A Radio Survey for Linear and Circular Polarization in Low-Luminosity Active Galactic Nuclei
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Abstract
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 x 10^4 rad m^-2 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.