Presentation #400.04 in the session “ISM/Galaxies/Clusters (Oral)”.
For years we have grappled with the “cooling flow problem” in galaxy clusters, where the massive reserves of hot (107 K) gas in the intracluster medium (ICM) have been universally observed to form stars with an efficiency of only 1-10%. Feedback from accreting active galactic nuclei (AGN) has been identified as the likely heating source capable of suppressing runaway cooling by up to two orders of magnitude. However, with the recent discovery of the Phoenix cluster exhibiting the only known pure cooling flow, the thermostat of AGN feedback appears to be broken in this and a few other highly accreting systems. In this study, we use new, deep Hubble observations to map out massive, intricate [O II] emission line nebulae in exquisite detail in a handful of the most rapidly star forming central cluster galaxies in the Universe. From these maps of cool (104 K) gas we can measure accurate star formation rates (SFRs), which we use together with systems from the literature spanning several orders of magnitude in ICM cooling rates (dM/dt, as measured from Chandra X-ray data) to determine the efficiency with which the ICM cools. We find a steeper than unity relationship between SFR and dM/dt, indicating perhaps a saturation point for AGN feedback in the strongest cooling systems (i.e. dM/dt > 1000 M⊙ yr-1), where feedback from the central AGN is being overwhelmed by the prodigious deposition of cooling material. We also use the maximum extent of cool gas as measured from our [O II] maps along with Hα measurements from the literature and compare these to features in the ICM to weigh in on the hotly-debated issue as to what criteria best predict the onset of thermal instability in the ICM.