We describe a series of observational studies of M81 which are presently underway. We hope that these studies will allow improved understanding of this one, simple, density-wave spiral galaxy.

We consider some results of attempts to trace the pattern of galactic spiral structure in HII regions, HI, and CO. There is really no adequate method available for solving this problem, a fact reflected in the lack of consensus regarding the “correct” spiral pattern. The newly-begun process of deriving galactic structure in CO seems to be recapitulating the history laid down by HI observers.

A variety of observations suggest that clouds of 106-107 M⊙ and extended regions of star formation are the best tracers of spiral structure in the Milky Way.

The CO emission within a few kiloparsecs of the Sun is dominated by a small number of very large molecular complexes, including those associated with the Orion Nebula (Thaddeus 1982), M16 and M17 (Elmegreen, Lada, and Dickinson 1979), and NGC7538 (Cohen et al. 1980). These complexes have masses from several 105 to 106 M⊙ and are generally very well-defined objects. They are also well endowed with HII regions, stellar clusters and associations, masers, and other Population-I objects whose distances can be measured. The complexes are thus valuable probes of the large-scale structure of the Galaxy.

In maps of the galactic structure based on the kinematical method (Fig. 1) several systematically heliocentric anomalies are found: 1. Assuming purely circular motion, the spiral arms are more tightly wound and the extent of neutral hydrogen is smaller in the northern galactic hemisphere than in the southern one. 2. With separate rotation curves for the north and the south, the arms become anomalously circular. 3. Consequently, there is a striking discrepancy with the stellar spiral structure. 4. There are long straight portions in the arms pointing towards the Sun. 5. There are abrupt knee-like features in the south. 6. Some arms seem to affect the structure of other, outer arms. 7. Conspicuously strong curvature of the arms is found in the north. 8. The HI-density is enhanced at symmetric longitudes on the far side. 9. With the northern rotation model HII-regions and HI avoid the southern tangential circle. 10. The Perseus Arm is displaced at 1 = 180°.

The interstellar medium shows turbulence whose velocity and density spectra are surprisingly like a Kolmogoroff law, in spite of the fact that the turbulence is generated mainly by microscopic explosions, rather than a cascade of energy from larger to smaller scales.

New aperture-synthesis observations of HI in absorption towards 3C10 with high velocity resolution (0.6 km s-1) and moderate angular resolution (1 arcmin) have been made with the Westerbork Synthesis Radio Telescope. These are an extension of the survey by Schwarz, Arnal & Goss (1980). A selection of the data has been studied in a preliminary way, covering the Perseus-Arm absorption feature (~ −50 km s−1 with respect to the Local Standard of Rest).

The small-scale structure of galactic neutral hydrogen may be statistically described by the spatial power spectrum of the 21-cm line. This latter may be readily observed by interferometer arrays since it is the squared modulus of the visibility function. We have observed the , region with the Westerbork Synthesis Radio Telescope (Crovisier and Dickey, 1983). Brightness fluctuations of the 21-cm line were detected in this region on scales as small as 1.7 arcmin (corresponding to less than 5 pc). The Westerbork observations, combined with single-dish observations made at Nançay and Arecibo, allow determination of the spatial power spectrum over a dynamic range of about 106 in intensity. The spectrum follows roughly a power law with indices ~ −3 to −2. An interpretation in terms of the turbulence spectrum is proposed by Dickey (1985).

Preliminary results of a 10-GHz radio-continuum survey of the galactic-plane region using the 45-m telescope at NRO are presented. An extensive study of a complex region at 22° ≦ ℓ ≦ 25°, |b|≳1° has been made.

Observation of absorption lines produced by interstellar gas is a straight-forward way to determine column densities and velocities along the line of sight of interstellar clouds. In practice, peculiar motions often mask galactic rotation and/or cause line blending. We have made a study of absorption lines of interstellar sodium covering a substantial part of the Galaxy at extremely high spectral resolution.

We report some preliminary results from the Massachusetts - Stony Brook CO survey of the first galactic quadrant using the 14-meter millimeterwave telescope of the Five College Radio Astronomy Observatory. The survey contains approximately 50 000 observations spaced every 3 arcminutes in 1 and b between longitudes 0° and 90° and latitudes −1° and 1°. We have mapped emission from giant molecular clouds (GMC) which we identified, in earlier more limited strip surveys of the galactic plane (Solomon, Sanders and Scoville 1979; Sanders 1981), on size scales from a few parsecs to hundreds of parsecs, in order to determine the degree of clustering and organization into large-scale features. In addition to the characteristic size of 20 – 60 pc for individual GMC, we find clustering of clouds on a scale of from 100 to 300 pc.

A segment of the Milky Way in Norma, from ℓ = 327° to 335°, |b|≦ 1°, has been studied as part of the Columbia CO survey of the fourth galactic quadrant. Description of the entire survey is given by Cohen elsewhere in this volume.

