Conventionally, two major components of spiral galaxies are distinguished: (1) a “spheroidal component”, which includes the central bulge and the surrounding halo, and (2) a “disc component”, which is basically flat and circular but is often warped at the edges. Other structures, such as bars or rings, may also be present, but they are probably of less fundamental significance for the processes of galaxy formation, and therefore they will not be considered here. The distinction between bulge and disc components corresponds approximately to the classical distinction between Population II, as exemplified by the globular clusters which have a nearly spherical distribution, and Population I, which is confined to the galactic plane and is relatively young and metal-rich. However, unlike the “classical” Population II objects which are old and metal-poor, the inner part of the bulge of our galaxy and of other galaxies is in fact metal-rich, and may even contain some young stars.

The dynamics of the gaseous component in disk-shaped galaxies is thought to play an important governing role in star formation, molecule formation, and the degree of development of spiral structure. The prospect that density waves and galactic shock waves are present on the large-scale has received support in recent years from a variety of observational studies. Large-scale shocks provide a most promising mechanism for driving star-forming and molecule-forming events on the scales of many kpc along spiral arms. Such shocks may also govern the kinematics and relative distribution of various galactic tracers. This is particularly apparent in M81 and other external spirals, because of our “bird’s-eye-view” perspective, and for the tracers of HI and CO in our own Galaxy. Comparison of the CO observations with model simulations based on the precepts of the density wave theory shows that these precepts are supported by several observational results.

My major topic will be the evolutionary changes in disk galaxies caused by the continuing process of star formation. The task of interpreting observations to derive the past rate of star formation is treacherous, as I am sure will be evident in the various observational papers presented at this colloquium. Therefore, I will mention only briefly some of the basic aspects of models which have been used to discuss that past evolution.

Heavy metal content is, apart from the kinematical behaviour, one of the important parameters of the population characteristics of stars. Spectroscopic observations of some galaxies including our own give the evidence for the gradient of heavy elements abundances towards the galactic center and the galactic plane (Peimbert 1975, Van den Bergh 1975 review papers). Simultaneously several authors (Kuroczkin 1974, Khokhlova 1976) found that the deficiency (or enrichment) increase with the mass number of the element.

H2O and OH maser sources are observed in the direction of compact regions of HII and IR stars and molecular complexes. Besides, the maser sources are observed in the direction of star formation regions and are associated with Herbiga-Aro objects and T-Tau stars.

As most of today approaches to the problem of galactic structure are based on the consideration of the two main components of a galaxy, halo and disk, it was thought that even simple models of such kinds of double structures, based only on the application of general theorems, could be of some use, at least for simplified outlooks concerning galac tic evolution. With this aim in mind, we have undertaken the treatment of bodies constituted by two different axially deformed polytropic structures, homocentrical and coaxial, with arbitrary values for the two masses MI and MII, the two radii aI and aII of the undeformed objects (aI aII) and polytropic indices nI and nII, interacting with each other only gravitationally. The flattening of the spheroids is assumed to be due to solid body rotation and tidal interaction, and the treatment follows closely the method used for such single structures by Chandrasekhar and Lebovitz.

In this paper we discuss a simple model for the chemical evolution of galactic disks, aiming at understanding the observed metallicity gradients in spiral galaxies. The model in corporates both the recent results of Talbot and Arnett (1975), who proposed a rate of star formation driven by the surface mass density, and those of Lynden-Bell (1975), who investigated the consequences of significant infall of gas into the system. A similar attempt was made by Chiosi (1977) for the solar neighbourhood. The result was that the metallicity distribution of nearby stars can be easily interpreted if the time scale for accretion of gas into the solar pool, or in other words the duration of the disk formation phase, is of the or der of 2-3 109 ys. Such a result is qualitatively in agreement with the models of Larson (1972), Ostricker and Thuan (1975), and also is implicit in the collapse models for the formation of disk galaxies, Larson (1976).

The local chemical inhomogneity of the interstellar medium at a given time is an important factor in models of galactic chemical evolution. It can affect both the G-dwarf metallicity problem (Tinsley 1975, Talbot and Arnett 1973) and the correlation of the abundances of different elements (Tinsley 1976). Observational evidence of abundance gradients in our own Galaxy, and some other galaxies (summarised by Peimbert 1975, van den Bergh 1975), implies that in homogeneities over a large length scale must be created and survive during galactic evolution. Brief consideration of the mixing of the Galactic disk (e.g. Edmunds 1975, 1976) suggests

The origin and dynamics of magnetic fields depend crucially on the dynamics of a gas in the Galaxy. In turn the magnetic field is needed to isotropize the cosmic rays (the influence on chemistry) and is of importance for the formation of stars (local dynamics). Thus the appearance of this topic at the Colloquium is justified.

