4. Summary and discussion
We found a close connection between the spectral and structural evolution of the radio outburst in III Zw 2. While the self-absorption frequency remained constant, we observed no change on VLBA-scales. The excellent consistency of the relative sizes during the first three epochs on the few micro-arcsecond level demonstrates the good quality of our data. The quick drop in peak frequency after 1998 November marked the beginning of a strong structural change. The spectral peak dropped from 43 GHz to 15 GHz within a few months and the VLBA-maps show a rapid expansion with an apparent expansion velocity of 1.25 c. The fact that spectral and spatial evolution are so closely linked also demonstrates that we are dealing with real physical expansion.
With its faint extended radio structure and its spiral host galaxy III Zw 2 is clearly not a radio-loud quasar, but has properties very typical of luminous Seyfert galaxies or radio-quiet quasars. The detection of superluminal motion in this galaxy now clearly shows that we are dealing with a relativistic jet on sub-pc scales. To our knowledge this is the first detection of superluminal motion in a spiral galaxy with a Seyfert nucleus. The maximum aspect angle of from the superluminal motion (see Sect. 3) is in good agreement with the orientation based unified scheme (e.g. Antonucci 1993) for AGN where Seyfert I galaxies are seen under intermediate or small aspect angles, so that the nucleus is not obscured by a dusty torus.
For the question of the nature of the radio-loud/radio-quiet dichotomy this means that radio-weak and radio-loud quasars can indeed have central engines that are in many respects very similar. Their optical properties are almost indistinguishable and both types of quasars can produce relativistic jets in their nuclei. The finding of superluminal motion supports the hypothesis of Miller et al. (1993) and Falcke et al. (1996a) that RIQs are relativistically boosted intrinsically radio-weak AGN.
While these general conclusions seem to be fairly robust, some characteristics of the outburst need to be modeled in more detail. The initial phase of the flux density rise with its millimeter-peaked spectrum and no detectable expansion perhaps has to be explained similar to the physics of Gigahertz-Peaked-Spectrum (GPS) sources, i.e. ultra-compact hotspots pumped up and powered by a jet interacting with the interstellar medium or the torus. The rapid expansion thereafter could have marked the phase where the jet breaks free and starts to propagate relativistically into a lower-density medium. Another explanation of the initial phase could be a fast jet moving through quasi-stationary components as proposed for sources like 4C 39.25 (e.g. Alberdi et al. 1993). In any case, this dramatic structural change should go together with a change of the polarization vector. This must be checked by future experiments.
The initial slow expansion has also possible implications for the interpretation of other Seyfert galaxies. For example, in observations of the Seyfert galaxies Mrk 348 and Mrk 231 (Ulvestad et al. 1999) only sub-relativistic expansion was found. Hence, one could raise the question whether the region of relativistic expansion in these two sources is at even smaller scales or whether they just happened to be in a `slow' phase, similar to III Zw 2 early on. If true, it is still possible that Seyfert jets are launched relativistically but are slowed down and disrupted significantly already on the sub-parsec scale. Therefore it would be important to follow the spectral evolution of these sources in more detail.
© European Southern Observatory (ESO) 2000
Online publication: June 5, 2000