Cool-Core Cycles and Phoenix

TL;DR

This paper proposes that the high star formation rate in the Phoenix cluster's core results from a late cooling phase in an AGN feedback cycle, where intense cooling and star formation precede a future AGN outburst that will suppress cooling.

Contribution

It introduces a new scenario where short-lived cooling and star formation phases are integral to the AGN feedback loop in massive galaxy clusters.

Findings

Phoenix is likely in a late cooling phase before an AGN outburst.

The high SFR state lasts less than 100 million years in Phoenix.

Simulations support the timing and nature of this cooling cycle.

Abstract

Recent observations show that the star formation rate (SFR) in the {\it Phoenix} cluster's central galaxy is $\sim 500$ M$_\odot$ yr$^{-1}$. Even though {\it Phoenix} is a massive cluster ($M_{200} \approx 2.0\times 10^{15}$ M$_\odot$; $z\approx 0.6$) such a high central SFR is not expected in a scenario in which feedback from an active galactic nucleus (AGN) maintains the intracluster medium (ICM) in a state of rough thermal balance. It has been argued that either AGN feedback saturates in very massive clusters or the central supermassive black hole (SMBH) is too small to produce enough kinetic feedback and hence is unable to quench the catastrophic cooling. In this work, we present an alternate scenario wherein intense short-lived cooling and star formation phases followed by strong AGN outbursts are part of the AGN feedback loop. Using results from a 3D hydrodynamic simulation of a standard cool-core cluster ($M_{200}\sim 7\times10^{14}$ M$_\odot$; $z=0$), scaled to account for differences in mass and redshift, we argue that {\it Phoenix} is at the end of a cooling phase in which an AGN outburst has begun but has not yet arrested core cooling. This state of high cooling rate and star formation is expected to last for $\lesssim$ 100 Myr in {\it Phoenix}.