Abstract
Context. The relaxed cool-core Phoenix cluster (SPT-CL J2344-4243) features an extremely strong cooling flow, as well as a mini halo. Strong star formation in the brightest cluster galaxy indicates that active galactic nucleus (AGN) feedback has been unable to inhibit this cooling flow. Aims. We aim to study the strong cooling flow in the Phoenix cluster by determining the radio properties of the AGN and its lobes. In addition, we used spatially resolved radio observations to investigate the origin of the mini halo. Methods. We present new multifrequency Very Large Array 1–12 GHz observations of the Phoenix cluster, which resolve the AGN and its lobes in all four frequency bands as well as the mini-halo in the L and S bands. Results. Using our L-band observations, we measure the total flux density of the radio lobes at 1.5 GHz to be 7.6 ± 0.8 mJy, and the flux density of the mini halo to be 8.5 ± 0.9 mJy. Using high-resolution images in the L and X bands, we produced the first spectral index maps of the lobes from the AGN and find the spectral indices of the northern and southern lobes to be −1.35 ± 0.07 and −1.30 ± 0.12, respectively. Similarly, using L- and S-band data, we mapped the spectral index of the mini halo, and obtain an integrated spectral index of α = −0.95 ± 0.10. Conclusions. We find that the mini halo is most likely formed by turbulent re-acceleration powered by sloshing in the cool core due to a recent merger. In addition, we find that the feedback in the Phoenix cluster is consistent with the picture that stronger cooling flows are to be expected for massive clusters such as this one, as these may feature an underweight supermassive black hole due to their merging history. Strong time variability of the AGN on Myr timescales may help explain the disconnection between the radio and the X-ray properties of the system. Finally, a small amount of jet precession of the AGN likely contributes to the relatively low intracluster medium re-heating efficiency of the mechanical feedback.
Highlights
The emission of strong X-ray radiation by the intracluster medium (ICM) in galaxy clusters suggests that this medium should often cool down rapidly: within a timescale of ∼109 years or fewer (e.g., Fabian 1994)
As we find that both the magnetic field configurations obtained by assuming a hadronic model and the relatively flat spectral index of the mini halo are inconsistent with the values generally reported in literature, we conclude that our results disfavor a pure hadronic origin of the radio emission, we cannot exclude that proton-proton collisions played a role in the origin of seed electrons for the re-acceleration
Jansky Very Large Array observations, enabling the radio lobes of the active galactic nucleus (AGN) and the mini halo in the Phoenix cluster to be studied in detail at frequencies from 1 to 12 GHz
Summary
The emission of strong X-ray radiation by the intracluster medium (ICM) in galaxy clusters suggests that this medium should often cool down rapidly: within a timescale of ∼109 years or fewer (e.g., Fabian 1994). The generally accepted solution to this problem is that feedback from active galactic nuclei (AGN) supplies energy to the ICM in the form of radiation and jetted outflows of plasma, thereby preventing the medium from cooling down (e.g., Brüggen & Kaiser 2002; McNamara & Nulsen 2007; Fabian 2012) Studying this feedback process is essential to our understanding of the formation and evolution of galaxies, as it plays a critical role in the cooling of the ICM and the star formation in galaxies across cosmic time (e.g., Matteo et al 2005; Croton et al 2006; Menci et al 2006; Sijacki et al 2007; Lagos et al 2008; Ciotti et al 2010; Mathews & Guo 2011; Vogelsberger et al 2014; Rasia et al 2015). The ICM is often dense enough to keep the jetted outflows from the AGN contained, which allows this mechanical form of feedback to be studied in detail (McNamara & Nulsen 2012)
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