments.

Findings

Star formation occurs in complex, dynamic environments influenced by feedback and galaxy motion.

Presence of young, massive stellar populations with high star formation rates.

Old stellar populations dominate, with no significant warm gas detected.

Abstract

We present Keck Cosmic Web Imager observations of stellar populations in three galaxies at the centers of cooling flow clusters. All three host rich molecular gas reservoirs and show prominent Balmer absorption from $30-100$ Myr-old stars consistent with long lasting star formation. Two systems, A1835 and PKS 0745-191, have extended young stellar populations in their centers with recent star formation rates of 100 M$_{\odot}$ yr$^{-1}$ and 8 M$_{\odot}$ yr$^{-1}$, respectively. In A1835 we uncover a massive blueshifted clump of young stars moving at high speed with respect to the gas and central galaxy. We suggest this feature is a young population that formed in a gaseous outflow that has detached from its natal gas and is falling back toward the galaxy. This result, combined with a companion study (arXiv:2404.02212) tracing nebular emission which presumably cooled from the hot X-ray atmosphere, indicates that star formation is proceeding in a dynamically complex environment resulting from the central galaxy's motion with respect to the cooling clouds and motion induced by feedback from the central radio jets. In RX J0820.9+0752 intermediate age stars are found in a filament outside of the nucleus with no discernible star formation at the center of the galaxy. All projected galaxies are composed of old stellar populations with deep D4000 breaks and are devoid of detectable warm gas. While in some instances they may be interacting gravitationally with the central galaxy, they cannot have donated the upward of $10^{10}$ M$_{\odot}$ of molecular gas found in these systems.