We aim at an unbiased census of the radio halo population in galaxy clusters and test whether current low number counts of radio halos have arisen from selection biases. We construct near-complete samples based on X-ray and Sunyaev-Zeldovich (SZ) eff
ect cluster catalogues and search for diffuse, extended (Mpc-scale) emission near the cluster centers by analyzing data from the National Radio Astronomy Observatory Very Large Array Sky Survey. We remove compact sources using a matched filtering algorithm and model the diffuse emission using two independent methods. The relation between radio halo power at 1.4 GHz and mass observables is modelled using a power law, allowing for a dropout population of clusters hosting no radio halo emission. An extensive suite of simulations is used to check for biases in our methods. Our findings suggest that the fraction of targets hosting radio halos may have to be revised upwards for clusters selected using the SZ effect: while approximately 60 per cent of the X-ray selected targets are found to contain no extended radio emission, in agreement with previous findings, the corresponding fraction in the SZ selected samples is roughly 20 per cent. We propose a simple explanation for this selection difference based on the distinct time evolution of the SZ and X-ray observables during cluster mergers, and a bias towards relaxed, cool-core clusters in the X-ray selection.
We present results from a joint X-ray/Sunyaev-Zeldovich modeling of the intra-cluster gas using XMM-Newton and APEX-SZ imaging data. The goal is to study the physical properties of the intra-cluster gas with a non-parametric de-projection method that
is, aside from the assumption of spherical symmetry, free from modeling bias. We demonstrate a decrease of gas temperature in the cluster outskirts, and also measure the gas entropy profile, both of which are obtained for the first time independently of X-ray spectroscopy, using Sunyaev-Zeldovich and X-ray imaging data. The contribution of the APEX-SZ systematic uncertainties in measuring the gas temperature at large radii is shown to be small compared to the XMM-Newton and Chandra systematic spectroscopic errors.
