Revisiting Scaling Relations for Giant Radio Halos in Galaxy Clusters

Abstract

Many galaxy clusters host Megaparsec-scale radio halos, generated by ultrarelativistic electrons in the magnetized intracluster medium. Correlations between the synchrotron power of radio halos and the thermal properties of the hosting clusters were established in the last decade, including the connection between the presence of a halo and cluster mergers. The X-ray luminosity and redshift limited Extended GMRT Radio Halo Survey provides a rich and unique dataset for statistical studies of the halos. We uniformly analyze the radio and X-ray data for the GMRT cluster sample, and use the new Planck SZ catalog, to revisit the correlations between the power of radio halos and the thermal properties of galaxy clusters. We find that the radio power at 1.4 GHz scales with the cluster X-ray (0.1-2.4 keV) luminosity computed within R 500 as P 1.4 ∼ L 2.1±0.2 500. Our bigger and more homogenous sample confirms that the X-ray luminous (L 500 > 5 × 10 44 erg s −1 ) clusters branch into two populations -radio halos lie on the correlation, while clusters without radio halos have their radio upper limits well below that correlation. This bimodality remains if we excise cool cores from the X-ray luminosities. We also find that P 1.4 scales with the cluster integrated SZ signal within R 500 , measured by Planck, as P 1.4 ∼ Y 2.05±0.28 500 , in line with previous findings. However, contrary to previous studies that were limited by incompleteness and small sample size, we find that "SZluminous" Y 500 > 6 × 10 −5 Mpc 2 clusters show a bimodal behavior for the presence of radio halos, similar to that in the radio-X-ray diagram. Bimodality of both correlations can be traced to clusters dynamics, with radio halos found exclusively in merging clusters. These results confirm the key role of mergers for the origin of giant radio halos, suggesting that they trigger the relativistic particle acceleration.

Lofar, Vla, and Chandra Observations of the Toothbrush Galaxy Cluster

“…A similar correlation with radio power is found (e.g., Basu 2012;Cassano et al 2013) for the integrated Sunyaev-Zel'dovich (SZ) effect signal, given by Y SZ . Y SZ traces the integrated pressure along the line of sight.…”

“…For radio halos, a correlation has been found between cluster X-ray luminosity (L X ) and radio halo power (e.g., Cassano 2010; Cassano et al 2013). A similar correlation with radio power is found (e.g., Basu 2012;Cassano et al 2013) for the integrated Sunyaev-Zel'dovich (SZ) effect signal, given by Y SZ .…”

“…From the 66 spectra we find that the spectral index across the radio halo over a 0.8 Mpc 2 region stays rather constant. Following Cassano et al (2013), to properly take the measurement uncertainties into account, we compute a raw scatter from the mean spectral index of 0.07. This implies that the intrinsic spectral index scatter is 0.04  , given that measurement errors from map noise alone would result in a spread of 0.06 ( 0.06 0.04 0.07 2 2 + » ).…”

Chandra and JVLA Observations of HST Frontier Fields Cluster MACS J0717.5+3745

“…Polarized emission from the radio halo was detected by Bonafede et al (2009). The radio luminosity (1.4 GHz radio power) is the largest known for any cluster, in agreement with the cluster's large mass and high global temperature (e.g., Cassano et al 2013).…”

The First Murchison Widefield Array low-frequency radio observations of cluster scale non-thermal emission: the case of Abell 3667

“…We may independently test this value for the radio halo flux density by applying the radio halo-X-ray scaling relation of Cassano et al (2013). By applying this relation, using an X-ray luminosity of L x = 9 × 10 44 ergs s −1 , we find an expected radio halo power of 1.11 × 10 24 W Hz −1 at 1.4 GHz which corresponds to a total flux density of 180 mJy.…”