Analyses of the distribution of far infrared point sources in the Galactic bulge have suggested that from a face-on perspective the bulge has a bar like shape. Here, we investigate how a rotating bar-like bulge affects the global gas dynamics in a disk and compare the longitude-velocity (LV) maps from selfgravitating hydrodynamical simulations with observed maps of neutral hydrogen and carbon monoxide in the Galaxy. We found that the features on the numerical LV maps depend strongly on four factors: the pattern speed of the bar, the position angle of the Sun, the strength of the bar potential and the ratio of the gas mass to total dynamical mass. We conclude that our Galaxy has a rotating, weak, bar-like bulge (a/b ∼ 0.8) observed from nearly end on (θp < 20°). The allowed range of pattern speed of the bar is surprisingly narrow (∼ 20 km s–1 kpc–1) and is consistent with recent observations of bulge stars. Selfgravity of the interstellar matter is needed to account for some of the observations.

Since the center of our own galaxy is the closest region of this type, it should provide a proving ground for our understanding of such regions. And fortunately, recent developments in radio, infrared, and x-ray astronomy allow us to look at the center of our galaxy in considerable detail – a privilege not possible in the visible region because of the rather dense dust clouds surrounding it. We find there many complex and interesting phenomena, including evidence for a compact central mass of a few million solar masses from the dynamics of stars and gases. However, its existence has been doubted because the radiant energy coming from the very center is much less than normal theoretical expectations for a black hole of such a mass. This discrepancy has led to speculation whether the velocity measurements could be misleading, whether the concentrated mass is in some form other than the expected black hole, whether the radiation might be taking place in an exotic form, being directed away from us or happening to be low at the moment, or whether our theories of such radiation are as yet incomplete. The general characteristics of the very central region of our galaxy will be reviewed along with the dynamic evidence for a large concentrated mass and comparison of the radiation it emits with that of various theoretical models.

Sgr A∗ is the unique 1 Jy flat spectrum radio point source located at the dynamical center of the Galaxy and in the very center of the central star cluster (Eckart et al. 1993). Due to its unusual appearance it has long been speculated that this source is powered by a supermassive black hole–an object whose presence has been suspected to reside in the nuclei of many other galaxies as well. Its mass is believed to be as large as M • ∼ 2 m 106M⊙ (e.g. Genzel & Townes 1987) while a lower limit of M • > 200–2000M ⊙ can be inferred from the low proper motion of Sgr A∗ (Backer – this volume). The currently best dynamical arguments for the million solar mass black hole stem from detailed stellar kinematic studies (Haller et al. 1995).

We argue that the wind from IRS 16 and He I stars in the central 1 pc of the Galaxy is responsible for the peculiar features of accretion onto a putative black hole at the Galactic center. What makes Sgr A∗ unique is not that it is just underfed but, in addition, it has a much lower efficiency of accretion and possibly a lower mass, compared to the AGN case.

Recently, I summarized arguments, both old and new, that the Galactic center has experienced a starburst in a recent past (Ozernoy 1994a). Here I propose a likely mechanism – collisions between giant molecular clouds – that might induce (recurrent) starbursts. Taken together, these two approaches seem to indicate that the history of the central part of our Galaxy can be described as recurrent starbursts or intermittent Seyfert activities/starbursts.

The best hypothesis for the energy source of the compact, nonthermal radio source in the center of our galaxy, Sagittarius A∗ (Sgr A∗), is accretion onto a massive black hole from winds emanating from nearby luminous stars. The hole mass, accretion rate, and ultimate fate of accreted matter are uncertain. In this report I give a summary of recent critical observations. The interpretation of these results supports this general hypothesis, and begins to place constraints on model parameters. If so, then Sgr A∗ is a miniature version of extragalactic AGNs in a quiescent state.

We discuss the radio spectrum of Sgr A∗ in the frequency range between ≈ 1 GHz and ≈ 1 000 GHz, show that it can be explained by optically thin synchrotron radiation of relativistic electrons, and point toward a possible correlation between the spectrum of Sgr A∗ and larger-scale (≲ 50 pc) radio emission from the Galactic Center region.

A personal reflection is presented of David Allen's research into the content of our Galactic nucleus.

The Galactic Center (GC) star cluster is a compact stellar system. The surface brightness distribution at infrared has core radius of about 0.8 pc (Rieke & Lebovsky 1987) when the contribution from bright stars are removed. On the other hand number counts of infrared sources in the high resolution speckle image by Eckart et al. (1993) at 2.2 μm gives 0.15 pc for the core radius of the surface density distribution. At present it is not clear which is more appropriate for the mass distribution.

We have obtained R ≈ 570 resolution K band spectra of eight sources in the Galactic Center, including four sources within the IRS 16 cluster, IRS 13, IRS 1W, and the compact He I emission line sources AF (also known as AHH) and AHH NW. We have also obtained R ≈ 570 H and K band spectra of nine galactic and LMC early–type mass–losing stars, including Ofpe/WN9 and WN stars. The spectra of both the Galactic Center sources and the comparison stars show a wide range of behavior in the He I (1.70 μm, 2.06 μm, 2.11 μm) and H I (Brackett series) lines. We find significantly larger He I equivalent widths in the AF source and two galactic early type mass losing stars than in any of the LMC stars. Several of the Galactic Center He I sources are found to have higher He I velocity widths than any of the galactic or LMC early type mass losing stars. At least one source, IRS 13, shows a strong red wing to the He I 2.06 μm emission.

We have modeled the mid-infrared emission from the Galactic Center using our array camera images at eight wavelengths. The results suggest that the high infrared luminosity of the region is provided by a cluster of luminous stars. There is no direct indication in the new model results of a very luminous object or “central engine” near Sgr A∗.

