It is well known that in dynamical systems governed by long-range forces, the secular evolution of the system is driven predominantly by collective effects. In the current paper, we report the discovery of such a collective dissipation process induced by a self-sustained spiral density wave, which leads to important secular evolution effects in spiral galaxies.

The study of stability of the stellar disks of flat galaxies is the first step towards an understanding of the phenomena of spiral structure. To explain the spiral pattern, Lin and Shu (1964) further developed the Lindblad's idea of density waves by considering the spiral structure as the collective effect in a self-gravitating system. As an extension to the original Lin-Shu theory, unstable spiral modes have been obtained from an asymptotic analysis (e.g., Bertin [1980]). In such “modal theory”, the Jeans-type instability (even weak) is important to the maintenance of spiral structure.

Until recently, only collisionless models have been investigated in the kinetic treatment of stellar disk stability (e.g., Fridman and Polyachenko [1984]). This is due to the fact that the frequency of ordinary binary stellar gravitational (elastic) encounters in the Galaxy is much smaller than the variation of the gravitational field for the process being studied. However, in the pioneering paper Spitzer and Schwarzschild (1951) proposed a different kind of encounter: interaction of stars with gas clouds of the interstellar medium having a mass of rougly 106 M⊙. In recent years this hypothesis was partially confirmed by observations: it was discovered in the Galaxy a few thousand giant molecular clouds of mass Mc ≥ 105 M⊙. Other evidence of dynamical relaxation of the star–cloud disk in the solar neighborhood was found by Grivnev and Fridman (1990); the time of relaxation was estimated equal to τ = (2 – 4) × 109 years. Hence the study of collisional star–cloud system is not only of academic interest – on the time span t ≥ 109 years an actual galaxy may be a collisional ensemble of stars and clouds.

It is evident that the knowledge of not only positions and velocities of objects belonging to different populations but also their osculating galactic orbits can clarify the structure and history of our Galaxy. Action variables being adiabatic invariants are especially useful for these purposes (Binney, May 1986). One of them is a usual area integral (and it can be found without any orbit calculation). The second action variable is connected with an energy integral but for 3–D orbits and arbitrary potentials it cannot be found analitically. Eccentricities resulting from orbit calculations can be used here (Lynden-Bell 1963). Comparison of eccentricities defined with different ways shown that differences between them are practically negligible (Kutuzov 1985, 1987). As for the third action variable and the corresponding third isolating integral orbit calculations can confirm its existence and control the validity of various approximations. Numerical technique allowing to find it and working at any potential is now in progress (Binney, Kumar 1993).

We have analyzed the distribution of residual velocities of nearby stars (within 200 pc of the Sun) looking for space variations on the velocity ellipsoid. We used a sample of 1071 main sequence stars of spectral types B, A and F selected from the Hipparcos Input Catalogue [7] with uvbyHβ photometric data. Ages have been calculated following [1]. Six subsamples with 8.07 ≤ log(age) ≤ 9.45 have been considered.

In order to investigate a possible origin for stars with high peculiar velocities in the thick disc of our Galaxy, the dynamical evolution of 16 000 three-dimensional triple systems which consist of a binary with equal or comparable masses of components M 1 and M 2 and a low-mass third body M 3 is considered. We examine an extensive range of initial conditions with positions of the body M 3 randomly distributed around and inside the binary orbit. M 3 was given the initial radial velocity V 0 with respect to the center of inertia of the binary. The following dynamical system of units is used in this work: the unit of distance is the semi-major axis of the binary orbit, the unit of time is the period of the binary; the universal constant of gravity is unity. In these units the total mass of the close binary is 4π2.

Triple stars and galaxies are referred to as having hierarchical or non-hierarchical structure. Thus, the dynamically stable and unstable triple systems are usually distinguished observationally by the ratio of the larger and smaller angular separation. However, (i) projection effects may hide the true configuration; and (ii) apparent hierarchical forms may occur in both types (Anosova 1986).

In order to obtain an objective criterion for dynamical isolation of binaries within systems of large multiplicity we study numerically the dynamical behavior and average parameters of stable hierarchical triple systems containing a close binary. Using the new criterion for hierarchical stability of triple systems with different mass ratios of components (Kiseleva, Eggleton, Anosova 1994; Kiseleva, Eggleton, Orlov 1994) the perturbing force from the outer body on the close inner binary is estimated. On this basis, the critical separations are obtained when both inner and outer orbits are practically not perturbed. Because the dispersion of masses has a very strong influence on dynamical processes in N-body systems, mass ratios of subsystems, and sometimes within subsystems, should always be taken into account.

