The ionized gas within a few parsecs of the center of our galaxy is contained primarily in a number of thin elongated filaments (Ekers, this volume). Infrared and radio line observations have succeeded in determining the gas velocities along the majority of these filaments. A comparison of the line emission velocities with general kinematic models indicates that the motion of the filaments between 0.3 and 2 pc from the center is consistent with predominantly orbital motion.

Sub-arcsecond (down to 0.1″ × 0.2″) radio continuum observations using the VLA2 in a number of configurations have been carried out in order to investigate the fine-scale morphological details of the ionized gas and the distribution of spectral index along the triskelian-shaped figure of Sgr A West. In addition to finding a number of isolated patches of thermally-emitting gas and an absorbing feature at λ6cm within three arcminutes of the Galactic center, we have observed:

1) radio continuum emission from IRS-7, implying that the stellar wind from this supergiant is externally ionized. An improved position for this object was obtained.

2) the circular mini-cavity located along the east-west bar of Sgr A West. This feature has a diameter of 2-arcseconds and may have been created by a spherical wind, the source of which is yet to be identified; the seemingly most plausible candidate, IRS-16, is offset by 3″ from the center of the cavity.

Spectral index maps having a resolution of 0.7″ × 0.3″ were made from scaled array observations at λ2cm and 6cm. They show that the eastern arm has a spectral index near −0.1, while the northern arm and the bar have positive spectral indices, indicating perhaps a partial opacity effect. The spectral index of IRS-7 is +0.6, consistent with that expected from a completely ionized stellar wind.

VLA data of the Galactic center at 6 cm from the A, B, and C configurations have been combined to produce a high quality image at 0.76″ × 0.41″ resolution with a dynamic range of ≈ 5000. The new combined array image enables a clear definition of the diffuse and filamentary structures in Sgr A West. The bulk of the image processing was carried out with the National Center for Supercomputing Applications' Cray XMP-48 at Urbana-Champaign.

Spectropolarimetric studies of Sgr A West at 10μm reveal intrinsically polarized emission from the northern arm. The polarization, of amount 7–8 %, is attributed to emission from dust grains which are aligned by a magnetic field directed along the line of the northern arm. Along the eastern arm the polarization is smaller and indicates either a weaker field or one directed closer to the line of sight.

Polarized emission has also been observed at 100 μm from aligned grains in the ring of molecular gas which apparently encircles Sgr A. The aligning magnetic field lies in the plane of the ring and is probably toroidal. The correspondence between the polarization position angles at 10 and 100 μm suggests a model in which the northern arm is a disrupted filament from the molecular ring.

While the uncertain magnetic properties of grains prevent a reliable direct estimate of field strength, circumstantial evidence suggests it is larger than elsewhere in the Galaxy and possibly large enough to influence the dynamics of the region.

Seeing-limited 11.6-μm images of the central 1 parsec (~20 arcsec) of the Galactic Center have been obtained with a new 58 × 62 pixel Si:Ga array camera system. Previous array observations (Gezari et al. 1985) were used to derive the color temperature structure of the region. This higher spatial resolution work resolves new structure, showing a strong similarity (and notable discrepancies) between the 11.6-μm emission and the 6-cm continuum VLA maps (cf., Morris and Yusef-Zadeh 1986) and closely correlated Brackett a infrared array images (Forrest et al. 1986). Careful comparison of the dust and ionized gas emission distributions shows most of the compact 11.6-μm dust sources are displaced from the gas peaks.

High resolution spectra of the 4.64 micron 1–0 R2 line of CO toward infrared sources in the galactic center reveal absorption components identified with features in radio and millimeter wave spectra of molecular lines. They demonstrate that the +20 km/s cloud is situated in front of the Sgr A West and the +50 km/s cloud is behind it. The spectra of IRS 3 and IRS 7 have strong absorptions near +50 km/s, which may originate in the 2 pc radius circumnuclear ring; if so then the western side of the ring lies in front of the galactic center. A less likely interpretation is that these objects, which are only a few arc-seconds from IRS 16, are behind the +50 km/s cloud and, thus, well behind the galactic center.

