AGN in Seyfert galaxies
Seyfert galaxies as a class were identified by Carl Seyfert in 1943 (ADS abstract). He noticed that some spiral galaxies had extraordinarily bright nuclei with emission lines broader than normal galaxy rotation would allow (several 1000 km/s). Similar to powerful radio galaxies and quasars, supermassive black holes have been found in many Seyferts, but they produce only little radio emission and so are radio-quiet objects. The reason for this is largely unknown, and many explanations have been suggested.
I am trying to collect some observational evidence to decide whether the radio-quietness is intrinsic to the AGN (i.e., circumnuclear material is ejected with less energy than in quasars), or whether the ejecta are slowed down by interactions with the surrounding medium. Proper motion measurements with VLBI can help to measure jet speeds close to the AGN, and there is a trend that these speeds are sub-relativistic even very close to the AGN, and so intrinsically slow, compared to the highly relativistic jets in quasars.
On the other hand, there is some evidence that a significant fraction of Seyferts have flat or inverted radio spectra, which is probably caused by free-free absorption in a foreground medium. This indicates that there is a lot of material in the AGN's vicinity, possibly acting on the radio ejecta. Furthermore, there is no obvious correlation between the position angles of the radio ejecta on pc scales and on kpc scales, indicating that the ejecta are deflected on scales of 10 pc to a few hundred pc.
I was involved in observations of the Seyfert 2 galaxy NGC3079 during four epochs. We found that the components are moving with respect to each other, but unfortunately, do not exactly know which of the components moves in which direction with respect to the sky (this is a consequence of the technique of self-calibration in VLBI). There is solid evidence for the location of the AGN through the velocity dispersion of water maser emission, but assuming that this point is stationary yields relative motions which are counter-intuitive.
Other projects include a single-dish flux density monitoring of a loosely defined sample of Seyferts with the Effelsberg telescope and further studies of NGC3079 and Mrk1210.
Collaborators: Alan Roy, Thomas Krichbaum, Neil Nagar, Andrew Wilson, Heino Falcke, Ed Colbert, Ray Norris, Arno Witzel, Klaus Fricke
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A four-colour VLBI image of the Seyfert 2 galaxy NGC3079. The frequencies 1.7 GHz, 2.3 GHz, 5.0 GHz and 15.4 GHz are represented as red, yellow, green and blue, respectively. The different colours indicate the brightness of the radio-emitting regions, and the colour gradient from left to right (i.e., from east to west) may indicate substructure in a foreground free-free absorber.
Last modified: Thu Apr 1 20:01:11 CEST 2004