PROBING THE OFF-STATE OF CLUSTER GIANT RADIO HALOS
- CSIRO Astronomy and Space Science, P.O. Box 76, Epping NSW 1710 (Australia)
- Minnesota Institute for Astrophysics, School of Physics and Astronomy, University of Minnesota, 116 Church Street SE, Minneapolis, MN 55455 (United States)
- INAF-Istituto di Radioastronomia, Via P. Gobetti 101, I-40129 Bologna (Italy)
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 L{sub X} > 10{sup 44} erg s{sup -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 L{sub X} > 3 x 10{sup 44} erg s{sup -1} show a 6{sigma} detection of Mpc-scale diffuse emission with a 1.4 GHz luminosity of (2.4 {+-} 0.4) x10{sup 23} W Hz{sup -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 x 10{sup 23} W Hz{sup -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.
- OSTI ID:
- 21565383
- Journal Information:
- Astrophysical Journal Letters, Vol. 740, Issue 1; Other Information: DOI: 10.1088/2041-8205/740/1/L28; ISSN 2041-8205
- Country of Publication:
- United States
- Language:
- English
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