We have used the Estec/Utrecht heterodyne submillimetre receiver and the ESO 3.6-m telescope at La Silla (Chile) to observe the CO(2–1) transition at 230 GHz (1.3 mm) in the Magellanic Clouds. We used a beam of 2 arcmin HPBW (corresponding to linear resolutions of 32 and 46 pc for LMC and SMC respectively), the system temperature was 2200 K (DSB) and the overall system efficiency was 0.55. In this paper we summarize the results, which are published in more detail elsewhere (cf. Israel et al., 1982, 1983; Israel, 1984).

We assemble principal constituents of the morphology of the Orion Arm within 1500 pc as a starting point for the study of progression of star formation.

A major study of the molecular gas surrounding young star clusters is underway. We are using the Columbia University 1.2–m millimeter-wave telescope to observe emission from the J=1→0 rotation transition of 12CO in the vicinities of 128 open star clusters. The survey region around each cluster is at least 10 cluster diameters in size, typically ≳ 5 square degrees. Sensitivity is sufficient to detect lines as weak as 1 K over a range in velocity ± 83 km/s centered on the cluster velocity and with a velocity resolution of 0.65 km/s. Clusters in this sample have well-determined distances ranging from 1 to 5 kpc, and ages ≳ 100 million years (Myr).

The structure and kinematics of the neutral hydrogen associated with Gould's Belt have been studied using data of high velocity resolution and large latitude extent covering ℓ = 10o − 350°. The data comprise the Berkeley Survey of Neutral Hydrogen (Weaver and Williams 1973, 1974), and an unpublished survey by Kerr, Bowers, Kerr, and Jackson (1983) using the 60-foot Parkes telescope. The latter is a fully-sampled survey of the region ℓ = 240° to 350°, b = −10° to +10°.

The ionized gas within 2 parsec from the centre region is distributed in spiral-like features which appear to emanate from a nucleus. This nucleus is close to the ultra-compact radio source and the infrared source IRS 16, but does not exactly coincide with either. The spiral is strongly inclined to the galactic plane. The arms are clumpy. The spiral appears to rotate, but there appear to be also large radial motions.

The phenomena show some resemblance to what is observed on a very much larger scale in the nuclei of radio galaxies and quasars.

An exceptionally powerful, variable source of hard X- and gamma-rays, GCX, has been found in the direction of the centre. This may be the true galactic nucleus. Strong, variable emission of the 511-keV electron-positron annihilation line has also been observed to come from the central region.

The evidence for a central black hole is discussed. It is not compelling. The energy required for the strong infrared radiation emitted by the central few parsecs is probably coming from a large number of luminous stars, and due to a recent burst of star formation. The observed state of ionization indicates the burst to have occurred about one million years ago.

The distribution of the dense molecular clouds within 300 pc from the centre is very asymmetric: practically all lie at positive longitudes. Their motions differ radically from circular motions. The layer of clouds is tilted relative to the galactic plane. The so-called “+40-km/s cloud” lies in front of the nuclear spiral, and is therefore moving towards the centre. Different interpretations of the systematic motions are considered. The outspoken asymmetry excludes a static condition. If the “molecular ring” at R ~ 150 parsec is actually an expanding feature, its origin might be sought in the same burst of star formation proposed to explain the state of ionization and the infrared radiation of the nuclear region.

We have observed the HCN(J=1–0) line in the vicinity of the galactic center with the 18″ beam of the Nobeyama 45-m telescope. Profiles were taken at 53 points within 1 arcmin from the galactic nucleus [R.A. = 17h42m29.29s, Dec. = −28°59′17.6″ (1950)] with a 10″ grid (see figure 1). A SSB cooled-mixer receiver (TRX = 600 K) and a wideband AOS (acousto-optical radiospectrometer) with 250 kHz resolution were used.

Aperture-synthesis observations of the H76α and H110α recombination lines are presented for the inner (3pc) region of the Galactic Nucleus. The large line width measured with single dishes (Pauls et al., 1974) is caused by well-ordered large-scale motions of the ionized gas. The velocities of the NeII clumps (Lacy et al., 1980) fit well into our smooth velocity field and smooth intensity distribution. We suggest therefore that the cloud picture of the NeII gas is (at least partly) invalid.

Using the VLA, we obtained matched-array continuum observations at 6 and 2 cm. An angular resolution of ~1″ and an rms sensitivity of ~0.05 mJy were achieved for a sample of 17 nearby spiral galaxies. Spectral-index maps derived for the nuclear regions reveal a mixture of thermal and nonthermal activity. The use of high angular resolution and high frequencies was the key to the success in detecting thermal activity. Conversion from thermal fluxes to total number of ionizing photons suggests that star formation is very active in some of these cores (inner 500 pc), with a rate typically 10 times greater than in our own nuclear region. A number of the nuclear regions appear to be dominated by extended nonthermal emission with a steep spectrum. Among these, some are closely associated with thermal emission and hence are consistent with supernova activity. However there are sources exhibiting aligned structures, suggesting possible connections with a central active nucleus. In any case, at our achieved sensitivity level 16 out of 17 galaxies were detected in the radio continuum.