The Galactic Magnetic Field in the Quasar 3 C 216

Multifrequency polarimetric observations made with the Very Long Baseline Array of the quasar 3C 216 reveal the presence of Faraday rotation measures (RMs) in excess of 2000 rad m−2, in the source rest frame, in the arc of emission located at ∼ 140 mas from the core. Rotation measures in the range −300 – +300 rad m−2 are detected in the inner 5 mas (∼ 30 parsecs). While the rotation measure near the core can be explained as due to a magnetic field in the narrow line region, we favor the interpretation for the high RM in the arc as due to a “local” Faraday screen produced in a shock where the jet is deflected by the interstellar medium of the host galaxy. Our results indicate that a galactic magnetic field of the order of ∼ 50 μG on a scale > 100 pc must be present in the ambient medium. Subject headings: galaxies:active — galaxies:individual(3C 216=0906+430) — galaxies: jets — radio continuum: galaxies — galaxies:ISM — galaxies: nuclei 1. Polarimetric Parsec-Scale Observations of High RM Radio Sources While typical extragalactic radio sources have Faraday rotation measures (RMs) of order 10 rad m−2 at arcsecond resolution, there exists ∼30 sources with RMs in excess of 1000 rad m−2 (see Taylor, Barton & Ge 1995; Carilli et al. 1997; Athreya et al. 1998). Approximately 40% of such sources are unresolved at arcsecond resolution. The parsec-scale study of the RM distribution in these radio sources exhibiting an excess of RM is important in order to understand the scale at which such high RMs originate, and to explain the origin of the phenomenon itself. High rotation measures could originate either within the source itself, given a sufficiently dense thermal component, or they could be due to an external Faraday screen of thermal gas and magnetic field. IRA-CNR, Via Gobetti 101, I-40129 Bologna, Italy; [email protected] NRAO, Box 0, Socorro, NM 87801, USA; [email protected] – 2 – A large fraction of radio sources with high RM on the arcsecond scale, i.e. ∼ 40% of the total, are compact steep spectrum radio sources (CSSs), while the rest are extended radio galaxies. For a few extended radio galaxies, such as Cygnus A and Hydra A, both located at the center of clusters of galaxies, it has been proposed that a highly magnetized intracluster gas is responsible for the high RM detected (see respectively Dreher, Carilli & Perley 1987 and Taylor & Perley 1993). Recent polarimetric observations of a few high RM radio sources carried out at parsec-scale resolution with the Very Long Baseline Array (VLBA) revealed that the RM distribution differs considerably from source to source. In the CSS quasar OQ172 (Udomprasert et al. 1997) the high RM observed on the arcsecond scale originates within the central parsecs, where values around ∼ 40000 rad m−2 are observed. It is therefore likely that the nuclear environment (i.e. the Narrow Line Region, or NLR) is responsible for the high RM observed. Taylor (1998) studied 4 quasars with normal to low RMs on the arcsecond scale, and for three of them found RMs in excess of 1000 rad m−2 within a few mas of the core, while the RM drops abruptly by over an order of magnitude farther along the parsec-scale jet. Again, it is likely that the nuclear environment is responsible for the RM distribution observed. Similar results were obtained also for 3C 138 (Cotton et al. 1997). Finally Aaron et al. (1998) studied the RM distribution in the CSS quasar 3C 309.1, and found small RMs and little depolarization, and concluded that the NLR environment in this quasar is “normal” and uniform. The quasar 3C 216 was found to have high RMs in the single dish surveys of Tabara & Inoue (1980) and in VLA observations made by Taylor, Ge & O’Dea (1995). In this paper we present and discuss the parsec-scale RM distribution based on multifrequency polarimetric VLBA observations carried out at 3.6 cm and 6 cm. 3C 216 is associated with an optically polarized quasar located at z=0.668. This object is peculiar, since it exhibits properties typical both of blazars and of CSS sources. Detailed observations carried out at parsec-scale resolution (Barthel et al. 1988, Venturi et al. 1993) revealed superluminal motion in the inner part of the VLBI jet, which argues in favor of a small orientation of the VLBI structure to the line of sight. The parsecand kiloparsec-scale radio emission are misaligned by ∼ 110◦. High frequency VLA polarimetric observations (Taylor, Ge & O’Dea 1995) showed that the source is strongly polarized, and that the high RM originates mainly in the arcsecond core and in the northeastern component. In particular their results indicate that the polarized flux in the nuclear components peaks at ∼ 150 mas southeast of the core. A Hubble constant H0 = 65 km sec −1 Mpc−1 and q0 = 0.5 will be used throughout this paper. With this choice of the cosmological parameters, at the distance of 3C 216 1 mas corresponds to 6.1 parsec.