From observations of the galactic center using various techniques radioastronomy, millimeter waves (molecules) – infrared and gamma rays, the interstellar matter of this region* appears to have been strongly processed into stars : the gas density is much lower than in the solar neighbourhood. From CO measurements one knows that there are many molecular clouds such as SgrB2 where stars are forming now. From IR measurements, there are some indication that low mass stars are relatively more numerous in such regions than in the external regions of the galaxy. Finally the heavy element abundances show three important features (i) the possibility of strong enhancements in elements such as N and in a less extent 0 and Ne (the so called abundance gradients), (ii) Some specific enhancements of isotopes such 43C, 44N and also 47O relative to 42C, 45N and 43O (iii) Deuterium seems to have a lower abundance than in other parts of the galaxy such as the solar neighbourhood. Simple models of chemical evolution have been designed to account for such features and are rewiewed here.

For the purpose of this Paper, I will define the central region of the galaxy as being the inner four kiloparsecs. I make this definition for three reasons:

I will now discuss these points in some greater detail.

This paper is a progress report on studies of coupled dynamical and chemical evolution of galaxies. We have focused our attention on the occurence of galactic hot winds. Such hot winds have been studied by Mathews and Baker (1971) for elliptical galaxies. Recently the detection of an iron X-ray emission line in clusters of galaxies give some support to their existence (Vigroux, 1977). On an other hand, the large radial flow in the galactic center might be explained by such hot wind. We shall present here a preliminary version of our evolution model and the results obtained for the early evolution of a 1.2 × 1012 Mʘ galaxy.

An investigation of the central regions of the Galaxy has been made with an angular resolution of ~10 arcmin with the radio telescopes at Jodrell Bank using the spectral lines of HI (λ21 cm), OH (λ18 cm) and H2CO (λ6 cm). Observations of radio recombination lines in the range (λ21 to 125 cm) have also been taken. These data taken together provide information on the velocity field and gas distribution in the galactic centre region. A continuing programme of spectral line observations of the galactic centre is being pursued at Jodrell Bank.

Many speculations related to a hypothesis that there is a very massive black hole at the centre of our Galaxy have appeared during the last years. This hypothesis is based, firstly, on the opinion that the formation of a massive black hole as a result of dynamical evolution of galactic nuclei is inevitable and, secondly, on the attempts to explain the nature of the “point source” at the Galactic centre (see Oort, 1977 and references therein). However both these approaches could not predict with any confidence the black hole mass. The estimates range from 107-1011 Mʘ (Novikov and Thorne, 1973) to 104 Mʘ (Shklovskii, 1976).

The fundamental difficulty of explaining the spiral structure of galaxies by material arms is the winding dilemma which restricts the life-time of a spiral to about one rotation period. This dilemma is resolved by the density wave concept. However, the density waves are damped by various processes. We calculated the life time of the density wave for our Galaxy (τ = 8.108 yrs) as well as for 24 nearby spiral galaxies with well-known data. It appears that the density wave can explain the existence of spiral arms for only about three rotation periods. After that time the energy of the wave has to be re-supplied. For this, various mechanisms have been proposed, e.g.

• 1) tidal origin – which requires rather special circumstances

• 2) resonant interaction – which is energetically insufficient

• 3) local Jeans-type instability in the outer regions – which also may not be able to provide sufficient energy

• 4) explosions of the nucleus – which involve high inclinations to the plane.

What we call Population II is composed by the stars populating the halo of our Galaxy, for which one finds evidences for an underabundance in the heavy elements and/or for an evolutionary age as large as (roughly) 1010 years. Similar star populations are at present recognized in the halo of other spyral galaxies, in elliptical galaxies as well as in smaller systems, such as the Magellanic Clouds and the so-called Dwarf Galaxies as the ones in Draco and in Ursa Minor. Then Population II looks like representing a very general constituent of present Universe.

Observational evidence related to the chemical composition across the disk of the Galaxy is reviewed. The H2 density distribution derived for the Galaxy is poorly known, consequently it is still not possible to compare theoretical models of the chemical evolution of the Galaxy with the gaseous density distribution. The H2 density distribution is particularly sensitive to the fraction of carbon atoms embedded in CO molecules and to the possible presence of a C/H abundance gradient.

Preliminary estimates are made of the absolute yields of abundant nuclei synthesized in observed stars. The compositions of helium stars of mass 3 ≤ Mα / Mʘ ≤ 64 are presented, taken at the instant of instability. These stars of mass Mα are identified with stars of main sequence mass M. The amount of synthesized matter for each mass M ≥ Mʘ He is estimated (Table 1). Using a variety of choices for the initial mass function (IMF) the yield per stellar generation is calculated. For standard choices of the IMF the absolute and relative yields of 12C, 16O, 20Ne, 24Mg, the Si to Ca group and the iron group agree with solar system values, to the accuracy of the calculations.

The thickness of the disc distribution of young objects is shown to be significantly smaller than that of the interstellar gas over the whole Galaxy, as predicted by the Schmidt’s law of star formation. The radial distribution of the density of young objects at z = 0, compared with that of the gas, is also consistent with the Schmidt’s law. The exponent in this law is found to be between 1.5 and 2. A brief discussion of simple models of galactic evolution using the Schmidt’s law is made.

The basic material for this preliminary investigation is formed of the Northern stare of Gliese’s catalogue, completely measured down to the K5 spectral type in the Geneva system. The physical properties and new spatial velocities are derived and, after removal of non members of the local sphere, 230 stars can be used here.