We have found two extreme GPS sources with peak frequencies of 20 and 30 GHz. The sources are both associated with X-ray bright, low redshift (z < 1) quasars. We have undertaken a search for similar sources using the Green Bank 5 GHz, NVSS 1.4 GHz surveys and follow-up observations with the Ryle Telescope at 15 GHz and the VLA up to 43 GHz.

Recently Readhead et al. (1996b) have presented empirical evidence that CSOs evolve into larger double sources many kpc in extent. The implications of this are explored analytically and numerically. It is found that pure luminosity evolution on the small scales of CSOs is unlikely in the presence of any ambient density gradients. However, the transition from CSO to MSO may involve a nearly constant lobe advance speed as suggested by Readhead et al.. This seemingly unphysical result can be seen to result from the interaction of the propagating jet with its surrounding medium.

A VLBA map of CTD 93 at 2 cm shows the morphology to be better described as a core-jet configuration than as a symmetric compact double structure.

PKS 1718–649 has a classic GPS spectrum and is identified with the galaxy NGC 6328. At a distance of 56 Mpc, PKS 1718–649 becomes the nearest known GPS radio source.

Using Hubble Space Telescope WFPC2 images of 3CR compact steep spectrum (CSS) radio sources and literature radio maps makes detailed comparison between optical and radio structures possible. All CSS sources for which the optical and radio orientation can reliably be measured, display good alignment between the optical and radio emission. This alignment is observed down to the lowest redshift in the sample, at z ~ 0.1. Optical emission from CSS galaxies is found to be dominated by this aligned component, especially at higher redshifts. Both the alignment and the relative brightness of this component indicate that CSS sources have large amounts of matter co-spatial with the radio structure. In order to assess the nature of the aligned light, we calculated line emission contamination in the passband using ground based spectra. From this we can conclude that at least in some sources (notably 3C 213.1, 3C346, and 3C 380) line emission is not important. The detailed optical–radio correspondence makes optical synchrotron the most likely mechanism for these three sources.

VLBI observations of IC 5063 are presented.

The Seyfert 1/starburst/IR galaxy Mrk 231 (UGC 08058), at z = 0.0422, has been imaged with the VLBA at frequencies ranging from 1.4 GHz to 22 GHz. The radio image shows a triple source of total size 50 mas (30h−1 pc), with a self-absorbed central component and two detached lobes. The northern lobe, located 12h−1 pc from the core, is free-free absorbed at the lowest frequencies; there also seems to be free-free absorption in the southern component. The linear scale over which this absorption takes place is several times larger than that seen in NGC1275/3C84.

The axis of the 50-mas radio source is approximately 60° from the elongation of the central component on the 1-mas scale. The spectrum of this central component becomes optically thin between 5 and 8.4 GHz, and resolves into a small triple source of size ~ 1.0h−1 pc at 15 GHz. There is no hint of a jet or other structure directly connecting this central component to the outer VLBI lobes.

We report on VLA and VLBI observations of the nucleus of the nearby spiral galaxy M81. The VLA observations show the flux density of the nucleus to be variable by 50%. The VLBI observations indicate that the structure of the nucleus of M81 is somewhat variable on timescales of weeks.

M81 has been shown to have a compact flat-spectrum core with a possible steep-spectrum jet. We report on position determinations of the brightness peak of the nucleus relative to the position of the early supernova 1993J with uncertainties as low as 0.08 mas. At early epochs, the supernova was largely pointlike at any frequency and therefore an ideal phase reference. We describe how VLBI astrometry at several frequencies could be used to support a model with a core and a one-sided jet for the nucleus of M81.

Many nearby galaxies show optical evidence for low-luminosity AGNs that are far less luminous than classical Seyfert nuclei and QSOs. LINERs, the most common variety of such emission-line objects, comprise ~ 1/3 of nearby galaxies, and may serve as an important “missing link“ between normal and Seyfert galaxies. To shed light on the physical origin of LINERs, which remains controversial, we are conducting a high-resolution continuum survey of a representative sample of galaxies using the VLA to search for compact radio cores. We additionally discuss the dramatic radio variability in the LINER nucleus of M81.

We present preliminary results from the observations of the Galactic Center compact source, Sgr A* at 3 and 2 millimeter wavelengths using the Nobeyama Millimeter Array to monitor flux density variations on timescales shorter than a month. Such high spatial resolution observations at millimeter wavelengths are important to shed more light on the origin of the variability and the nature of this compact source. Our observations indicate the flux density varies at least by ~30% in one to two weeks at 3 mm.

The VLA was used at 1.5-2 arcsec resolution at 20cm to search for compact continuum sources in the inner regions of 374 UGC galaxies.

We present radio interferometric observations of a well defined sample of IRAS galaxies with warm far-infrared colors — 60 µm Peakers (60PKs). The core radio power of 60PKs is intermediate to that of “normal” Seyfert 2 galaxies and radio ellipticals, and follows the same relationship with respect to total radio emission as low and high power radio galaxies. This is consistent with the suggestion that 60PKs represent nascent radio elliptical galaxies.

The evidence for a subparsec-scale disk in the nucleus of NGC 4258 is reviewed in the context of multi-epoch VLBA observations. The detection of jet continuum emission and of maser proper motions in these data bolster the case for a nuclear disk.

We propose that the observed high-velocity maser emission from circumnuclear disks arises in spiral shocks, which can naturally account for several peculiar findings in the spectrum and spatial distribution of the maser emission.

We present multi-frequency observations towards the nucleus of NGC 3079 using both Japan VLBI Network (J-Net) and the VLBA including phased VLA and Effelsberg. We detected two continuum components at 1.4 and 8.4 GHz, and determined the spectral indices for each component. Further, we found several H ɪ absorption features towards the continuum components whose velocity gradient is opposite to the galaxy rotation. Assuming rotation, the central mass is estimated to be 3 × 106M⊙. The water maser distribution extends in North-South direction along the galactic disk, while no clear indication of a Keplerian rotating disk was found.

We report the first VLBI images of the NGC 1068 H2O maser that include the full 600 km s−1 spread of the emission. The structure of the source is suggestive of an edge-on massive disk that is probably bound by the central engine of this AGN.

We present VLBA observations of the H2O megamasers in the LINER galaxy NGC 1052. The structure we observe suggests a different origin for the masers than those seen in NGC 4258.

Calculations are summarized for the spectral and spatial distribution of maser radiation that emerges from a turbulent Keplerian disk when viewed nearly edge-on. A close comparison is made with the refined observational data about the masing accretion disk around the presumed massive black hole at the nucleus of the galaxy NGC 4258.

Global VLBI imaging of the luminous IR galaxy Arp 220 reveals four major emission regions in the 1667 MHz line, each with complex spatial and velocity structure showing intriguing symmetries, suggestive of an AGN-related phenomenon. The continuum emission consists of about a dozen sub-mJy unresolved features which appear unrelated to the maser emission, and are interpreted as radio supernovae in an ongoing nuclear starburst.