The discrepancy between observed and predicted γ-ray emission toward the Galactic Center is attributed to a unique population of wide-line molecular clouds. The most prominent objects of this class show evidence of rotation and a significant stellar population. The observed 12CO emission traces the gravitational field produced primarily by stars, not molecular gas.

Preliminary results of a systematic survey of H78α, H91α and H98β emission from the inner 40′ of the Galactic center region are presented. This region consists of two prominent continuum features, the Sgr A complex and the radio continuum Arc. In spite of much nonthermal emission arising from these two features, we detected strong line emission with large line widths in more than half of the observed 130 positions. Many of the detections are new, in particular −50 km s−1 ionized gas linking the Sgr A complex and the Arc, β line emission from GO.1+0.08 (the arched filaments), and α line emission from the loop-like structures which surround the non-thermal filaments near G0.2−0.05. We find that much of the detected lines are probably associated with the −50 km s−1 and the 20 km s−1 molecular clouds, known to lie near the Galactic center. We present line profiles of a number of Galactic center sources including Sgr B1, Sgr C and Sgr D.

I present the results of hydrodynamical model calculations for the evolution of accretion disks in an originally soft potential well. We follow the onset of selfgravity in the accretion disk and the formation of a central object, and find that for an ample set of possible parameter combinations of the “host galaxy” the evolution runs such that today, i.e. after an evolution time of the order of 1010yrs, compact objects of about 106 M⊙ have formed in the centers of galaxies. In the first Section, I concisely describe the basic ideas; Sect. 2 is devoted to the presentation of the results of hydrodynamical model calculations; and in the last Section, I shall discuss these results, interpreting them with the help of an analytical model that shows why the evolving central object is so independent of the conditions in the galaxy.

The ionization of interstellar gas and the heating of dust near the galactic center are usually assumed to be dominated overall by the radiation emanating from young, massive stars. This paper questions that assumption by pointing to the paucity of direct evidence for current star formation and by considering alternative sources of ionization and luminosity. It is suggested that star formation can be inhibited by the strong, poloidal magnetic fields observed in the galactic center. The presence of some red supergiants (e.g., IRS7) can be understood if massive star formation occurs episodically.

The 20 → 3−1 E-type transition of 13CH3OH at 14.78 GHz has been detected towards four continuum sources: Sgr B2, two positions in Sgr A (the peaks of the ' + 20 km/s' and the '40 km/s' clouds), and W33. The NASA Deep Space Network 70-m antenna near Canberra, Australia, which has a 66 arcsec beam at this frequency, was used. A comparison of the 13C and 12C profiles for Sgr B2 indicates a rest frequency of 14,782.27 ± .03 MHz, 0.12 MHz above the laboratory value of Haque et al. (1974). For the Galactic Centre sources, the 12C/13C abundance ratios derived using the simplest assumptions lie in the range 30–40, higher than the 20–25 range derived from H2CO observations. For W33 the apparent value of ∼ 50 is lower than the value of ∼100 derived by Henkel et al. (1983) from H2CO. There may be no discrepancy, however, as W33 contains two velocity components — the higher velocity one at 36 km/s is more prominent in CH3OH and the lower 33 km/s more prominent in H2CO.

The distribution of H2O masers in the Sgr B2 core was observed with a 2.5′×2.5′ wide field and with 540 km s−1 total velocity coverage by the Nobeyama Millimeter Array. Thirty-nine resolved maser spots were detected with a relative positional accuracy of 0.3″, which are clustered into four separate regions. In Sgr B2 north, the cluster lies at the edge of the continuum ridge. One of the maser spots shows strong and wide velocity-spread emission, suggesting it may correspond to a center of star forming activity. In Sgr B2 main, the strong maser spots are projected just on the face of a compact HII region, and are red-shifted relative to the central velocity of the HII region. There are two possibilities to interpret our results in Sgr B2 (M). One is that the H2O maser spots are distributed around the HII region and are infailing to the HII region. The other is that the H2O maser sources are associated with the cloud in the foreground of the HII region.

