Radio-continuum surveys with SKA and LOFAR: a first look at the perspectives for radio mini-halos

Abstract

Context. Diffuse synchrotron radio emission has been observed in a number of cool-core clusters on scales comparable to that of the cooling region. These radio sources are called “mini-halos”. In order to understand their origin, which is still unclear, joint radio and X-ray statistical studies of large cluster samples are necessary to investigate the radio mini-halo properties and their connection with the cluster thermodynamics. Aims. We here extend our previous explorative study and investigate the perspectives offered by surveys in the radio continuum with the LOw Frequency ARray (LOFAR) and the Square Kilometre Array (SKA), in particular examining the effect of the intracluster magnetic field in the mini-halo region for the first time. Methods. By considering the minimum flux detectable in radio surveys and exploiting the Pradio − LX correlation observed for known mini-halos, we estimate the detection limits achievable by future radio observational follow-up of X-ray cluster samples, such as HIFLUGCS and eROSITA. This allows us to estimate the maximum number of radio mini-halos that can potentially be discovered in future surveys as a function of redshift and magnetic field strength. Results. Under the optimistic assumption that all cool-core systems host a mini-halo and that the radio versus X-ray scaling relation extends to systems with lower X-ray luminosity, we show that future radio surveys with LOFAR and SKA1 (at ∼140 MHz and ∼1.4 GHz) have the potential to discover ∼1000–10 000 radio mini-halo candidates up to redshift z =  1. This shows that these surveys may be able to produce a breakthrough in the study of these sources. We further note that future SKA1 radio surveys at redshift z > 0.6 will allow us to distinguish between different magnetic fields in the mini-halo region, because higher magnetic fields are expected to produce more powerful mini-halos, thus implying a larger number of mini-halo candidates detected at high redshift. For example, the non-detection with SKA1 of mini-halos at z > 0.6 will suggest a low magnetic field (B < few μG). The synergy of these radio surveys with future X-ray observations and theoretical studies is essential in establishing the radio mini-halo physical nature.