We have measured the linear polarization of a new large sample of extragalactic radio sources, and by combining these with polarization values already in the literature, we have been able to compute a large number of rotation measures, with improved quality. We have also investigated the depolarization properties of these sources and as a result have been able to identify most sources with a large internally generated Faraday rotation. Figure 1 shows the rotation measures of 475 extragalactic radio sources on an equal-area projection, after “cleaning out” the extragalactic effects to first order.

Parker (1966) has considered a simple equilibrium state of the interstellar gas and fields system, such that the magnetic field lines are parallel to the galactic plane; he has shown that this state is subject to the Rayleigh-Taylor instability. The gravitational pull of the stars on the gas and the buoyancy of the magnetic field tend to bend the field lines, allowing the gas to slide down towards the plane. The stability of equilibria with curved magnetic field lines remained to be considered.

There has been a great development in the past few years of observational data of various kinds:

1. (a) HI observations of high resolution and sensitivity (Cohen and Davies, Burton et al., Kerr et al.)

2. (b) Observations of molecules, in particular high-resolution observations of CO (Burton, Gordon, Bania, Liszt, Solomon et al.)

3. (c) Infrared and far-infrared

4. (d) Ionized gas: hydrogen recombination lines, and in particular Nell (with 4″ beam) (Mezger et al., Townes, Wollman et al.)

After reviewing the available observational material, we describe here a simple model of the distribution and kinematics of HI gas within 1.5 kpc of the galactic center. According to this model, most of the inner-Galaxy gas is smoothly distributed in a tilted disk, within which the perceived kinematics are consistent with axisymmetric rotation and expansion of approximately equal magnitude. The model subsumes in a coherent way many observed spectral features which were previously studied separately, without requiring important density enhancements or anisotropic ejection from the nucleus.

It is nearly twenty years since Rougoor & Oort (1960) reported the discovery of neutral hydrogen (HI) emission from the galactic nucleus, and I would like to summarize some of the progress that has been made since then in observing and interpreting this emission. One of the most surprising properties to be found is the tilt of the HI distribution relative to the galactic equator. This was noted originally by Kerr (1967), and recent measurements indicate that the tilt is present from within 200 pc of the centre out to a distance of some 2 kpc. (Cohen & Davies 1976, 1978). Because of the tilted distribution the properties of the nuclear emission cannot always be deduced from observations at zero galactic latitude. For example, Rougoor & Oort (1960) deduced a sharp cut-off in the negative-velocity “nuclear disk” emission at ℓ ≃ −4°, from their measurements near zero latitude, whereas the more extensive observations by Kerr (1967) show that the cut-off is only an apparent effect. In reality the emission continues to more negative longitudes, but at higher galactic latitude because of the tilt.

A complete, high-sensitivity survey of HI (Sinha 1978, in preparation) has been examined for model-independent morphological properties of the distribution of HI in the central region of the Galaxy. The dominant symmetry of the kinematics of HI is evident in Figure 1, where equivelocity contours of the permitted velocity edge (corresponding to 1 K antenna temperature) have been plotted on the longitude-latitude plane. The axis of symmetry is coincident with the Kerr-Sinclair ridge of 20-cm continuum radiation from this region. The positions of the maxima of the absolute velocity in the first and the fourth quadrants are aligned along a line inclined to the line of symmetry indicated in Figure 1. The HI gas, therefore, must have noncircular motion. A similar but not identical symmetry is evident in the distribution of the forbidden velocity edge of the profiles. The absolute magnitude of the velocity peaks and the area under the profiles differ in the two quadrants. The nuclear disk appears to be symmetrical in an angle-velocity diagram in which the angle is measured along the line of symmetry in Figure 1.

Emission and absorption spectra of several molecules have been used to trace the kinematic patterns of molecular material in the inner Galaxy. For a variety of reasons, the great majority of effort in this area has focussed on the region 357 ≳ ℓ ≳ 3° very near the plane b = 0°. Extending the observational coverage of the molecular distribution away from this plane and over a wider longitude range is a tedious process only now beginning and severely hampered by relatively small beamwidths and long integration times. Our knowledge of the arrangement of molecular material in the inner few kpc of the Galaxy is primitive and quite incomplete compared to that of the atomic gas sampled at λ21 cm.

