This review addresses the question of MHD phenomena in the galactic center, which are expectedly important in view of the large value of the magnetic field. A physical similarity with other MHD environments where magnetic fields are dominated by a dense driver and dominate a more tenuous halo is recognized. Known physics rules this type of coupling. Most proposed MHD theories of the galactic center fit, at least partly, in this general frame. One of them views Sgr A and its environment up to 50pc as an active corona, similar to that of the sun. whose driver is some central accretion disk, which may (or may not) be the molecular ring. This unified picture is outlined and is shown to naturally explain a number of otherwise puzzling observed features, such as the radio arc and bridge, possibly the ionized minispiral and some aspects of the general energy balance of this region.

I critically discuss three possible MHD mechanisms for the formation of Galactic center lobes (GCL) found by Sofue and Handa (1984) from the theoretical point of view. The three mechanisms I shall discuss are: (1) sweeping-magnetic-twist mechanism, (2) explosion in a disk with a vertical magnetic field, and (3) nonlinear Parker instability. I review the characteristics of these mechanisms, which are mainly obtained from nonlinear 2D MHD numerical simulations, and discuss their merits and demerits as possible mechanisms for the formation of GCL and related magnetic structures.

We critically review the status of our current understanding of galactic dynamo theory. In brief, a definitive model for the production of magnetic fields on galactic scales remains to be constructed.

Molecular clouds moving at Keplerian velocities can induce strong electric fields in ionized regions near the galactic center. The induced field can drive currents over equivalent circuits ~ 100 pc. along the highly ordered magnetic fields, B ~ 10−3G. Such current paths drive low-level ion acoustic turbulence, providing a resistance in the circuit. Small magnetic pinches form which are generally kink unstable, but can organize into larger, long-lived structures. Ohmic losses are energetically important in the molecular clouds, where high density regions should be most luminous. Electrons can accelerate in the induced fields to relativistic energies, yielding the radio luminosity. Electrodynamic flares may occur on year timescales. Such electrodynamic deceleration of clouds can powerfully increase accretion toward galactic centers and enhance their luminosities.

The energy storage and dynamics at the center of galaxies is explained using a new construct, the gravitationally bound current loop (GBCL), produced when the galaxy formed under gravitational collapse. Thin toroidal plasma around the slender intense relativistic current loop is bound to it by the Maxwell “frozen-field” condition, and also binds gravitationally to the central object (presumably a black hole). The Strong Magnetic Field model (SMF) explains directly the Milky Way (MW) galactic center radio observations of a vertical magnetic field perpendicular to the galactic disk and the extended radio arcs, as well as the production of successive radio blobs ejected from the compact cores of active galactic nuclei (AGN) or quasars.

This contribution is a summary of an impromptu and extremely interesting discussion on the nature of the forces acting on gas in the galactic centre. The origins of the gas motions in the central few parsecs are crucial for clarifying many important problems about galactic nuclei. Some of the many gas dynamics issues that were raised include: 1. are there explosive events in the galactic centre which produce outward moving gas clouds (see Heyvaerts, these proceedings)? 2. are the streamers of gas due to the tidal disruption of molecular clouds that stray in too deep in the galactic centre's tidal field (see Serabyn, these proceedings)? 3. is there circular gas motion which would be important in constraining the nature of the mass distribution inside scales of a few pc (see review by Townes, these proceedings)?

The discussion centered on gas dynamics in the molecular torus and ionized gas streamers on the 2–5 pc scale. Because reliable measurements of the magnetic field strength in the galactic centre are not yet available, several of the key points of contention will not be finally settled until the data are in. Be this as it may, the discussion boiled down to the issue of whether gravity or magnetic fields were responsible for the structures and morphology on this scale. Accordingly I will label the arguments of the two relevant camps by G (for gravitational fields as the predominant shaper of things) and M (for magnetic fields as the important agents).

We have observed SgrA at 332 MHz (92 cm) with a resolution of 12 arcsec (0.6 pc) using the four configurations of the VIA. These results illustrate the dramatic and almost unique variation of radio spectral index within the central 3–4 arcmin of the galactic center. SgrA East is a non-thermal shell source that could be a supernova remnant or a very low-luminosity example of a radio component associated with the active nucleus of a spiral galaxy. The most dramatic aspect of the new 332 MHz observations is the appearance of the the SgrA West spiral features in absorption against SgrA East. Based on these results, SgrA East is situated behind SgrA West, the center of the galaxy. The halo is in front of or surrounds the former sources. The HII regions to the east of SgrA East (1 = −0°.02, b = −0°.07) are probably associated with the 50 km/s molecular cloud. The 7 arcmin halo (20 pc) has a non-thermal spectrum with turn-over below 1 GHz.