The innermost 2 pc contain a rotating ring (“circumnuclear disk”) of molecular gas, neutral hydrogen, and dust with an embedded H II region called Sgr A West; a dense stellar cluster; and a compact nonthermal radio source Sgr A∗ (for a recent review, see Blitz et al. 1993). The clumped, spiral-shaped morphology of Sgr A West, sometimes called lthe mini-spiral”, has been a subject of numerous speculations concerning its origin (for a review, see Genzel & Townes 1987). Lacy et al. (1991) demonstrated that both the kinematics and shape of a part of Sgr A West can be fairly well approximated using an one-armed density-wave model.

A population of nonthermally-emitting radio filaments tens of parsecs in length has been observed within a projected distance of ∼130 pc of the Galactic center. More or less perpendicular to the Galactic plane, they appear to define the flux lines of a milligauss magnetic field. The characteristics of the known filaments are summarized. Three fundamental questions raised by these structures are discussed: 1) Do they represent magnetic flux tubes embedded within an ubiquitous, dipole magnetic field permeating the inner Galaxy, but which have been illuminated by some local source of relativistic particles, or are they instead isolated, self-sustaining current paths with an approximately force-free magnetic configuration in pressure equilibrium with the interstellar medium? 2) What is the source of either the magnetic field or the current? and 3) What is the source of the relativistic particles which provide the illuminating synchrotron radiation? We are nearer an answer to the the last of these questions than to the others, although several interesting models have been proposed.

Ever since the Galactic Center Arc was resolved into its component filaments a decade ago, it has been clear that its linear structure arises from the influence of a strong magnetic field. However, the origin and nature of the contributory phenomena have remained elusive. Since what is seen is synchrotron emission from relativistic particles, of prime interest is a knowledge of the acceleration mechanism involved. Interferometric imaging of the molecular gas in the vicinity of the Arc has now provided a tantalizing clue to the Arc's origin: molecular clumps coinciding with the endpoints of a number of the Arc's filaments point to these clumps as the source of the relativistic particles. This suggests that as dense molecular clumps course through the ambient magnetic field at the Galactic Center, magnetic energy is liberated in their leading layers via field reconnection, precipitating rapid acceleration of free charges to high energy.

We probe the relationship between the scattering and Faraday screens toward the Galactic center by examining the scattering properties of Sgr A∗ and OH/IR stars and by measuring the rotation measure distribution of an extended nonthermal filament. The unusually large rotation and scattering measures, the evidence for anisotropic scattering, and the anisotropy in the rotation measure structure function, suggest that the Faraday and scattering media are located near the Galactic center. The results presented here are consistent with a turbulent medium of magnetized thermal electrons with the outer scale of electron density and magnetic field fluctuations being of the order of 5 × 10–4 and 2 pcs, respectively.

The star cluster in the Galactic center (GC) provides an exceptional test-ground for the evolution of stars in extreme environments. Its understanding is an important step for the study of stellar populations in starburst galaxies and AGN's. In this work we address the question if the central cluster can be understood by standard evolutionary scenarios, or whether other processes (collisions, mergers, atmosphere stripping etc.) need to be invoked. A more detailed account of this work will be published elsewhere.

Using the SEST, we have observed 554 positions with a spacing of 45″ in the C18O(J = 1 → 0) and HNCO(Jkk′ = 505 → 404) lines. The data covers most of the Sgr A region including the Arc. Many of the dominant clouds in the GC region (see e.g. Güsten et al. 1981, A&A 103, 197; Bally et al. 1987, ApJS, 65, 13) are readily identified in the total integrated C18O and HNCO maps (Fig. 1). The results will be published in A&AS and will include intensity maps with 5 km s–1 velocity resolutions, as well as galactic longitude-velocity and galactic latitude-velocity maps. J = 2 → 1 C18O observations are planned for selected regions. The objectives of the project are twofold: 1. Geometrical, morphological, and dynamical relationships between the molecular regions and the radio continuum sources. 2. Heating mechanisms in GC molecular clouds.

The inner few hundred parsecs of the Galactic Center contains ∼10% of the molecular ISM in the Galaxy. The conditions in this gas are significantly different from those in molecular clouds elsewhere in the Galaxy. Typical temperatures, densities, and internal velocity dispersions are higher (Güesten 1989). There is also evidence for a large amount of molecular gas which is not bound to distinct clouds (Stark et al. 1989). High velocity bulk gas motions and velocity discontinuities open up the possibility of a role for powerful large-scale shocks in ISM excitation. The very different nature of the dense ISM in the inner Galaxy make it useful as a laboratory for physical effects in the interstellar medium and a proving ground for ideas about the interaction of gas and stars in the nuclei of other galaxies.

The Galactic Centre candidate Sgr A∗ may exhibit a 40 km/s radial velocity component, which is not observed for OH/IR stars around the centre. This could be interpreted as orbital motion of one member of a binary massive black hole. In other galaxies such pairs may be inferred from radio jet precession and emission line profiles.

X-ray imaging spectroscopic observations near the Galactic center region were carried out with the ASCA satellite. We found two bright spots very close to the Galactic center (Sgr A∗); one is extended and has a soft spectrum associated with strong emission lines from highly ionized irons, while the other is a point-like object with a harder spectrum and a larger absorption. We also found extended emission with K-shell transition lines from highly ionized Si, S, Ar, Ca and Fe. Remarkable feature found with ASCA is an extended emission of 6.4 keV lines of low ionization irons. The 6.4 keV line fluxes are found to be well correlated to the region of cool clouds. We interpret that the 6.4 keV line is due to florescence from the cool clouds irradiated by strong (and obscured from our line of sight) X-ray beams.