First isolated using the 21-cm HI spectral line, the 3-kpc arm exhibits the largest non-circular motion of any large-scale gaseous structure in the Galaxy (see e.g. Rougoor, 1964). Carbon monoxide observations best delineate the 3-kpc arm because the CO features are both narrower in velocity and also less confused with emission from more local gas than are HI observations. Because CO is a tracer of molecular hydrogen, the HI and CO data can in principle provide a good estimate of the total gaseous mass of the 3-kpc arm. A previous CO study of the inner Galaxy (Bania, 1977) suggested that, unless it is significantly tilted with respect to b=0°, the 3-kpc arm cannot be a continuous ring structure because emission at extreme positive velocities which should be produced by ring segments lying farther than 10 kpc from the Sun is absent. Consequently, the latitude distribution of CO in the 3-kpc arm has been studied by surveying 12CO emission over the region 350°<ℓ<25° at b=0° and ±0°.33. The survey positions are separated by Δℓ=1°. This coarse angular sampling resolution is deemed sufficient because the typical 3-kpc arm CO cloud has a linear dimension of 100 pc (Bania, 1977). Such clouds will subtend ∼0°.4 at the maximum line-of-sight distances expected for the extreme positive velocity 3-kpc arm emission (∼14 kpc).

The centimeter wavelength continuum radiation seen toward the Galactic center (Figure 1) is a mixture of thermal (free-free) and nonthermal (synchrotron) radiation which originates in the nucleus and along the line-of-sight. In this review we discuss only the thermal emission (also see Mezger 1974 and Oort 1977). High-frequency radio continuum and recombination line observations show that the thermal radiation comes from extend, low-density (ELD) HII, and a number of giant “radio HII regions” (see Mezger 1978 for definitions). The approximate half-power contour of the ELD HII (labelled EI in Fig. 1), probably represents a superposition of evolved and expanded HII regions. Thermal radiation outside EI comes predominantly from along the line-of-sight (see Pauls and Mezger 1975).

Infrared observations have provided considerable information about the structure and energetics of the galactic center. The stellar bulge dominates the mass and luminosity of the nuclear region. The luminosity of the bulge is strongly peaked toward the center, even within the central parsec. Dust in the nuclear disk absorbs the power output of the central portion of the bulge and reemits it in the far infrared. Near the center, molecular clouds move in a plane apparently tilted toward the Sun. Within the central few parsecs, or core, the inclination may be as large as 45°. The total power output of the core is about twice that of the bulge population alone. The source of excess luminosity is uncertain, but evidence points to ongoing star formation associated with the Sgr A molecular complex.

Mira variables are attractive for galactic centre studies because (1) they are numerous there; (2) they have high luminosities (MK ∼ −7.5) in the infrared where they can be seen through heavy absorption; (3) good radial velocities can be obtained from emission lines even for very faint objects; (4) at least in the solar neighbourhood and in globular clusters the period seems a good indicator of age and/or chemical composition.

Studies of the stellar distribution in the inner region of our Galaxy have been seriously hampered at optical wavelengths by strong interstellar extinction. The extinction decreases considerably at infrared wavelengths, allowing us to look deep into the Galaxy. Motivated by this, we have tried to observe the near infrared brightness distribution of the central region of the Galaxy (Okuda et al., 1977, Maihara et al., 1978, Oda et al., 1978). Similar observations have been carried out by Hayakawa et al., (1976), Ito et al., (1977), and Hofmann et al., (1977). These observations have provided valuable information on the distributions of stars and dust in the inner Galaxy (Hayakawa et al., 1977, Maihara et al., 1978, Oda et al., 1978).

Based on current 2.4-micron observations of the Galaxy (see Okuda et al. in this Symposium), we have proposed a specific model for the bulge component. This model is a concentric spheroid with an axial ratio of ∼0.5; ρ(a)∞(a2+ac2)−1exp(-(a/ao)2), where ao = 2.5 kpc and ac = 0.14 kpc respectively. A constant mass-to-luminosity ratio M/Lv≃7.6 is assumed, which yields the relevant rotational velocity in the inner region (Figure 1); the absolute velocity is normalized to 250 km s−1. This value of M/Lv is likely to meet with giant-rich synthetic models for the nuclear bulge of M31.

