A Radio Survey for Linear and Circular Polarization in Low Luminosity Active Galactic Nuclei

We conducted a Very Large Array survey of eleven low luminosity active galactic nuclei for linear and circular polarization at 8.4 GHz. We detected circular polarization in one source (M81*) and linear polarization in 3 sources. Sensitivity limits were ∼ 0.1% for both modes of polarization in 9 of 11 sources. The detections confirm the importance of nonthermal emission in LLAGN. However, detection rates for circular and linear polarization are lower for these sources than for more powerful AGN. Fractional linear polarization in detected sources is also lower than in more powerful AGN. The weak linear polarization in the survey sources indicates their overall similarity to Sgr A*. Confusion with thermal sources, depolarization and weaker, less extended jets may contribute to these differences. We detect a rotation measure & 7 × 10 rad m for NGC 4579. This may arise from magnetized plasma in the accretion, outflow or interstellar regions. Inverted spectra are present in both M81* and Sagittarius A* and absent from all sources in which circular polarization is not detected. This suggests that optical depth effects are important in the creation of circular polarization. Subject headings: galaxies: active — galaxies: individual (NGC 3031, NGC 3516, NGC 4278, NGC 4374, NGC 4486, NGC 4552, NGC 4579, NGC 4589, NGC 5216, NGC 6500, I 1024) — radio continuum: galaxies — polarization — radiation mechanisms: non-thermal — techniques: polarimetric National Radio Astronomy Observatory, P.O. Box O, 1003 Lopezville, Socorro, NM 87801; [email protected] Radio Astronomy Laboratory, University of California, Berkeley, CA 94720; [email protected] Max Planck Institut für Radioastronomie, Auf dem Hügel 69, D 53121 Bonn Germany; [email protected] Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16803; [email protected]