We present new 30″ resolution J=2–1 12CO observations of the molecular gas located close to a group of non-thermal radio emitting filaments recently detected near the Galactic center (Bally and Yusef-Zadeh, 1989). The new data provide some support for the association of the non-thermal filaments G359.54+0.18 with the projected edge of a molecular cloud.

A near-infrared survey has been conducted of 0.55 square degrees around the Galactic center with the 1 m telescope of Siding Spring Observatory, Australia. From the detected sources, 39 objects which are bright (K<7.6) and red (H-K>1.4) and 11 objects slightly fainter (K~8) have been selected. Subsequently, their 1–20 μm photometry and 2–3.5 μm spectra at a resolution of λ/Δλ∼150 have been obtained with the IRTF atop Mauna Kea, Hawaii.

An unusual radio source exhibiting an axisymmetric, cometary morphology was recently reported by Yusef-Zadeh and Bally (1987) near the Galactic center. This source, G359.3–0.82, consists of a bright head containing a compact source followed by a tail exhibiting sinuous structure. Radio emission is highly polarized and has a nearly flat spectrum between λ6cm and λ20cm. Its location in the sky, spectrum, and lack of resemblance to any other extragalactic radio source suggested to us that this radio source is a Galactic object possibly lying near the Galactic center. New high-reslolution radio images obtained using the VLA confirm the remarkable morphology and strengthen the distinction between G359.3–0.82 and any known extragalactic radio source. The characteristics of G359.3–0.82 suggest that it may be a nonthermal radio wake produced by an object moving through the interstellar medium at a high relative velocity.

Preliminary results are presented from a 4.8-GHz VLA study of 5 far-IR sources within 1° of the Galactic center. Sgr-D and FIR-27 appear to be nearly featureless radio sources similar to young blister type HII regions, and FIR-21 has a filamentary structure similar to bright rimmed HII regions or Crab-like supernova remnants. Also presented are additional observations of Sgr-C.

Radio continuum observations of the galactic center region have revealed a number of vertical structures running across the galactic plane, most of which are reasonably attributed either to vertical magnetic fields or to energy release out of the galactic plane. We review the observed radio structures and discuss their properties and origins with a particular attention to the unusual manifestation of energy release in the galactic center. The relation of the continuum structures to the expanding and/or contracting molecular gas rings is also discussed.

We report the detection of a 4-kpc long, highly collimated radio feature emanating from the galactic center. This feature, which may be cylindrical in shape, is some 200 pc in diameter and extends almost perpendicular to the galactic plane. The structure may possibly be the remnant of a one-sided or highly asymmetric jet from the nucleus, or it might be a magnetic tornado produced by a twisted poloidal magnetic field between the disk and halo.

Symmetrical structures do exist, in the Galactic Centre region, In this article we attempt to summarize their properties and draw the attention of the scientific community to the advantages of taking them into account when working with models of the Centre of our Galaxy, Our work is corroborated by two new maps of the region at 10.7 GHz.

Recent studies of the Galactic center environment have revealed a wealth of new thermal and nonthermal features with unusual characteristics. A system of nonthermal filamentary structures tracing magnetic field lines are found to extend over 200pc in the direction perpendicular to the Galactic plane. Ionized structures, like nonthermal features, appear filamentary and show forbidden velocity fields in the sense of Galactic rotation and large line widths. Faraday rotation characteristics and the flat spectral index distributions of the nonthermal filaments suggest a mixture of thermal and nonthermal gas. Furthermore, the relative spatial distributions of the magnetic structures with respect to those of the ionized and molecular gas suggest a physical interaction between these two systems. In spite of numerous questions concerning the origin of the large-scale organized magnetic structures, the mechanism by which particles are accelerated to relativistic energies, and the source or sources of heating the dust and gas, recent studies have been able to distinguish the inner 200pc of the nucleus from the disk of the Galaxy in at least two more respects: (1) the recognition that the magnetic field has a large-scale structure and is strong, uniform and dynamically important; and (2) the physics of interstellar matter may be dominated by the poloidal component of the magnetic field.