I will define the central region of the Galaxy as being the inner four kiloparsecs. The distinguishing characteristics of this region are:

1. 1) The dominance of a central spheroidal component in the mass distribution – a bulge.

2. 2) An apparent deficiency of gas, at least between radii of 500 pc and 4000 pc.

3. 3) High non-circular gas velocities. Now let us consider these characteristics in some detail.

To explain the features in the range 340° ≤ 1 ≤ 22° which show expansion velocities, elliptical stream-lines have been proposed that happen to expose expansion components towards the sun. Three general observations, however, make this interpretation doubtful:

During the past 10 years an hypothesis about the presence of a massive black hole at the center of our Galaxy (Lynden-Bell, 1969) has been an object of many exciting speculations. This hypothesis is based, firstly, on attempts to explain the nature of the “point radio source” at the galactic center (as well as a presumed much more powerful activity of the galactic nucleus in the remote past), and, secondly, on the opinion that the conditions in the course of dynamical evolution of galactic nuclei are favorable for the formation of massive black holes. However, both these approaches did not succeed in predicting with any confidence the black hole mass at the center of the Galaxy. The estimates available are based on indirect arguments and range from 107-1011 M⊙ (Novikov and Thorne, 1973) to 104 M⊙ (Shklovskii, 1976). A recent dynamical approach using NeII infrared observations of the galactic center (Wollman et al., 1977) has indicated that the black hole mass does not exceed 5×106 M⊙ (Oort, 1977), although this value may well be due to a very dense star cluster whose brightest members only are seen in the infrared.

There is a reasonable chance of finding a (probably X-ray) pulsar in a short-period orbit around the galactic center. Such a pulsar can provide a test distinguishing a central black hole from a supermassive object or spinar. It also makes available a good clock in a region of space in which GM/Rc2 is much larger than solar system values, thus allowing strong-field tests of general relativity.

Observations at various wavelengths are considered for extragalactic nuclei and are compared to how our galaxy would appear at comparable distances. The starlight from our nucleus is similar to that from the spirals in the Virgo Cluster. Our nucleus would show no sign of activity to a distant observer, neither unusual color, nor emission lines, nor excess infrared radiation. For example, the luminosity in Hβ emission is about 1038 ergs s−1, which is 100 times fainter than that in the faintest Seyfert galaxy or emission line galaxy. It is also emphasized that there is no evidence from X-ray data for a massive, condensed object in the Galactic nucleus.

Of the eleven galaxies with detected CO emission, eight have bright nuclear CO sources: M82, NGC 253, M51, NGC 5236, NGC 1068, Maffei 2 (Rickard et al. 1977a,b), NGC 6946, and IC 342 (Morris and Lo 1978). Two have disk-population CO sources and no detectable nuclear source (M31 and M81, Combes et al. 1977a), and one has no obvious nucleus (LMC, Huggins, et al. 1975). Nuclear maxima thus appear to be the rule for galaxies with extensive molecular components, and such a peak is also seen in our Galaxy (e.g., Bania 1977). In Figure 1, I compare the CO data for the nuclei of M82 and M31 with spectra of the Galactic nucleus as it would be seen at their respective distances. (The Galactic spectra were synthesized from the date of Bania [1977], and assume a uniform z-distribution of 30 pc width.) The Galaxy is roughly intermediate, being about one-fifth the intensity of M82 and more than six times the intensity of M31.

Continuum observations of NGC 6946 strongly suggest the presence of an exponential non-thermal disk component. When applied to galactic 408 MHz observations, the scale length is found to be between 5 and 6 kpc. Continuing the parallel with NGC 6946, a total luminosity of 2 × 1010 L⊙ is derived. Observations of the edge-on galaxies NGC 891 and 4631 show the existence of flattened halos with a steep spectrum around these galaxies. This is consistent with the constraints that are known for the galactic radio halo. On the basis of the radial distribution of HI in the Galaxy and in M81 three regimes can be defined: a central one which is hyperdeficient in hydrogen; a middle one which is deficient in HI, but not necessarily in total hydrogen; and an outer one which is neither deficient in HI, nor in H. Density wave theory, combined with gas flow dynamics appear a powerful tool in interpreting the kinematical data. No rotation curve is yet known to become Keplerian in the outer parts. Many external galaxies are warped, like our own; one of them, NGC 5907, does not have a visible companion.