Radio Studies of Circumnuclear Gas in Active Galaxies

Doctoral thesis, 2001

In this thesis we have used radio interferometry in order to perform high resolution spectral line studies of circumnuclear gas in AGN and starburst galaxies. Amongst the studied objects is the Seyfert 2 NGC5793, for which we find the high opacity HI absorption to be consistent with a 50-100 pc radius atomic disk aligned with the host galaxy disk. Such large scale HI disks are commonly observed in the radio quiet Seyferts. Disk-like distributions of gas are also frequently observed in radio loud objects. One example first revealed by HST observations is the ~240 pc radius circumnuclear disk of dust and gas in the FRI NGC4261. Consistent with this HST disk, our VLBI investigations of the HI absorption suggest the presence of a thin atomic disk at radius ~6 pc. Our results further agree with the fact that HI absorption in radio loud objects preferentially occurs in sources with symmetric pc-scale morphology. Even though disks probably are the most common cause of HI absorption, there are also examples where the HI instead is associated with jet-cloud interactions. This might be the case in the CSS 3C216, and is most likely the case for the FRII 3C234, both reported on in this thesis. The most favoured explanation to the compact sizes of GPS/CSSs is that they are young sources evolving in a power law density medium. Our studies of the HI absorption in these objects find that small GPS objects (<1 kpc) have larger HI column densities than the larger CSSs (1-10 kpc). This anti-correlation can be explained by both a spherical and a disk distribution with a radial power law density profile. Since most detections occur in galaxy classified objects, we argue that if the unified schemes apply to the GPS/CSSs a disk distribution is more likely. Our derived gas densities are consistent with those estimated from ram-pressure confinement of the lobes and VLBI measured expansion velocities, and these results therefore strongly support the youth model. Instead of accretion, some active galaxies are powered by intense starformation. One of the very few such sources imaged by VLBI so far is IIIZw35 for which, in our VLBI observations, we find compact continuum sources that are consistent with being either RSNe or SNRs. In addition a diffuse continuum component is observed which can be explained by emission from old SNRs. In this galaxy we have also mapped the OH megamaser emission and found a ring like distribution. Such a ring can account both for the diffuse as well as the compact masers observed, and is the best confirmation so far for the standard OH megamaser model, albeit on a more compact scale than expected.