Cool-core cycles and Phoenix

Recent observations show that the star formation rate (SFR) in the Phoenix cluster's central galaxy is similar to 500 M-circle dot yr(-1). Even though Phoenix is a massive cluster (M-200 approximate to 2.0 x 10(15) M-circle dot; z approximate to 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 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 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 similar to 7 x 10(14) M-circle dot; z = 0), scaled to account for differences in mass and redshift, we argue that 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 less than or similar to 100 Myr in Phoenix.