The need for high angular resolution is emphasised, especially in the context of programs to understand massive black holes and the processes in their environment.

Observations over the past decade have verified, beyond reasonable doubt, that most galactic nuclei contain massive black holes. Hole masses are being measured and firm evidence for spin is being sought. Attention is now returning to the study of how gas flows around black holes and how energy is released both from the accreting gas and from the hole itself in the form of radiation, relativistic jets and non-relativistic, hydromagnetic winds. Some of the different possibilities currently under investigation are briefly reviewed and some recent clues from radio scintillation, polarization and X-ray observations are discussed. It is argued that observations of persistent circular polarisation in Sgr A* support the presence of an ordered disk magnetic field. It is also conjectured that adiabatic, sub- and super-critical accretion flows are demand-limited, not supply-driven and are associated with large mass outflow as appears to be the case in Sgr A*. This principle may have to be modified when a massive black hole forms in a protogalaxy and this modification may account for the proposed hole mass-bulge velocity dispersion relation. The final stages of this process may release sufficient wind energy from the nucleus to prevent disk formation.

I review recent numerical and analytic work on the magnetohydrodynamic (MHD) model of jet formation in active galactic nuclei, with an emphasis on producing the highly relativistic outflows and high radio luminosities observed in the most powerful sources.

Measurements of the proper motions and radial velocities of stars in the central cluster of the Milky Way have revealed the presence of a 2-3 million solar mass black hole at the position of the compact radio source Sagittarius A* (SgrA*). The overall stellar motions do not deviate strongly from isotropy and are consistent with a spherical isothermal stellar cluster. Speckle spectroscopy with SHARP at the NTT and slit spectroscopy with ISAAC at the VLT suggests that several of them are early type stars. This is consistent with the idea that these stars are members of an early type cluster with small angular momentum and therefore are now in the immediate vicinity of SgrA*. Most recent data now allows to measure the curvatures of the stellar orbits for a few of the stars that are closest to the center and have the largest proper motions of up to 1400 km/s. The curvatures indicate that the stars indeed orbit the central compact object and will allow to further determine its mass and compactness.

We show and discuss results and prospects of high-resolution imaging of the supermassive black hole candidate Sgr A*. We also briefly review the latest observational and theoretical progress for this source. The latest millimeter-VLBI observations show compact radio emission from within a region of about 15 Schwarzschild radii. This compact component is most likely responsible for the so-called sub-mm bump in the spectrum and perhaps even for the recently discovered circular polarization discovered up to 43 GHz and some X-ray emission through synchrotron self-Compton emission. Most importantly, however, the sub-mm emission from Sgr A* opens the door to observe, for the first time, the event horizon of a black hole directly with VLBI at sub-mm wavelengths.

At the center of the Milky Way lurks a unique compact nonthermal radio source, Sgr A*. It is thought to be powered by a 2.6 × 106 solar mass black hole that is accreting the stellar winds from the numerous early-type stars that exist in the central parsec. However, until recent high resolution Chandra observations, Sgr A* had never been unequivocably detected at wavelengths shorter than the sub-millimeter. We present a spherical accretion model which is consistent with both the flux and steep spectral shape of the X-ray emission from Sgr A*.

We present a map of the Galactic center in the J=l-0 line of SiO covering the region mapped with the ASCA satellite in the 6.4 keV Fe line. We find a correlation between the spatial distribution of the Fe 6.4 keV line and the SiO emission. The SiO abundance increases by more than a factor of 20 in the regions with strong Fe 6.4 keV line. This indicates that the Fe 6.4 keV line mainly arises from molecular clouds with large gas phase abundance of refractory elements. We discuss the implications of the correlation on the origin of the hard X-rays, and the heating and the chemistry of the molecular clouds in the GC.

As part of the Chandra GTO program we are monitoring and surveying M31 using the HRC and ACIS cameras. These observations have resolved the nuclear X-ray source into five separate sources, one of which is very soft and may (or may not!) be associated with the central super-massive black hole. In addition, the superb spatial resolution and low scattering of the Chandra telescope allows us to unambiguously resolve the diffuse emission from the point sources. This emission is clearly softer than the point sources, and also increases with temperature radially. The monitoring nature of the observations allows detailed study of the variability of the point sources.

The spiral galaxy M81 hosts the closest extragalactic AGN besides that in Cen A that can be imaged with VLBI. Ground and Space-VLBI observations have shown that M81's center consists of a stationary core and a short, jittery, one-sided jet. The jet appears wavy, reminiscent of a helical structure. The central engine of M81 has qualitative similarities to those of powerful AGN of radio galaxies and quasars, and may represent a scaled-up version in power and size of the largely hidden nucleus of our own Galaxy.

