EDP Sciences
Free access
Volume 417, Number 3, April III 2004
Page(s) 925 - 944
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20040019

A&A 417, 925-944 (2004)
DOI: 10.1051/0004-6361:20040019

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

E. Middelberg1, A. L. Roy1, 2, N. M. Nagar3, 4, T. P. Krichbaum1, R. P. Norris5, A. S. Wilson6, H. Falcke1, 7, E. J. M. Colbert8, A. Witzel1 and K. J. Fricke9

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

(Received 28 November 2002 / Accepted 14 January 2004)

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 $(0.92\pm0.07)~c$ between the two central components separated by 282 pc and, in  NGC 5506 , we find a $3~\sigma$ upper limit of  0.50 c for the components separated by  $3.8~{\rm pc}$. 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  $0^{\circ}$ and  $90^{\circ}$. The frequent misalignment could result from jet precession or from deflection of the jet by interaction with gas in the interstellar medium.

Key words: galaxies: seyfert -- galaxies: active -- galaxies: jets

Offprint request: E. Middelberg, emiddelb@mpifr-bonn.mpg.de

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© ESO 2004