Star densities on a galactic scale are traced by far infrared emission of dust heated by young stars and by the 2.4 μm radiation of stars in the red giant phase. Coherent results are obtained, pointing to a very strong star formation rate during the last ∼200 My in a ring 5 kpc from the galactic center. A steepening of the initial mass function compared to that observed in the solar vicinity is also suggested.

All sky surveys of the radio continuum emission give us the basic information on the distribution of the nonthermal emission in the Galaxy. At metre wavelengths, where nonthermal emission is dominant, good angular resolution is difficult to attain. For many years the best surveys near 2 m wavelength gave us a picture of the galaxy with ∼ 2° resolution. At centimetre wavelengths, where arc min resolution is available, the intense HII regions dominate the radio sky. Supernova remnants have a distribution somewhat similar to that of the discrete HII regions and must be delineated by various methods in high resolution galactic plane surveys in the decimetre wavelength range.

Recent pulsar surveys have increased the number of known pulsars to well over 300, and many of them lie at distances of several kpc or more from the sun. The distribution of pulsars with respect to distance from the galactic center is similar to other population I material such as HII regions, supernova remnants, and carbon monoxide gas, but the disk thickness of the pulsar distribution is rather greater, with <|z|>≈350 pc. Statistical analysis suggests that the total number of active pulsars in the Galaxy is a half million or more, and because kinematic arguments require the active lifetimes of pulsars to be ≲5×106 years, it follows that the birthrate required to maintain the observed population is one pulsar every ∼10 years (or less) in the Galaxy.

Within the last few years, γ-ray astronomy has shifted from the discovery phase to the exploratory phase, thanks to the SAS-2 and COS-B satellites. The strongest feature of the γ-ray sky is the overwhelming emission of the galactic disc; even the radiation observed away from the galactic plane appears to be predominantly galactic, on the basis of its latitude dependence (Fichtel et al., 1978). Nevertheless, extragalactic γ-ray astronomy is not hopeless: the γ-radiation of the nearby quasar 3C273 has been very recently detected (Swanenburg et al., 1978). A brief summary of the present status of the galactic γ-ray astronomy follows.

Density waves are probably the most general phenomenon producing spiral structure in disk galaxies. The density-wave theory is able to give a rather successful interpretation of the observed spiral structure and of the related kinematics in external galaxies and in our Galaxy. We assume that the reader is familiar with the basic concepts of density-wave theory; for recent reviews see e.g. Kalnajs (1978), Lin (1975), Lindblad (1974), Marochnik and Suchkov (1974), Roberts (1977a,b), Rohlfs (1977, 1978), Shu (1973), Toomre (1977), Wielen (1974a). In Section 2, we discuss the proper theoretical devices which should be used for a meaningful comparison between observations and density-wave theory. The other sections are devoted to a comparison of density-wave theory with some relevant observations, mainly in our Galaxy.

It is generally believed that the constituents of a galaxy like ours are formed from an isolated gaseous assembly at successive stages, starting from a spheroidal sybsystem down to the flattest subsystem.

A number of questions was posed at this Symposium: why is the density of molecules highest near R = 5 kpc? Why is star formation most active near 5 kpc? The life-time of the molecular clouds near 5 kpc is about 2·108 years - where does the gas come from? Why is almost no gas observed between R = 1 and 3 kpc?

A concentrated nuclear bulge with about 30% of the galaxy mass is sufficient (Lin, 1975; Berman and Mark, 1978) to eliminate strong bar-forming instabilities which dominate the dynamics of the stellar disk. Weak bar-like or oval distortions might remain depending on the model. In such systems self-excited discrete modes give rise to global spiral patterns which are maintained in the presence of differential rotation and dissipation (cf. especially the spiral patterns in Bertin et al., 1977, 1978). These spiral modes are standing waves that are physically analyzable (Mark, 1977) into a superposition of two travelling waves propagating in opposite directions back and forth between galactic central regions and corotation (a resonator). Only a few discrete pattern frequencies are allowed. An interpretation is that the central regions and corotation radius must be sufficiently far apart so that a Bohr-Sommerfeld type of phase-integral condition is satisfied for the wave system of each mode. The temporal growth of these modes is mostly due to an effect of Wave Amplification by Stimulated Emission (of Rotating Spirals, abbrev. WASERS, cf. Mark 1976) which occurs in the vicinity of corotation. In some galaxies one mode might be predominent while other galaxies could exhibit more complicated spiral structure because several modes are present. Weak barlike or oval distortions hardly interfere with the structure of these modes. But they might nevertheless contribute partially towards strengthening the growth of one mode relative to another, as well as affecting the kinematics of the gaseous component.

Discrete spiral modes of density waves have been calculated for models of the Milky Way system. The calculation is based on a fluid dynamical formulation. (See Lau and Bertin, 1978.) The equilibrium assumes a rotation curve given by Schmidt in 1965. The surface density σ of the active disk is less than the projected value implied by the Schmidt model, because of the presence of the spheroidal component and of the finite thickness of the disk. One such model (left figure) supports a spiral mode with a pattern speed of 13.6 km/sec/kpc. The spiral pattern (right figure) is very similar to that calculated earlier by Lin and Shu (1967) who used only the short wave. The perturbation density on the contours shown equals 1/5 of the maxima. There are other unstable modes. The modes are sensitive to the assumed Q profile. The superposition of these modes opens the way to the explanation of the complicated features observed but not well explained by the single spiral pattern computed earlier.

