Extragalactic radio sources with very large Faraday rotation

Abstract

There are several radio sources with Faraday rotation measures larger than 500 rad m−2 (ref. 1). Because most of these sources are located near the galactic plane, the large rotation measures can be attributed to the galactic medium2. However, a few sources at high galactic latitude also show large rotation, implying intrinsic Faraday rotation within the source. Until now, no sources have been discovered to have a rotation measure > 1,000 rad m−2. Using a multichannel polarimeter at 10 GHz attached to the 45-m radio telescope at Nobeyama, we have observed about 100 extragalactic radio sources which were suspected to have large rotation from our polarization catalogue1. Four of these 100 sources were found to have an intrinsic rotation measure > 1,000 rad m−2. Here we report rotation measures which are two orders of magnitude larger than those observed so far for most extragalactic radio sources. Two of these four sources are classified as compact steep-spectrum sources.