Detection of a cosmic microwave background decrement towards a cluster of mJy radio sources

We present the results of radio, optical and near-infrared observations of the field of TOC J0233.3+3021, a cluster of milliJansky radio sources from the TexOx Cluster survey. In an observation of this field with the Ryle Telescope (RT) at 15 GHz, we measure a decrement in the cosmic microwave background (CMB) of −675±95 μJy on the RT's≈0.65 kλ baseline. Using optical and infrared imaging with the McDonald 2.7-m Smith Reflector, the Calar Alto 3.5-m telescope and the United Kingdom Infrared Telescope (UKIRT), we identify the host galaxies of five of the radio sources and measure magnitudes of R≈24, J≈20, K≈18. The cosmic microwave background (CMB) decrement is consistent with the Sunyaev–Zel'dovich (SZ) effect of a massive cluster of galaxies, which if modelled as a spherical King profile of core radius θC=20 arcsec has a central temperature decrement of 900 μK. The magnitudes and colours of the galaxies are consistent with those of old ellipticals at z∼1. We therefore conclude that TOC J0233.3+3021 is a massive, high redshift cluster. These observations add to the growing evidence for a significant population of massive clusters at high redshift, and demonstrate the effectiveness of combining searches for active galactic nuclei (AGN) ‘signposts’ to clusters with the redshift-independence of the SZ effect.