We have mapped the source complex Sgr A (containing Sgr A West and East) at λ2.8mm, 1.3mm and 350μm using the IRAM 30m MRT (Pico Veleta, Spain) and the 3m IRTF (Mauna Kea, Hawaii) telescopes. Detailed results have been published or will be published elsewhere (Mezger et al., 1986, Paper I; Zylka and Mezger, 1988, Paper II; Mezger et al., 1988, Paper III).

We have obtained a K band image of the central 30 × 40 arcminutes of the Galaxy at a scale of 1.4″/pixel using a 256 × 256 Pt: Si Schottky barrier diode array detector provided by the Hughes Aircraft Company. The excellent cosmetic quality and large field of this device provide an unprecedented view of the inner Galaxy. Images of the central 10 arcminutes at a scale of 0.9″/pixel in the H (1.65μm) and K (2.2μm) bands produced with the same detector array have been combined to produce a color picture, which clearly shows the circumnuclear molecular ring in absorption; this picture demonstrates directly that the southwestern side of the ring lies in front of, and the northeastern side behind, the Galactic center.

HI absorption studies of the Sgr A complex based upon recent VLA2 observations, show that the 40 km s−1 molecular cloud lies behind the Sgr A West region, and that the cloud may be physically associated with the Sgr A East non-thermal shell. The absorbing gas over the range +30 to +60km s−1 lies mainly between the eastern edge of the Sgr A East shell and the western boundary of a cluster of HII regions. There is a pronounced velocity gradient over this region, and at 40 km s−1 similar morphology is apparent between the distribution of atomic hydrogen and the edge of the Sgr A East shell. The physical association of the 40 km s−1 cloud and the Sgr A East shell, and the placing of the 40 km s−1 cloud behind Sgr A West, are consistent with recent evidence which has demonstrated that Sgr A West lies infront of Sgr A East. The HI optical depth maps of the circum-nuclear disk plus a new HI feature associated with the northern extension of Sgr A West will be presented.

We report NH3 observations of the Sgr A complex region including Sgr A West and the 20 km/s and 50 km/s molecular clouds (M–0.13–0.08 and M–0.02–0.07) using the Nobeyama Millimeter Array and the 45m telescope. NH3(1,1) and (2,2) lines were simultaneously observed to estimate the kinetic temperature. Our results suggest strong interaction between the molecular clouds and the continuum sources in the Sgr A complex. The interaction with continuum sources might be an important factor in determining the physical conditions of molecular gas in the galactic center region.

VLA observations in the (J, K)=(3,3) line of ammonia reveal new structures in the Galactic center region. An approximate ring of emission is centered on the central ionized streamers. This ring, seen previously in millimeter-wave interferometer maps, is very clumpy in the ammonia emission, with size scales ≲10″ (0.4 pc). The clumps show good spatial and velocity agreement with the ionized gas, and are warm with brightness temperatures exceeding 30 K. A comparison of the (3,3) to (1,1) ratio indicates considerably higher gas temperatures. This circumnuclear ring may not be the dominant feature in the mass distribution of the circumnuclear gas. A streamer, immediately to the south of the Galactic center, connects the gas complex at lII= −4′ (~10 pc) directly to the Galactic center. This streamer may define the path for gas flow into the nuclear region.

A 3′ × 6′ field centered ~4′ south of the Galactic center and a ~2′ × 4′ field east of the Galactic center have been mapped using the 4–1 → 30 E line (36.169 GHz) of methanol (CH3OH). Line profiles typically consist of broad (~10–20 km s−1) components, as well as narrow (~1 km s−1), spike-like components. The narrow features are bright and unresolved with 60″ resolution, and are probably due to maser activity. We found maser activity immediately adjacent to the non-thermal “wisp” (SgrA-E) ~4′ south of SgrA-West, the secondary knot at SgrA-F, and the eastern edge of SgrA-East. Position-velocity diagrams suggest that the molecular material has been shocked to a higher velocity in the same vicinities. We propose that the maser activity, and the shift in velocity south of the Galactic center, heretofore interpreted as an increasingly steep velocity gradient toward the Galactic center, may be due to the impact of a SNR and the resulting shock of the ambient molecular material. The dynamical effects on the molecular gas of such a shock may have implications on gas feeding toward the nucleus.