The evidence for a large systemic motion of the neutral ring at the centre of the Galaxy is reviewed, indicating that the local standard of rest is probably receding from the centre at around 40 kms−1.

Observations of the stellar cluster in the central 10 pc of the Galaxy are reviewed. The stellar density law derived from the observed light distribution and the effects on this density law of variable extinction, the possibility of a varying mass-to-light ratio, and the current debate as to the core radius of the cluster are all important for establishing the true mass distribution of the stellar cluster. The presence of the supergiant IRS 7 in the Galactic Center establishes that some recent star formation has occurred, but the age and extent of a possible starburst are still being established. The kinematics of the stellar cluster show predominantly velocity dispersion, in contrast to the systematic gas motion observed, yet the total mass distributions derived from stellar and gas kinematics agree reasonably well. The core radius of the cluster is critical to establishing whether or not a central dark mass is required to explain the total mass distribution.

A study of the Galactic Center stellar population is continuing with a sensitive 2μm CCD camera. Using a 64 × 64 detector array, background limited images are recorded with modest amounts of observing time (tobs ≈ 20 sec to reach K=13). Magnitudes have been extracted using DAOPHOT from repeated imaging of the central 5′ × 5′ to search among approximately 1500 stars for long period variables (LPV's, P > 200d), particularily Miras. Miras have a well defined period-luminosity relationship as well as one in period-mass. This program investigates the nature of highly luminous stars at the Galactic Center. Presently 12 variables have been found and have several characteristics consistent with Miras. They have a maximum bolometric luminosity of −4.4 mag, which supports the case that high luminosity stars in the central 6 pc are young supergiants.

The determination of the mass distribution in the central few parsecs of the Galaxy, primarily from the ionized gas dynamics, is discussed. The gas motions are described and interpreted assuming that the orbits are determined by gravitational forces. It is shown that there is good evidence for a central mass of ~ 2 × 106 M⊙. The primary uncertainty in this conclusion results from the possibility of significant magnetic forces. In the absence of corroborating evidence, the case for a massive black hole cannot be considered proven.

We present the results of a project to map the profile of the 2.3 μm CO V = 2–0 band-head in the integrated starlight in the central 10 pc of the Galaxy. This is the first detailed determination of the kinematics of the faint stars that are the major constituent of the mass of the stellar cluster. The stars exhibit systematic rotation in the same sense as Galactic rotation, with VLSR increasing with Galactocentric radius. The stellar velocity dispersion generally dominates the rotation and shows clear evidence for a radial gradient, in the sense of σ decreasing with Galactocentric radius. The data are consistent with the dynamical center of the Galaxy being located at IRS 16 (to an accuracy of ± 10″). The mass distribution has been derived via the theory of stellar hydrodynamics and is shown in Fig. 1. For an assumed core radius of 10″ (~0.4 pc), the best fitting model is a combination of a central mass of 2.5 × 106 M⊙ and a stellar cluster with a density dependence of r-2.1 (Fig. 1). If the core radius is small (~1″) then the mass distribution could be just due to a stellar cluster; a central condensed mass is not required to model the data in that case.

The preliminary results of a search for OH/IR stars in the Galactic Centre using the VLA are presented. The goal of this project is to determine the mass density distribution from a few parsecs up to about 120 pc of the Galactic Centre using stellar kinematics. Up to now, 125 OH/IR stars have been found. The distribution of the stars is clearly elongated with the major axis parallel to the galactic equator. The stars show a rotation of 117 km s−1 deg−1 with a dispersion of 90 km s−1. Two preliminary models have been applied in order to determine the enclosed mass, resulting in M(r)=4.0·106·r1.4 Mo, where r is the galactocentric distance in parsecs.

A high resolution K-band (2.2 μm) image of the Galactic center was acquired as part of an effort to develop computer simulations of a lunar occultation of the IRS 16 region that was previously observed in 1986 September. Through our computer simulations and recent observations of another occultation that occurred in 1988 September, confident identifications of the objects detected during the 1986 September occultation were acquired. In particular, IRS 16C, 16SW, and 16NE were all identified and found to be unresolved (<0.05″ in angular extent).

