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J.S. Ulvestad, J.M. Wrobel, A.L. Roy (NRAO), A.S. Wilson (UMd), H. Falcke, T. Krichbaum, G. Bower, A. Zensus (MPIfR)
The two Seyfert galaxies Mrk~231 and Mrk~348 have been imaged at two epochs with the Very Long Baseline Array, with separations of approximately 1.7~yr. Mrk~231 is a Seyfert 1 and Ultra-Luminous Infrared Galaxy, while Mrk~348 is a Seyfert 2 galaxy with a hidden broad-line region observed in polarized light. In addition to two-sided radio structures on scales of tens of parsecs, both contain central double structures that are apparently one-sided ``jets'' less than a parsec in length. These double sources have been imaged with resolutions of 0.3~pc in Mrk~231 and 0.1~pc in Mrk~348. The nuclear components in both galaxies can be identified by their radio spectral properties and their strong high-frequency variability between the two epochs. All components have brightness temperatures Tb\geq 109~K, indicating the dominance of synchrotron emission rather than radiation from the accretion torus or from star formation.
Comparison of 15-GHz images yields separations increasing by 0.1~mas between epochs for the central double sources in both galaxies, corresponding to apparent linear velocities of 0.2c for Mrk~231 and 0.05c for Mrk~348 (assuming H0 = 75). Upper limits are approximately twice the measured speeds (more detailed error analysis is under way), indicating that both galaxies have apparent jet speeds far below c on sub-parsec scales. These results may indicate that Seyfert galaxies have jets that are initially slower than those in many radio galaxies and quasars, rather than having rapid jets that are slowed or disrupted in the narrow-line region. The low speeds, when combined with the apparently one-sided jets (sidedness ratios >10:1) imply that the radiation may be intrinsically one-sided, that one side is completely obscured (possibly by strong free-free absorption), or that the measured motions represent a pattern speed rather than the true jet velocity.
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