Testing the Limits of AGN Feedback and the Onset of Thermal Instability in the Most Rapidly Star-forming Brightest Cluster Galaxies

Abstract

We present new, deep, narrow- and broadband Hubble Space Telescope observations of seven of the most star-forming brightest cluster galaxies (BCGs). Continuum-subtracted [OII] maps reveal the detailed, complex structure of warm (T ∼ 104 K) ionized gas filaments in these BCGs, allowing us to measure spatially resolved star formation rates (SFRs) of ∼60–600 M ⊙yr−1. We compare the SFRs in these systems and others from the literature to their intracluster medium cooling rates ( M ̇ cool ), measured from archival Chandra X-ray data, finding a best-fit relation of log ( SFR ) = ( 1.66 ± 0.17 ) log ( M ̇ cool ) + (−3.22 ± 0.38) with an intrinsic scatter of 0.39 ± 0.09 dex. This steeper-than-unity slope implies an increasingly efficient conversion of hot (T ∼ 107 K) gas into young stars with increasing M ̇ cool , or conversely a gradual decrease in the effectiveness of AGN feedback in the strongest cool cores. We also seek to understand the physical extent of these multiphase filaments that we observe in cluster cores. We show, for the first time, that the average extent of the multiphase gas is always smaller than the radii at which the cooling time reaches 1 Gyr, the t cool/t ff profile flattens, and that X-ray cavities are observed. This implies a close connection between the multiphase filaments, the thermodynamics of the cooling core, and the dynamics of X-ray bubbles. Interestingly, we find a one-to-one correlation between the average extent of cool multiphase filaments and the radius at which the cooling time reaches 0.5 Gyr, which may be indicative of a universal condensation timescale in cluster cores.

Multiple Shock Fronts in RBS 797: The Chandra Window on Shock Heating in Galaxy Clusters

“…In this context, Cavagnolo et al (2011) noted filamentary structures in the residual optical image of the BCG extending 8-10 kpc southward. These structures were recently confirmed by Calzadilla et al (2022), who produced continuum-subtracted [O II] maps of nebular emission using the Hubble Space Telescope. The overlay of [O II] contours on the t cool /t ff map (see lower right panel in Figure 5) reveals that cool gas at ∼10 4 keV is coincident with the region where the minimum in t cool /t ff is found (within ∼20 kpc S from the center).…”

“…The low-entropy (30 keV cm 2 ) and short cooling time (t cool 1 Gyr) ICM is preferentially found between and behind the X-ray cavities, which may indicate that the bubbles are stimulating cooling. In this context, the region with the shortest cooling time (∼440 Myr) and minimum t cool /t ff ratio (∼18) is located within 20 kpc S of the center, where filamentary patches of [O II] nebular emission tracing star-forming gas were recently detected (Calzadilla et al 2022). 3.…”

“…Alternatively, the turbulence injected by the nuclear activity may trigger local compressions of the gas (e.g., Brighenti & Mathews 2002;Gaspari et al 2011Gaspari et al , 2012. In such cases, as thermal instabilities may ensue, further cooling of the ICM can be stimulated, creating warm or cold filaments (e.g., McDonald et al 2010;Russell et al 2019;Calzadilla et al 2022;Temi et al 2022;Olivares et al 2022).…”

“…In the lower panels, cyan ellipses show the positions of the X-ray cavities. In the lower right panel, white contours fromCalzadilla et al (2022) trace the morphology of the central [O II]-emitting nebulae, and the red dashed wedge shows the region where the minimum in t cool /t ff is found. Relative uncertainties in temperature, pressure, and cooling time are on the order of ∼10%, while those in t cool /t ff are on the order of ∼20%.…”

SPT-CL J2215−3537: A Massive Starburst at the Center of the Most Distant Relaxed Galaxy Cluster

“…The latest gain and charge transfer inefficiency corrections were applied, with improved background screening for the VFAINT telemetry mode. Modeling of the global ICM properties was done as in Calzadilla et al (2022), and the thermodynamic profiles are shown and discussed in Section 3.1.…”