PROBING THE OFF-STATE OF CLUSTER GIANT RADIO HALOS

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Abstract
We derive the best characterization to date of the properties of radio quiescent massive galaxy clusters through a statistical analysis of their average synchrotron emissivity. We stacked 105 radio images of clusters from the 843 MHz Sydney University Molonglo Sky Survey, all with LX > 1044 erg s–1 and redshifts z < 0.2, after removing point-source contamination and rescaling to a common physical size. Each stacked cluster individually shows no significant large-scale diffuse radio emission at current sensitivity levels. Stacking of sub-samples leads to the following results: (1) clusters with LX > 3 × 1044 erg s–1 show a 6σ detection of Mpc-scale diffuse emission with a 1.4 GHz luminosity of (2.4 ± 0.4) ×1023 W Hz–1. This is 1.5-2 times lower than the upper limits for radio quiescent clusters from the GMRT Radio Halo Survey and is the first independent confirmation of radio halo bi-modality. (2) Clusters with low X-ray concentrations have a mean radio luminosity (2.6 ± 0.6 × 1023 W Hz–1) that is at least twice that of high X-ray concentration clusters, and (3) both of these detections are likely close to the low-level "off-state" of giant radio halos (GRHs) in most or all luminous X-ray clusters, and not due to contributions from a much smaller subset of "on-state" GRHs following the radio/X-ray luminosity correlation. Upcoming deep radio surveys will conclusively distinguish between these two options. We briefly discuss possible origins for the "off-state" emission and its implications for magnetic fields in most or all luminous X-ray clusters.
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