The radio source 0108 + 388 is a canonical example of a class of extragalactic radio sources, referred to as Gigahertz-Peaked Spectrum (GPS) sources, whose spectra peak at high frequencies. There are two competing models for the cause of the high frequency turnover: free-free absorption (f-f) of the lower frequency radiation by ionized gas in the host galaxies (e.g. van Breugel 1984), or synchrotron self-absorption (SSA) due to exceptionally large magnetic fields, (e.g. Hodges, Mutel, & Phillips 1984).

NGC 3079 has very luminous water megamaser from the nucleus, the peak of the spectrum being blueshifted by 180 km s−1 from the systemic velocity of the galaxy (Vsys = 1131 km s−1) (Henkel et al. 1984, Haschick & Baan 1985). Core-jet like continuum structure is also found in the nuclear region (Irwin & Seaquist 1988). No velocity drift for main features of water maser (VLSR = 941–975 km s−1) has been shown (Nakai et al. 1995). However, the drift was recently detected for the maser of 1190 km s−1 (Nakai 1997). HI and OH absorptions are detected in the nucleus (Haschick & Baan 1985, Irwin & Seaquist 1991). Thus this galaxy is very unique object to investigate water masers, continuum structure and absorption features all together with VLBI.

Megamasers provide a powerful tool to probe the innermost few parsecs of active galactic nuclei (AGNs). In particular, for AGNs with large inclination angles, only maser sources enable to directly study the structure and dynamics of the region inside the supposed obscuring tori, which are considered to be optically thick from the infrared to the X-ray bands. For example, the VLBI observations of the water megamaser sources at the center of NGC 4258 have provided not only the compelling evidence for the presence of a central supermassive black hole but also the detailed information on the spatial and velocity distribution of the maser sources (Miyoshi et al. 1995; Nakai et al. 1995).

In the Autumn of 1991, Nakai et al.(1993) began monitoring observations of water mega masers including NGC4258 using Nobeyama 45 m telescope. The 45 m telescope was equipped with wide band cooled HEMT receiver (22 GHz) and wide band spectrometers (AOS). Nakai used 8 AOSs which covered 4480 km/sec in velocity. It was from the wide-band system that the high velocity maser components were detected unexpectedly which locate ±1000 km/sec from the known features of the NGC4258 (Claussen et al. 1984). The finding was in May 1992 when busy Nakai got time checking the data deeply, half year had passed since the beginning of the observations.

We argue that the velocity dispersion of the stars is likely a monotonically decreasing function of radius along the minor axis in the central regions of cuspy galaxies. We show that then a central mass concentration (a black hole) must be present and calculate its minimum mass, M⊙. This lower bound is relevant and entirely consistent with current mass estimates.

The force-free approximation is a very useful method in studies of the global electromagnetic structure of the magnetospheres around the relativistic astronomical bodies. However, its application has almost been restricted to the stationary and axisymmetric configuration. In this work, we remove this restriction and construct a general method for treating the force-free electromagnetic field.

We observed the nuclear region of NGC 4261 (3C270) with the VLBA to determine the morphology of the central radio source on parsec scales. Our highest angular resolution image at 8.4 GHz shows a very narrow gap in emission just east of the radio core (on the counterjet side), which we interpret as an absorption feature caused by a small, dense inner accretion disk whose width is less than 0.1 parsec. If the inclination of this inner disk is close to that of the much larger-scale disk imaged by HST, it becomes optically thin to 8.4 GHz radiation at a deprojected radius of about 0.8 pc. September 1997 VLBA observations at higher frequencies should allow us to determine the radial electron density distribution of the inner disk.

The radiative winds from a geometrically thin accretion disk are studied. The effect of radiation drag which causes in the intense radiation fields around the accretion disk is examined recently. Then, we numerically consider the radiatively-accelerated accretion-disk winds which consist of ionized gas particles, taking into account radiation drag of the order of v/c.

The effects of large-scale galactic winds in active galaxies may be far-reaching. It has been suggested that the Hubble sequence can be understood in terms of a galaxy's greater ability to sustain winds with increasing bulge-to-disk ratio. The large-scale circulation of gas associated with these galactic winds might help explain the mass-metallicity relation between galaxies and the metallicity-radius relation within galaxies. Galactic winds probably contribute non-negligibly to the cosmic X-ray background and may be involved in the quasar absorption-line phenomenon. The cosmological implications of the wind phenomenon have been widely explored in the context of proto-galaxies and quasars. The extremely energetic galactic winds that were likely associated with galaxy formation almost certainly played a key role in heating and ionizing the intergalactic medium at high redshifts and may have created the seeds for the large-scale structure we see today.

High spatial resolution, near-infrared imaging and spectroscopy of the nuclear star cluster obtained in the last few years have given key new insights about the structure, evolution and mass distribution in the Milky Way Center. The central parsec is powered by a cluster of hot, massive stars. Their characteristics imply that there was an active phase of star formation a few million years ago, probably triggered by the infall and collapse of a very dense gas cloud. Other such starburst episodes may have taken place between 100 and 300 million years ago. Measurements of radial and proper motions for more than 200 stars show that stellar velocities increase with a Kepler law down to a scale of a light week from the compact radio source Sgr A∗. The data make a compelling case for the presence of a compact, central dark mass of about 2.6×106 M⊙. Simple physical considerations show that this dark mass cannot consist of a stable cluster of stars, stellar remnants or substellar condensations. Energy equipartition requires that at least five percent of the dark mass (≥105 M⊙) must be associated with Sgr A∗ itself and likely is enclosed within less than 8 light minutes. If one accepts these arguments it is hard to escape the conclusion that Sgr A∗ is indeed a massive black hole at the core of the Milky Way.