Using the infrared grating spectrometer FIGS on the Anglo-Australian Telescope we have mapped the inner parsec of the Galaxy. We used spectral resolutions of 1000 to observe He I λ2.058 and Brackett α, and 400 for complete K-band spectra.

The source IRS16 has been observed with the FTS at the Canada-France -Hawaii Telescope, in July 1987 and May 1988, in an attempt of spectral resolution associated with spatial resolution. Three positions were observed with a 2.5 arcsec aperture, centered on the sub-structures SW, NE and NW. Brα and Brγ spectra were obtained at respective velocity resolutions of 210 and 140 km s−1, allowing resolution of the line profiles. The detection of hydrogen emission was marginal on NW, and strong on SW and NE, showing a net velocity difference of the gas of 160 km s−1. The Brα line on SW has very extended wings suggesting two components of different origin. These observations imply a difference of physical nature of the objects located in the vicinity of Sgr A∗.

H I Br α (4.05 μm), [Ne II] (12.8 μm), and [Ar III] (9.0 μm) emission lines from Sgr A West were mapped with ~100 km/s and 2″ resolution. The observations were made with the U. T. infrared echelle spectrometer (IRSHELL) on the IRTF in April 1988. The instrument simultaneously measures 10 64-point spectra along a 2 × 10″ slit. Data analysis is still in progress; preliminary results are presented here.

An important means of studying the unusual activity within the central ~0.15 parsec of the galaxy is to obtain detailed information on the high velocity ionized gas there. This gas was first reported by Hall, Kleinmann, and Scoville (1982), who observed the He I line at 2.06 μm. Subsequent observations of this line and the Br ∝ and Br γ lines of H I (4.05 μm and 2.17 μm, respectively) by Geballe et al. (1984, 1987) have defined the coarse spatial and spectral properties more accurately. Briefly, the broad (i.e., |v| > 400 km/s) line emission, as observed at velocity resolutions as high as 400 km/s and angular resolutions as high as 2.5″ (1) extends approximately to +/– 700 km/s (e.g., see Fig. 1), (2) is spatially resolved, with a characteristic dimension of 3″, (3) is centered approximately on IRS 16C, and (4) appears to be due neither to rotational motion nor to a simple radial flow from or onto a single compact object. These properties are difficult to understand in terms of simple models, and point out the necessity for further measurements at higher spectral and spatial resolutions.

We review the current observational status of Sgr A∗, the compact nonthermal radio source at the galactic center. Sgr A∗ is a unique radio source at a unique location of the Galaxy. It is unlike any compact radio source associated with known stellar objects, but it is similar to extragalactic nuclear compact radio sources. The positional offset between Sgr A∗ and IRS16 places little constraint on the nature of the underlying energy source, since IRS16 need not be the core of the central star cluster. Sgr A∗ is still the best candidate for marking the location of a massive collapsed object.

We investigate the long-term flux density variations of the compact radio source Sgr A∗ at the galactic center by combining recent VLA observations with previous Green Bank interferometer data. We present radio flux density light-curves for Sgr A∗ at 20, 11, 6 and 3.7 cm from 1974 to 1987. Long-term variability with a timescale of at least 5 years is seen at 20 cm and there is evidence for more rapid variations at the shorter wavelengths. The variability timescales at 20, 11 and 6 cm fit the λ2 scaling predicted by the theory of refractive scintillation suggesting that the variability could be due to this cause. However, the timescales are relatively short, implying an unusually high velocity in the scattering screen. The modulation index of the variability is large and relatively independent of wavelength.

A young star cluster is a less contrived explanation than a massive black hole for many of the features seen in the Galactic center. However from a Copernican point of view, this explanation is less attractive than a black hole. The evidence for a ~ 106M⊙ black hole is becoming progressively less convincing, but the case against it is no stronger. We describe the development of a singular star cluster, as well as the processes of stellar disruption, merging, and gas accretion in such a cluster. Recently merged stars and tidally stripped giants may be detectable within an arcminute of the Galactic Center. We examine the physics of star formation in the inner parsecs of the galaxy, and the problem of maintaining the two parsec molecular torus.