Waking the monster: The onset of AGN feedback in galaxy clusters hosting young central radio galaxies

Aims: The investigation of the feedback cycle in galaxy clusters has historically been performed for systems where feedback is ongoing ("mature-feedback" clusters), that is where the central radio galaxy has inflated radio lobes, pushing aside the intracluster medium (ICM). In this pilot study, we present results from "pre-feedback" clusters, where the central newly active radio galaxies (age < 10³ yr) may not yet have had time to alter the thermodynamic state of the ICM.
Methods: We analyze Chandra and MUSE observations of two such systems, evaluating the hot gas entropy and cooling time profiles, and characterizing the morphology and kinematics of the warm gas.
Results: Based on our exploratory study of these two sources, we find that the hot gas meets the expectations for an as of yet unheated ICM. Specifically, the entropy and cooling time of pre-feedback clusters within 20 kpc from the center fall below those of mature-feedback clusters by a factor ∼2. We speculate that with an estimated mechanical power of ∼10⁴⁴ − 10⁴⁵ erg s⁻¹, the two young radio galaxies may restore the entropy levels in a few tens of millions of years, which are typical values of power outbursts and lifetimes for radio galaxies in clusters. Conversely, the properties of the gas at ∼10⁴ K seem to remain invariant between the two feedback stages, possibly suggesting that the warm gas reservoir accumulates over long periods (10⁷ − 10⁸ yr) during the growth of the radio galaxy. We conclude that the exploratory results obtained from our analysis of two cluster-central young radio galaxies are crucial in the context of understanding the onset of active galactic nuclei feedback, and they provide enough motivation for further investigation of similar cases.