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

TL;DR

The study investigates star formation histories and stellar dynamics in the central galaxies of three cool-core clusters (RX J0820.9+0752, A1835, PKS 0745-191) to understand how cooling gas and AGN feedback shape star formation. It employs KCWI integral field spectroscopy to map stellar continua, velocities, and ages across the inner tens of kiloparsecs, distinguishing stellar light from nebular emission and comparing to ALMA molecular gas, KCWI nebular kinematics from Gingras 2024, and X-ray cavity data. Key results include substantial young stellar populations: SFRs of about $100\ M_{\7odot} \,\mathrm{yr}^{-1}$ in A1835 and $8\ M_{\7odot} \,\mathrm{yr}^{-1}$ in PKS 0745-191, while RX J0820.9+0752 shows no central star formation and only intermediate-age stars in a filament; A1835 also hosts a blueshifted, massive clump of young stars moving rapidly relative to gas, interpreted as a detached inflowing/outflowing population in a feedback-affected flow. Overall, the results reveal star formation occurring in dynamically complex environments driven by the central galaxy's motion relative to cooling clouds and jet-induced flows, with all galaxies dominated by old populations and little to no warm gas, implying that the cooling flow problem involves distributed and feedback-regulated star formation that can proceed in filaments and clumps rather than purely centrally.

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.