Motion and properties of nuclear radio components in Seyfert galaxies seen with VLBI

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: emiddelb;aroy;tkrichbaum;awitzel@mpifr-bonn.mpg.de
² Geodätisches Institut der Universität Bonn, Nussallee 17, 53115 Bonn, Germany
³ Kapteyn Institute, University of Groningen, Landleven 12, 9747 AD Groningen, The Netherlands e-mail: nagar@astro.rug.nl
⁴ INAF, Arcetri Observatory, Largo E. Fermi 5, Florence 50125, Italy
⁵ Australia Telescope National Facility, PO Box 76, Epping, NSW, Australia e-mail: rnorris@atnf.csiro.au
⁶ Department of Astronomy, University of Maryland, College Park, MD 20742, USA e-mail: wilson@astro.umd.edu
⁷ ASTRON, PO Box 2, 7990 AA Dwingeloo, The Netherlands e-mail: falcke@astron.nl
⁸ John Hopkins University, Department of Physics and Astronomy, 3400 North Charles Street, Baltimore, MD 21218, USA e-mail: colbert@pha.jhu.edu
⁹ Universitäts-Sternwarte, Geismarlandstrasse 11, 37083 Göttingen, Germany e-mail: kfricke@uni-sw.gwdg.de

Corresponding author: E. Middelberg, emiddelb@mpifr-bonn.mpg.de

Received: 28 November 2002
Accepted: 14 January 2004

Abstract

We report EVN, MERLIN and VLBA observations at 18 cm, 6 cm and 3.6 cm of the Seyfert galaxies NGC 7674, NGC 5506, NGC 2110 and Mrk 1210 to study their structure and proper motions on pc scales and to add some constraints on the many possible causes of the radio-quietness of Seyferts. The component configurations in NGC 7674 and NGC 2110 are simple, linear structures, whereas the configurations in NGC 5506 and Mrk 1210 have multiple components with no clear axis of symmetry. We suggest that NGC 7674 is a low-luminosity compact symmetric object. Comparing the images at different epochs, we find a proper motion in NGC 7674 of between the two central components separated by 282 pc and, in NGC 5506, we find a upper limit of for the components separated by . Our results confirm and extend earlier work showing that the outward motion of radio components in Seyfert galaxies is non-relativistic on pc scales. We briefly discuss whether this non-relativistic motion is intrinsic to the jet-formation process or results from deceleration of an initially relativistic jet by interaction with the pc or sub-pc scale interstellar medium. We combined our sample with a list compiled from the literature of VLBI observations made of Seyfert galaxies, and found that most Seyfert nuclei have at least one flat-spectrum component on the VLBI scale, which was not seen in the spectral indices measured at arcsec resolution. We found also that the bimodal alignment of pc and kpc radio structures displayed by radio galaxies and quasars is not displayed by this sample of Seyferts, which shows a uniform distribution of misalignment between and . The frequent misalignment could result from jet precession or from deflection of the jet by interaction with gas in the interstellar medium.