Checking the density-wave theory against observations of our own Galaxy has proven very difficult, as witnessed also at this Symposium. Less ambiguous results, however, are obtained for other galaxies. These results involve a) calculating convincing models for a sample of 25 fairly well observed spirals (Roberts et al. 1975) and b) locating the compression zones on the inner edges of the spiral arms.

By considering the interaction of a single stellar orbit with a weak cos 2Φ potential it is shown that in the central regions of galaxies with slowly rising rotation curves, the elongations of the orbits will align along any potential valley and oscillate about it. This effect is more pronounced for elongated orbits. In such regions any pair of orbits will naturally align under their mutual gravity and so a bar will form. The gravity of this bar will drive a spiral structure in the outer parts of the galaxy where differential rotation is too strong to allow the orbits to be caught by the bar. The spiral structure carries a torque which slowly drains angular momentum from the bar, gradually making its outline more eccentric and slowing its pattern speed. In the outer parts of the bar only the more eccentric orbits align with the potential valley; the rounder ones form a ring or lens about the bar. As the pattern speed slows down, the corotation resonance and outer Lindblad resonance, which receive the angular momentun, move outwards. The evolution of the system is eventually slowed down by the weakness of these outer resonances where the material is rather sparse.

A mechanism of triggering star formation by galactic shocks is discussed.

A model has been devised for the orbits of molecular clouds in the Galaxy. The molecular clouds are assumed to be launched from the two-armed spiral-shock wave, to orbit in the Galaxy like ballistic particles with gravitational perturbations due to the density-wave spiral-potential and each cloud is assumed to produce an identical cluster of stars. A comparison of the model with observations suggests that each cloud radiates detectable 12C160 (J = 1 → 0) spectral line radiation from birth to an age of 30 million years and that stars are seen in the cloud 15 million years after its birth. The model has been tested by comparing its predicted velocity-longitude diagram for CO against the observed one for the Galaxy and by comparing the model's predicted surface brightness in the UBV photo metric bands against observed surface photometry for Sb and Sc galaxies.

Observations of the 2.6-mm line of C12O16 were made in the galactic plane from 1=105° to 150° at 1° intervals. The total velocity coverage is 83.2 km s−1 centered at −40 km s−1 with resolution equal to 0.65 km s−1. The observations were made first in June 1976 and confirmed in October 1977 after some improvement of the observing facilities at the Aerospace Corporation. The integration time in each direction is 15 minutes, giving 30-noise dispersions of 0.9K. Among the 46 directions, there are 18 CO detections. Twelve of them have CO components moving with the velocities typical of those expected for the Perseus Arm. Most of them are situated in or near the 0 associations or HII regions in the Perseus Arm, while the others lie in the regions rich in young star clusters. All the velocities observed are in excellent agreement with those expected in the post-shock region in the context of the shock formation model of the spiral density-wave theory. These molecular clouds may be viewed as being formed in the compression phase of the cloud medium as the individual clouds enter the shock front of the intercloud medium. Their line-of-sight velocities are in general a few km s−1 more negative than those of the HII regions, the young star clusters, and the O associations in the Perseus Arm. This suggests that these young objects may be formed prior to the observed CO clouds. The CO velocities are in agreement with those of the HI ridge. One CO cloud is observed at 1=148°, although it is well known that all the tracers of the spiral arm come to an abrupt stop at 1=140°.

In this review the large-scale dynamics of the gas in a model disk galaxy which has an oval, bar-like distortion (or bar structure) in the inner parts is reviewed from the standpoint of recent gas dynamical studies, both steady state and time evolutionary.

We show that there is an important four-armed term in the response of a flat galaxy to an imposed two-armed spiral field near the Inner and Outer Lindblad Resonances.

I have tested the reliability of certain approximations involved in the asymptotic WKBJ density wave description of the inner Lindblad resonance (=ILR) of our galaxy.

Recent work on the distance scale of the Galaxy has largely been in the direction of refining previously established methods. The RR Lyrae variable stars appear to be better distance indicators than was once thought and they have been used in determining Ro, the distance to the Galactic center. Ro is probably somewhat less than 10 kpc but greater than 7 kpc. Most methods point to a value near 8.5 kpc.

In his pioneer study of the RR Lyrae variable stars near NGC 6522, in the relatively unobscured window close to the galactic center (1 = 0.9, b = −3.9), Baade (1963) was limited by the high zenith distance of the galactic center as seen from Palomar, and suggested that southern hemisphere observations would be of value. Subsequent studies of the region have been based on the variables found in Baade's original search. Southern hemisphere plates were taken by Hartwick et al. (1972), and their re-analysis of a sample of Baade's variables showed many periods to be in error. They did not, however, search the plates for new variables. Plaut (1973) re-analyzed all the variables but no search for new variables was done.

After a brief review of previous attempts at identifying the morphological type of our Galaxy by optical or radio methods, a new approach from surface photometry is described. A two-component model consisting of an R1/4 spheroid and an exponential disk is fit to the local galactic disk brightness inferred from star counts and to new observations of the brightness distribution in the bulge along the galactic prime meridian. All parameters are in close quantitative agreement with corresponding quantities for Sbc II galaxies and confirm the AB(rs) morphology first proposed in 1963 (IAU Symposium No. 20). An average of NGC 1073, 4303, 5921 and 6744 described in 1969 (IAU Symposium No. 38) closely approximates the photometric properties of our Galaxy.