The impact of the SZ effect on cm-wavelength (1–30 GHz) observations of galaxy cluster radio relics

Abstract

Radio relics in galaxy clusters are believed to be associated with powerful shock fronts that originate during cluster mergers, and are a testbed for the acceleration of relativistic particles in the intracluster medium. Recently, radio relic observations have pushed into the cm-wavelength domain (1-30 GHz) where a break from the standard synchrotron power law spectrum has been found, most noticeably in the famous "Sausage" relic. Such spectral steepening is seen as an evidence for non-standard relic models, such as ones requiring seed electron population with a break in their energy spectrum. In this paper, however, we point to an important effect that has been ignored or considered insignificant while interpreting these new high-frequency radio data, namely the contamination due to the Sunyaev-Zel'dovich (SZ) effect that changes the observed synchrotron flux. Even though the radio relics reside in the cluster outskirts, the shock-driven pressure boost increases the SZ signal locally by roughly an order of magnitude. The resulting flux contamination for some well-known relics are non-negligible already at 10 GHz, and at 30 GHz the observed synchrotron fluxes can be diminished by a factor of several from their true values. At higher redshift the contamination gets stronger due to the redshift independence of the SZ effect. Interferometric observations are not immune to this contamination, since the change in the SZ signal occurs roughly at the same length scale as the synchrotron emission, although there the flux loss is less severe than single-dish observations. Besides presenting this warning to observers, we suggest that the negative contribution from the SZ effect can be regarded as one of the best evidence for the physical association between radio relics and shock waves. We present a simple analytical approximation for the synchrotron-to-SZ flux ratio, based on a theoretical radio relic model that connects the nonthermal emission to the thermal gas properties, and show that by measuring this ratio one can potentially estimate the relic magnetic fields or the particle acceleration efficiency.

Relics in galaxy clusters at high radio frequencies

“…This also rules out a significant B-field variation within the CRE cooling time. The steepening found by Stroe et al (2016) for the Sausage and Toothbrush relics are based on two values at higher frequencies that, however, have large errors and may suffer from the SZ effect (Basu et al 2016). We note that the integrated flux densities of the Sausage and Toothbrush relics given in Table 4 of Stroe et al (2016) are based on the same maps as in this work, but are slightly different because the values in our Table 5 are obtained with somewhat different integration areas and an additional source subtraction in the case of the Toothbrush relic.…”

Shock Acceleration Model for the Toothbrush Radio Relic

“…Basu et al (2016) showed that the Sunyaev-Zel'dovich (SZ) decrement in the observed radio flux can be significant above 10 GHz for radio relics. We adopt their estimates for the SZ contamination factor for the Toothbrush relic given in their Table 1.…”

“…The error bars are given in the same tables. The solid black circles at 16 and 30 GHz are the data points, multiplied by factors of 1.1 and 1.8, respectively, which could represent the SZ-corrected fluxes (Basu et al 2016). electrons or preexisting soft-spectrum electrons, are accelerated by a strong shock with » M 3.0 s . The main results are summarized as follows: 1.…”

Alma-Sz Detection of a Galaxy Cluster Merger Shock at Half the Age of the Universe

“…5 (bottom panel). The fit suggests a narrow lognormal shape, with best-fit parameters [µ, σ] = [2.1, 0.9] kpc (see Basu et al 2016, Equation (9)), and a shock radius 882 kpc that is consistent with the SZ measurement. The observed broadening (Fig.…”

“…The effective resolution is 6.1 × 3.1 , making the relic sufficiently resolved in the direction of shock propagation to fit a radial brightness profile. We use a lognormal emissivity model (Basu et al 2016) that is projected from a 3D cone and smoothed to the ATCA resolution. The fit is shown in Fig.…”