Anatomy of a Cooling Flow: The Feedback Response to Pure Cooling in the Core of the Phoenix Cluster (1904.08942v1)

Abstract: We present new, deep observations of the Phoenix cluster from the Chandra X-ray Observatory, the Hubble Space Telescope, and the Karl Jansky Very Large Array. These data provide an order of magnitude improvement in depth and/or angular resolution at X-ray, optical, and radio wavelengths, yielding an unprecedented view of the core of the Phoenix cluster. We find that the one-dimensional temperature and entropy profiles are consistent with expectations for pure-cooling hydrodynamic simulations and analytic descriptions of homogeneous, steady-state cooling flow models. In the inner ~10 kpc, the cooling time is shorter by an order of magnitude than any other known cluster, while the ratio of the cooling time to freefall time approaches unity, signaling that the ICM is unable to resist multiphase condensation on kpc scales. When we consider the thermodynamic profiles in two dimensions, we find that the cooling is highly asymmetric. The bulk of the cooling in the inner ~20 kpc is confined to a low-entropy filament extending northward from the central galaxy. We detect a substantial reservoir of cool (10^4 K) gas (as traced by the [OII] doublet), which is coincident with the low-entropy filament. The bulk of this cool gas is draped around and behind a pair of X-ray cavities, presumably bubbles that have been inflated by radio jets, which are detected for the first time on kpc scales. These data support a picture in which AGN feedback is promoting the formation of a multiphase medium via a combination of ordered buoyant uplift and locally enhanced turbulence. These processes ought to counteract the tendency for buoyancy to suppress condensation, leading to rapid cooling along the jet axis. The recent mechanical outburst has sufficient energy to offset cooling, and appears to be coupling to the ICM via a cocoon shock, raising the entropy in the direction orthogonal to the radio jets.

• The paper demonstrates that AGN feedback in the Phoenix cluster, revealed through massive radio jets, enhances cooling flows and triggers elevated star formation.
• It employs multi-wavelength observations from Chandra, Hubble, and VLA to derive detailed thermodynamic profiles with steep temperature and entropy declines in the core.
• It highlights that the finite suppression of cooling enables multiphase condensation, challenging the view of AGN feedback as solely inhibitory in galaxy clusters.

Analyzing the Feedback Mechanisms in the Phoenix Cluster's Core

The paper presented by McDonald et al. focuses on the intricate processes at play in the Phoenix cluster, particularly within its core. Utilizing data from the Chandra X-ray Observatory, the Hubble Space Telescope, and the Karl Jansky Very Large Array, the researchers provide a comprehensive examination of the feedback mechanisms that are critical to understanding the formation and maintenance of cooling flows and the role of active galactic nuclei (AGN) in this context.

Core Detailed Observations

The Phoenix cluster is distinguished by its extreme cooling rate and star formation occurring in its central galaxy. This is driven by a cooling flow, which is manifesting at rates far exceeding what is typically observed in similar clusters. The deep observations achieved in this paper led to several key insights:

• Thermodynamic Profiles: The temperature and entropy profiles of the intra-cluster medium (ICM) depict a steep decline towards the cluster core. The entropy profile lacks the typical central entropy floor found in many clusters, resembling closely the predictions of a steady, homogenous cooling flow.
• AGN Feedback: Interestingly, the paper emphasizes the dual nature of AGN feedback—both heating and cooling. The AGN in the Phoenix cluster is propelling massive radio jets that create cavities in the X-ray emitting gas. These cavities appear to play a role in uplifting low-entropy gas, promoting a cooling and condensation process rather than halting it, resulting in a large reservoir of cold gas and elevated star formation rates.
• Finite Suppression of Cooling Rates: The observations suggest that the cooling in the cluster center is only moderately suppressed by the AGN, leading to a cooling rate that vastly exceeds that typically seen in comparable environments. This results in star formation correlating closely with the theoretical cooling rates.

Implications and Future Speculations

The paper's findings have significant implications for our understanding of galaxy cluster dynamics:

• Positive vs. Negative Feedback: This paper reveals that the traditional concept of AGN feedback as a purely suppressive mechanism is overly simplistic. The Phoenix cluster showcases a regime where AGN activity, through uplifting and stirring rather than purely heating, accelerates the cooling process.
• Conditions for Multiphase Condensation: With cooling times falling below free-fall times (t_cool / t_ff ≤ 1), conditions are conducive to multiphase condensation within the cluster core, offering a direct observation of a state rarely documented across other clusters.
• Potential for Future Models and Studies: This paper sets the stage for further studies that could adapt existing models of starburst-driven turbulence to cluster environments, examine the broader impact of asymmetrical feedback on cluster evolution, and utilize future high-resolution, multi-wavelength observations to validate these processes.

In conclusion, the Phoenix cluster provides an exceptional laboratory for studying the complex interplay of cooling flows and AGN feedback. These new findings challenge existing assumptions about the feedback cycle, providing fertile ground for future research that could redefine our understanding of cosmic structure formation.