Although the notion that the Milky Way galaxy contains a supermassive central black hole has been around for more than two decades, it has been difficult to prove that one exists. The challenge is to assess the distribution of matter in the few central parsecs of the Galaxy. Assuming that gravity is the dominant force, the motion of the stars and gas in the vicinity of the putative black hole offers a robust method for accomplishing this task, by revealing the mass interior to the radius of the objects studied. Thus objects located closest to the Galactic Center provide the strongest constraints on the black hole hypothesis.

In 1918, Harlow Shapley first noted that globular clusters were concentrated toward the constellation of Sagittarius, and hence the Sun was not near the center of the Milky Way. Since that time astronomers have expended considerable effort to determine the distance to the center of the Milky Way, because any change in the value of this distance, R0, has a widespread impact on astronomy and astrophysics. Beginning in 1991, we have conducted observations with the VLBA designed to make possible a program to measure the distance to the Galactic Center via a trigonometric parallax. This could be accomplished with the VLBA using Sgr A∗ as a phase reference for one or more (weaker) compact extragalactic sources. A time series of measurements of the position of Sgr A∗ relative to an extragalactic source should show the effects of the annual ≈ ±0.12 mas signature of the Earth's orbit around the Sun (trigonometric parallax), as well as the ≈ 6 mas yr−1 secular motion caused by the Sun's orbit around the Galactic Center.

Sgr A∗, the enigmatic compact nonthermal radio source located at the center of the Galaxy for many years has been considered as the signpost of a massive black hole (Rees 1982; Lo 1986; Falcke et al. 1997). Its properties are unique in the Galaxy, but it resembles other nuclear radio sources (Lo 1993). Efforts to delineate the source structure of Sgr A∗, in order to constraint the nature of the underlying energy source, have been ongoing since 1975 (Lo et al. 1975).

Time variability of Sgr A∗ at cm wavelengths is well known phenomenon and has been studied by several authors (e.g. Lo 1989; Zhao et al. 1992). On the other hand, variability at millimeter wavelengths is not well established. Wright & Backer (1993) reported significant flux variations at λ = 3.4mm in a month during the decay of a flare observed by the Very Large Array in 1990. However others found no variability at millimeter and submillimeter wavelengths (Gwinn et al. 1991; Zylka et al. 1995). Emission from extended component surrounding Sgr A∗ is significant even at millimeter wavelengths (e.g. Tsuboi et al. 1988), thus it is important to observe with a narrower beam to isolate the compact component for accurate measurement of variability of Sgr A∗. Here we present some results from an on-going millimeter monitoring program of Sgr A∗ with the Nobeyama Millimeter Array (NMA).

The Geneva group has computed a complete grid of stellar models from 0.8 to 60 M⊙ at a metallicity of Z=0.100 (i.e. 5 times solar, Mowlavi et al. 1997). These stellar models present interesting features such as the nearly complete evaporation of the most massive stars due to intense stellar winds. For instance, for Z >> 0.040, final stages of massive stars (M > 60 M⊙) might be white dwarfs rather than neutron stars or black holes.

We report the column density distribution to the Galactic Center region using the X-ray binary observations with the X-ray satellite ASCA, and demonstrate a new method of the total mass determination near the Galactic Center. The column densities are given by a simple form as a function of the angular distance from the Galactic Plane. Assuming a disklike mass distribution of 500 pc radius, we estimate the total mass to be ~ 6 × 107M⊙.

NGC 6251 is an E2 galaxy with mB = 13.6 mag (RC3) and heliocentric velocity 7400 km s−1(=106 Mpc for H0 = 70 km s−1Mpc−1). It is the host galaxy of one of the largest known radio sources in the sky: the radio lobes extend for 1°.2 (over 2 Mpc, Waggett et al. 1977, Readhead et al. 1978, Cohen and Readhead 1979, Jones et al. 1986, Jones and Wehrle 1994).

There is now compelling evidence for massive dark objects at the centers of severals galaxies. The possibility that these are black holes (BHs) fits well into the picture where quasars and AGNs are powered by accretion onto a massive BH, so that dead quasar engines should be hiding in many nearby galaxies.

The study of supermassive galactic black holes (BH) has moved beyond discovery to maturity. The are now ∼ 15 reliable detections. The mass of a central black hole apparently correlates with the mass of the hot component of its galactic host. It may be that every normal galaxy has a supermassive black hole carrying about 10−3 of its bulge mass, with important consequences for the structure and evolution of the core of the galaxy. The most recent major review is by Kormendy & Richstone (1995, KR).

Compact radio cores, which are often assumed to mark the presence of a super-massive black hole, are not only found in the nuclei of powerful quasars but also in nearby galaxies. While in quasars they are typically associated with relativistic jets, the nature of those cores in low-luminosity AGN is less clear. Here, I will briefly mention some of the recent theories (ADAFs or jets) and observations of the latter class of objects.