Introductory remarks for IAU Symposium No. 84 are made. These include a brief history of previous symposia dealing with galactic structure, basic research problems in this area, and parallels regarding our galaxy which can be drawn from extragalactic research.

A brief review of current theoretical views on how disk and spheroidal galaxies form and evolve provides the background for a summary of recent optical observations of external galaxies. Primary emphasis is placed on a discussion of the large-scale distribution and chemical composition of the stellar and gaseous constituents of relatively isolated galaxies. New studies of halo and disk surface-brightness distributions in spiral and SO galaxies are summarized. The “missing mass” in galactic halos, the relationship between disk size and luminosity, the nonexponential character of disk light distributions and very low-surface-brightness disk systems are highlighted in this section. Next discussed are observations which may provide insight into the factors which regulate the star-forming history of galactic disks and the post-astration appearance of spiral galaxies. Finally, the observed properties of relatively isolated disk galaxies are compared with those located in dense groups. It appears from this comparison that environment plays a significant role in governing the evolutionary history of a galaxy.

The main purpose of this session is to gather observational data which should ultimately yield a better understanding of the relationships between the interstellar matter and the stars in our Galaxy. Ideally we would like to know how the Galaxy evolves in this respect and in particular to understand quantitatively the laws of star formation: What is the relation between star formation and the density of gas? (for a study of this kind see Guibert, Lequeux, and Viallefond, Astron. Astrophys. 68, 1, where Schmidt's law is rediscussed); Does the Initial Mass Function (IMF) vary within the Galaxy? (for recent studies see Burki, Astron. Astrophys. 57, 135, and Puget, Serra, and Ryter, this Symposium). We are obviously still far from being able to give definitive answers to these questions, but I hope that we will obtain at least a better understanding of the basic ingredients: the distribution of interstellar matter and of young stars.

Through systematic surveys (Johansson et al., Caswell and Haynes, Bowers, Baud, Caswell et al.) some 200 OH masers have been detected that are presumably associated with long period variables of very late spectral type (>M5). Tentatively these stars will be called “OH/IR stars”. They are characterized by their strong emission in the 1612 MHz line which shows a double peak due to the expansion of the shell surrounding these stars. The velocity difference ΔV between the peaks is generally between 10 and 50 km s−1.

Millimeter wave observations of emission from the CO molecule have become, over the past eight years, the dominant method for determining the physical properties of dense interstellar clouds, composed primarily of molecular hydrogen and for exploring the structure and kinematics of the galactic disk. In this paper we briefly review the CO survey results in the literature (Section 2) and then present new results (Section 3-7) of an extensive 13CO and 12CO survey of the galactic distribution, size, mass and age of molecular clouds. The interpretation of this survey leads to a new picture of the interstellar medium dominated by very massive stable long-lived clouds which we refer to as Giant Molecular Clouds. We find that Giant Molecular Clouds (GMC's) with M = 105–3 × 106M⊙ are a major constituent of the galactic disk, the dominant component of the interstellar medium in the galaxy interior to the sun and the most massive objects in the galaxy. We find that the interstellar medium and star formation are dominated by massive gravitationally bound clouds in which stars and associations are forming but at a very low rate in comparison to the free fall time. The galactic distribution of the molecules as traced by CO emission is interpreted as the distribution of GMC's. As the most massive objects in the galaxy they are also basic to the dynamics of the disk.

We are undertaking at Columbia University an out-of-plane survey of 2.6 mm CO emission from the first quadrant of the galactic disk with a 1.2 meter telescope. With the 8′ beamwidth of this instrument at 2.6 mm (about that of the largest existing steerable instruments at 21 cm) it is possible to completely map the first quadrant in only a few hundred days of observation.

The CH distribution in the galactic plane has been determined through observations of the ground state main line transition 2π1/2, J=1/2, F=1-1 at 3335 MHz. The longitude ranges 10°≤ℓ≤60° and 60°<ℓ≤230 have been covered with the spacings 2°.5 and 5°, respectively. The derived radial distribution is similar to that of CO, although differencies may exist. From observations at ℓ=30° and 43°.26 the CH layer halfwidth is estimated to be 50 pc, in close agreement with that of CO. A CH displacement below the standard plane is observed in these directions.

This review of 21-cm studies of the galactic disk will be rather short, because several aspects of such studies are being covered by other speakers. These include the galactic rotation curve, the warp, the outer limits of the hydrogen layer, high-velocity clouds, large supernova remnants and comparisons between observations and theory with respect to density waves.