The Galactic Center Arc has been observed with the Effelsberg 100-m telescope at 32 GHz. The percentage polarization is of the order of 50%. The magnetic field structure is uniform in a direction parallel to the Arc.

Polarization measurements of the radio Arc were made with the VLA at 15 GHz. High frequency polarimetry made with high spatial resolution minimizes Faraday depolarization and reveals polarized filaments which correspond to the predominant filaments of the radio Arc. We notice a peculiar linear feature in the polarization map (“thorns”) which suggests the presence of a second magnetic field system. The total intensity maps show no evidence for an interaction between the two field systems, so the thorns may be foreground magnetized structures. However, if the two magnetic field systems do interact, it would allow a model in which the acceleration of relativistic particles takes place at their intersection. The accelerated particles would flow toward both ends of the radio Arc, and account for the intrinsic polarization observed along the entire length of the system. Thermal electrons responsible for the Faraday depolarization occuring at longer wavelengths may be supplied by the interaction of the streams of relativistic particles with relatively dense, ambient thermal clouds.

The H92α recombination line was observed at 8 GHz toward the “pistol-shaped” HII region G0.15–0.05 using the VLA2 in its most compact configuration. The line profiles of individual components of this source peak at VLSR=123 km/s and have total line widths of ~90 km/s. The kinematical structure of the “pistol” is unusual in that much of the neutral and ionized gas in this region is seen predominantly at either +50 or +20 km/s. The line width and radial velocity are the largest found in the Galactic center region with the exception of Sgr A West. We also found gas at VLSR=140 km/s associated with G0.18–0.04: the sickle-shaped feature which surrounds G0.15–0.05. The kinematic properties of G0.18–0.04 and G0.15–0.05 suggest that these two features are components of a single, but complex thermal system interacting with the nonthermal filaments of the radio Arc. In this regard, the width of the broad recombination line from G0.15–0.05, and its large radial velocity, might be explained as the interaction of streaming relativistic particles in the nonthermal filaments of the Arc impacting upon ambient gas clouds lying in the Galactic plane.

A cluster of luminous infrared sources has been found near the Galactic Center. It consists of five identical stars clustered in a compact volume, to be called an IR quintuplet. They are all highly reddened, strongly polarized and associated with deep absorptions of silicate band and CO vibration band. They seem to be a cluster of young stars newly born near the Galactic Center.

We have used the 20-pixel IR camera to observe thermal IR emission from dust associated with the radio continuum Arc near the Galactic center and the cluster of HII regions in the immediate vicinity of Sgr A East. We detected strong 10μm emission from the eastern and western arched filaments (G0.1+0.08), from an unusual pistol-shaped structure known as G0.15–0.05 and from the brightest member of the Sgr A East HII region. Spatial maps of these features at 10μm with a resolution of 4.1″ × 4.2″ are presented and are compared with 5-GHz radio images. We find a general spatial correlation between the ionized gas and the dust distributions. The ratio of IR to radio flux densities is significantly different in the eastern and western arched filaments, which suggests that the source of heating has a softer spectrum along the eastern arched filaments. In addition, the ratio of IR to radio flux densities, which is typically ~10 in normal Galactic HII regions excited by O stars, is at least a factor of two higher than this value in almost all the sources we have observed. This suggests that additional mechanisms other than trapped Lyman α radiation should be present in heating the dust, e.g. stochastic heating of small dust grains by energetic particles associated with the nonthermal filaments.

An HI absorption study of the central ~ 100 pc of the galactic centre was performed using the compact array of the VLA2. The results of these observations lead us to believe that the body of gas at 40–50 km s−1, known as the ‘40 km s−1 molecular cloud’, is placed behind the Arc. In addition, the distribution of gas around +20km s−1 across the Arc and its correlation with 3cm polarized emission suggests that it may be this 20km s−1 gas which is associated with the Arc and is depolarizing much of its synchrotron radiation. The kinematic structure of the negative velocity HI gas across the arched filaments implies an association between the neutral atomic gas and the ionized and molecular material.