We report VLA observations of H76α emission from the H II region in M–0.13–0.08, the Sgr A “15 km s−1 cloud.” The line to continuum flux ratio implies an electron temperature Te ~ 7500 K, slightly lower than measurements of Te in Sgr B2. The deduced number of early stars and the infrared luminosity are consistent with a normal IMF in this cloud.

This review summarizes our current knowledge about the interstellar matter between about 1.7 and 10 pc(a) from IRS16/SGR A∗, a region which now commonly is referred to as the circum-nuclear disk (CND). I will discuss first the spatial distribution of the neutral and partially ionized, interstellar matter, then its physical parameters and excitation and, finally its dynamics. For other recent reviews of the subject I refer to the articles by Güsten (1987), Genzel (1987), Genzel and Townes (1987) and Jaffe (1987).

We report aperture synthesis observations of the HCO+ and HCN J=1–0 molecular lines towards the Galactic center. These data complement existing HCN data and trace a dense molecular ring surrounding the ionized central 2 pc of the Galaxy. The new data are consistent with the model of a clumpy, almost complete ring which is inclined to the line of sight at 50 to 75 degrees. The same structure is seen in HCO+ and in HCN with the exception of an HCN feature at 60 to 100 km/s in the western part of the ring, which is not detected in HCO+ emission. The HCN and HCO+ are collisionally excited in clumps with densities around 10 cm and volume filling factor 1/3 to 1/30. H13 CN emission from the ring was detected at about 1/7 of the intensity of the HCN; the latter is optically thick and is mapping a combination of surface density and excitation temperature. The HCO+ emission shows deep absorption features associated with galactic structure along the line of sight. Absorption features corresponding to the 3 kpc arm, the inner disk and an expanding ring at −195 km/s can be seen in absorption against the Sgr A radio continuum.

Images of the central 36 arcseconds of the Galaxy in the Brackett gamma line of atomic hydrogen (2.17μm) at a spatial resolution of 0.6 arcsec and a velocity resolution of 110 km/sec were obtained using UKIRT. The velocity separation between adjacent frames is 55 km/sec; 17 different velocities channels were imaged. Several distinct kinematic components are evident within the clumpy distribution of the ionised gas. An image in the v=1-0 S(1) line of molecular hydrogen (2.12μm) clearly resolves the structure of the “molecular ring.”

Three rotational transitions of the CS molecule were observed toward the negative longitude half of the molecular disk which orbits about the galactic center between radii of 2 and 10 pc. A comparison of the relative line strengths allows a determination of the radial run of density in the circumnuclear disk. In order to observe all of the transitions with a similar beamsize, the two lower frequency transitions were measured with the IRAM 30-m telescope, and the highest frequency transition with the NRAO 12-m. Table I lists the observing parameters.

The 18-cm distribution of OH in the Galactic Center region near Sgr A has been mapped in all four of the 1612, 1665, 1667 and 1720 MHz OH absorption lines using the VLA with 4 arcsec angular resolution and 9 kms-1 velocity resolution. The OH gas at +50 and +20 kms-l is seen clearly in absorption against the shell structure of Sgr A East but not against the spiral structure of Sgr A West, possibly implying that this molecular gas lies between the two continuum components - behind Sgr A West and in front of Sgr A East. Inside the Circumnuclear Disk, there is a new neutral streamer which sweeps from the disk in towards Sgr A∗ as the observed radial velocity decreases from +78 to +16 kms-1. The streamer may have a negative-velocity counterpart on the opposite side of Sgr A∗.

The luminosity of the central 5 pc of the Galaxy -encompassing the inner regions of the rotating ring of dust and gas which surrounds the galactic center - emerges primarily at infrared wavelengths in the form of thermal emission from heated dust. The nature and location of the sources which heat the dust can be inferred from the spatial and temperature distribution of the thermal infrared emission (λ>20um), from studies of the ionized gas in this region, and from direct imaging in the near infrared. These observations show that the principal heating sources within this 5-pc region are concentrated within the central parsec of the Galaxy and indicate that the luminosity of these sources is within a factor of two of 107 LO. The near-infrared observations of the compact sources at the galactic center do not reveal a single dominant source but suggest instead that the several components of the IRS-16 complex, taken together, may contribute the bulk of the luminosity; however, the data also permit a single object to dominate the energetics of this region. We draw attention to the striking morphological similarities between the galactic center and the innermost regions of the 30 Doradus nebula in the Large Magellanic Cloud and speculate that the luminosity sources in the galactic center may resemble the early-type supergiants in 30 Doradus.