The ionized hydrogen regions seen on monochromatic Hα photographs of nearby spiral galaxies (e.g. Fig. 1) can be divided into four classes (Monnet, 1971): (1) bright, condensed, classical HII regions; (2) diffuse emission in the arms surrounding and connecting the classical regions (emission measure (EM) ∼ 150 cm−6 pc, for a gas electron temperature of 6000 K); (3) much fainter, diffuse emission extending over the entire disk (EM ∼ 50 cm−6 pc) in most Sc and Sd galaxies; (4) diffuse emission in the nuclear region.

Although radio recombination line and continuum observations are very useful for investigating galactic structure, it is well to remember their limitations. First, they only provide measurements of coordinates and velocities; a kinematic model is needed to derive the distance and thus the actual location of every nebula. In some directions, particularly all longitudes from ℓ=0°, kinematic distance estimates are prone to systematic errors arising from velocity crowding or uncertainties in the rotation curve, and are of little use in quantitative studies. I will only discuss the distribution of radio nebulae in the inner ∼100° of the galactic plane, since in this area kinematic analyses can give reasonable results and, in any case, here we must rely on radio observations for most of our information.

A new continuum survey of the galactic plane has been made with the Effelsberg 100-m telescope at a frequency of 4.875 GHz with a beamwidth of 2°.6. The data are available in the form of radio contour maps covering ℓ = 357°.5 to 60°, b = ±1°, together with a list of 1186 radio sources. (Altenhoff, Downes, Pauls and Schraml, 1978, Astron. Astrophys. Suppl. 35, 1).

A deep survey of formaldehyde absorption along the galactic plane has been made using the MkII telescope (beamwidth 9 arcmin). Observations were made with 1° spacing between ℓ = 14° and 36° and 2° spacing elsewhere in the longitude range 8 to 60. Integration times were typically 7 hr; the rms noise was 4 mK.

The Jet Propulsion Laboratory (California Institute of Technology) has recently begun preparations for a modest SETI (Search for Extra-Terrestrial Intelligence) project at radio wavelengths. The proposed project is two-fold: (1) to search the entire sky visible from Goldstone, California (site of one of the NASA Deep Space Network Stations), i.e. that north of δ ∼ −30°, at all frequencies between 1 and 24 GHz using a horn receiver and (2) to carry out a more sensitive sky search at a few selected frequency bands, using dedicated 26 and 10m antennas. The search will be for signals of the type unlikely to be produced by natural causes, e.g. narrow spikes in frequency space. The basic rationale behind this search is discussed by Murray, Gulkis and Edelson (1978).

The ratio R between visual extinction and colour excess, is slightly larger than 3 and does not vary much throughout our part of the Galaxy. The distribution of dust in the galactic plane shows, on the large scale, a gradient with higher colour excesses towards 1=50° than towards 1=230°. On the smaller scale, much of the dust responsible for extinction is situated in clouds which tend to group together. The correlation between positions of interstellar dust clouds and positions of spiral tracers seems rather poor in our Galaxy. However, concentrated dark clouds as well as extended regions of dust show an inclined distribution similar to the Gould belt of bright stars.

Ever since the pioneering work by Morgan and his collaborators (1952, 1953), it has been well known that the distribution of the associations of young stars, HII regions, and young clusters defines the optical spiral features. Although considerable progress has been made in spiral structure studies during these past 25 years, the basic picture of optical spiral structure has not been significantly altered. The three spiral features first described by Morgan are still recognized. Modern work on the various optical spiral features has strengthened and improved the definition of the optical features, especially to larger distances. Most of the improvements and any additions to the basic three-arm pattern have resulted primarily from observations of the spiral tracers in the Southern Milky Way. Specifically, the Sagittarius feature is now generally recognized as the Sagittarius-Carina arm which may indeed be a major arm of the Galaxy. It can now be traced optically to very large distances, up to 6 kpc or more in the direction ℓ = 290°. The Local arm (Cygnus-Orion) probably extends to 4 kpc in the direction of Puppis (ℓ ≅ 240°), and most astronomers would probably agree that our local spiral feature is not a major arm, but an inter-arm feature.

The use of modern photographic emulsions and techniques allows for the discovery of optical spiral tracers much fainter than was possible in the past. Our searches for such faint objects deal with the Puppis, Vela, Crux, Circinus, Norma and Ara regions of the Milky Way using Kodak plates (baked in dry nitrogen) obtained with the Curtis Schmidt-telescope at Cerro Tololo Inter-American Observatory. We have discovered OB stars as faint as 15 mag on IIIa-J plates obtained with the thin prism, and Hα emission-line objects (many of which are Be stars) as faint as 16 mag on 127-04 plates obtained with the 4° prism.

The photometric distances of 60 young galactic clusters (with sp < b3), all observed in UBV or RGU, have been calculated or recalulated according to “method A” (Becker, 1963). The galactic distribution of these clusters, shown in Figure 1, confirms their role of being good spiral tracers.