The physical conditions in the inner parsec of accretion disks believed to orbit the central black holes in active galactic nuclei can be probed by imaging the absorption of background radio emission by ionized gas in the disk. High angular resolution radio observations of several nearby galaxies at multiple frequencies have revealed evidence for free-free absorption by disks or tori of ionized gas. The depth and angular width of the absorption increases with decreasing frequency. The longest possible baselines are needed to provide adequate angular resolution at low frequencies where the effects of free-free absorption are most evident. Recent results from VSOP as well as ground-based VLBI observations of the nearby galaxy NGC 4261 illustrate the critical importance of high angular resolution at frequencies below 10 GHz.

VLBA observations of the two-sided emission structures in 3C 84 have been used to study the ionized gas in the vicinity of the presumed accretion disk on parsec scales. Strong free-free absorption is seen with a radial gradient, but only upper limits have been obtained on a stimulated recombination line at 23 GHz.

A discrete thin rotating emission-line disk is ideal for determining the mass of a supermassive black hole in the nucleus of a galaxy. One can use high resolution imaging in combination with a matrix of spatially resolved velocity measurements to confirm the simple geometry and kinematics of the disk. If the disk remains bright and orderly beyond the sphere of influence of the black hole, with motion driven predominantly by gravitational force, one can also measure the density of the surrounding matter in the nucleus. This is being done for the bright Liner galaxy NGC 3998.

Motivated by STIS observations of more than 50 nearby galactic nuclei, we consider long-slit emission-line spectra when the slit is wider than the instrumental PSF, and the target has arbitrarily large velocity gradients. The finite width of the slit generates complex patterns in the spectra that can be misinterpreted as coming from various physically distinct nuclear components, but when interpreted correctly, they can have considerable diagnostic power. For a thin disk in circular motion around a central galactic black hole (BH), a characteristic artifact occurs in the spectrum at the outer edge of the BH's sphere of influence. It betrays the presence of a BH, and allows us to develop a new method for estimating its mass, which gives higher sensitivity to BH detection than traditional methods.

We describe an on-going HST program aimed at determining the relationship between the nuclear black hole mass and bulge mass in spiral galaxies. We have selected a volume limited sample of 54 nearby spiral galaxies for which we already have ground based emission line rotation curves, CCD surface photometry and radio maps. We are now obtaining HST/STIS longslit observations of each of the galaxies in the sample in order to determine the nuclear Hα rotation curve at high (∼ 0.1) spatial resolution. We will use these data to measure the unresolved dark mass concentration at the nucleus of each object. Here we show the first results from observations of objects in the sample.

We have observed a complete sample of 21 nearby (D < 70h−1Mpc) Fanaroff & Riley Type I galaxies with HST/WFPC2 and detected dust disks and lanes in 19 of them. The radio jets are roughly perpendicular to the dust which is used to constrain the Doppler boosting factors of the radio jet and cores. The VLBA core flux correlates with the central Hα+[NII] flux which might indicate that the VLBA core is dominated by an isotropic component. Twelve galaxies show nuclear optical sources. We discuss various possible origins for this emission.

We describe preliminary results from our study of multi-scale structures in Centaurus A (NGC 5128) obtained using the Chandra X-ray Observatory HRC-I observations. The high-angular resolution Chandra images reveal X-ray multi-scale structures in this object with unprecedented detail and clarity. The region surrounding the Cen A nucleus, believed to be associated with a supermassive black hole, shows structures on arcsecond scales clearly resolved from the central source.

What causes the dichotomy between very powerful and very weak radio emission from AGNs? Perhaps the engines are the same but the jets get disrupted by dense ISM in radio-quiet objects, or else the engines are intrinsically different with jet power scaling with, say, black hole spin. To distinguish, one can look for interaction between the jets and the NLR and measure the jet speed close to the core using VLBI, before environmental effects become important. We find that in radio-quiet AGN, the jets appear slower and have a greater tendency to bend, and that one-sidedness and flat-spectrum cores are probably due to obscuration.

Parsec-scale VLBA imaging of five Seyfert galaxies with flat-spectrum radio nuclei was conducted to determine whether the flat spectrum represents thermal emission from the accretion disk/obscuring torus or nonthermal, synchrotron self-absorbed emission. Four of the five show emission consistent with synchrotron self-absorption, with intrinsic sizes ∼0.05-0.2 pc (or 104 gravitational radii for a 108 M⊙ black hole for the smallest). In contrast, NGC 4388, which was detected with MERLIN but not the VLBA, shows thermal emission with similar properties to that detected in NGC 1068. It is notable that the two Seyfert galaxies with detected thermal nuclear radio emission both have large X-ray absorbing columns, suggesting that columns in excess of ∼1024 cm−2 are needed for such disks to be detectable.

We present results of our study of the parsec-scale components of radio galaxies as observed at radio, optical and X-ray wavelengths, in the framework of the Unified Scheme.