We now stand at the threshold of the 21st century having witnessed perhaps the greatest era of astronomical discovery in the history of mankind. During the twentieth century the subject of astronomy was revolutionized and completely transformed by technology and physics. Advances in technology that produced radio astronomy, infrared astronomy, UV, X and γ ray astronomy, large telescopes on the ground, in balloons, aircraft and space coupled with advances in nuclear, atomic and high energy physics forever changed the way in which the universe is viewed. Indeed, it is altogether likely that future historians of science will consider the twentieth century as the Golden Age of observational astronomy. As a measure of how far we have come in the last 100 years, recall that at the turn of this century the nature of spiral nebulae and of the Milky Way itself as an island universe were not yet revealed. The expansion of the universe and the microwave background were not yet discovered and exotic objects such as quasars, pulsars, gamma-ray bursters and black holes were not even envisioned by the most imaginative authors of science fiction. The interstellar medium, with its giant molecular clouds, magnetic fields and obscuring dust was unknown. Not even the nature of stars, these most fundamental objects of the astronomical universe, was understood.

Four processes of molecular cloud formation are discussed: formation in swept-up shells, coagulation of smaller clouds, condensation in larger clouds, and compression in a supersonically turbulent medium. Examples and constraints for each process are given.

The outer Galaxy (defined as those parts of the Galaxy with galactocentric distances R>R0=8.5 kpc) in general and particularly the outermost regions (R ≳ 16 kpc; the far-outer Galaxy or FOG) has not received the attention that has been dedicated to the inner Galaxy and the solar neighbourhood.

The Galactic continuum spectrum from 5–96 cm−1 is derived from COBE/FIRAS observations. The spectra are dominated by warm dust emission, which may be fit with a single temperature along each line of sight in the range 16–21 K. A widespread, very cold component (4–7 K) with optical depth that is spatially correlated with the warm component is also detected. The nature of the cold component and its implications for the amount of very cold material in the Milky Way are discussed.

It is well known that the rotation curves of many galaxies remain flat well outside their optical disks. This leads to the conclusion that the visible mass in galaxies is only a small fraction of the total (see e.g. Faber & Gallagher 1979). Although we are certain that “dark matter” exists, its composition and distribution are still unknown.

In a recent paper Pfenniger et al. (1994) argued strongly for the possibility that dark matter could well be in the form of cold, predominantly molecular gas in a flat distribution. To have escaped previous detection in the outer parts of our Galaxy, the gas would need to have a low temperature and thus a low emissivity. Because most substantial heating sources of gas are associated with the presence of massive stars (UV radiation and cosmic rays) this requirement may be satisfied in the Galactic plane outside the optical disk.

Nearly all the star formation in the Milky Way and nearby spiral galaxies occurs in the giant molecular clouds (GMC). Inside the GMC's the units of star formation are the high density (≥ 103 cm –3) and high mass (≥ 103 M ⊙) clumps (Blitz, 1991). Once a GMC is “infected” by star formation many clumps form stars producing a star forming region. The formation of massive stars induces destructive processes, such as H2 dissociation, HI ionization, stellar winds and supernova explosions, thus self-limiting the lifetime of GMC to ∼ 3 107 years.

We present a model for the development of density structure in the neutral interstellar medium. In this model, bulk kinetic energy is injected mainly at small scales, by jets and expanding nebulae generated by young and/or massive stars. Subsequently, this bulk kinetic energy propagates to, and is dissipated on, larger scales.

This is in contrast to standard (incompressible) Kolmogorov turbulence, where kinetic energy is injected at larger scales, then propagates to, and is dissipated on, smaller scales. The sense in which the Second Law of Thermodynamics drives the propagation of turbulent energy is reversed in the interstellar medium because virialized self-gravitating gas clumps and ensembles of clumps (clouds) have negative effective specific heat.

The model is able to explain Larson's relations (between the mass, size, and velocity-dispersion of molecular clouds and clumps; Larson 1981), the maximum masses of giant molecular cloud complexes, the velocity dispersions observed (at low resolution) in face-on spirals like the Milky Way, and the apparent scaling of the interstellar magnetic field with density.

The analysis of the observational line-of-sight radial velocity field of molecular clouds, connecting with young stars, has strengthened the Fridman's hypothesis (1994) on the possible existence of anticyclone in the solar neighborhood. Anticyclones are located near corotation radius of the observed spiral arms, a number of which is equal to a number of vortices. Our calculations show that the four-vortices model fits observational data fairly well.

We shall not use any theoretical conception on the nature of spiral arms generation (bar, selfgravitational or hydrodynamical mechanisms, etc.). We shall base on the treatment of the observational data.

Table 1 summarizes results of programs to determine distances and metallicities of high-velocity clouds (HVCs, see Wakker 1991, IAU Symp 144, 27 for a general review). Reliable absorption is reported for one stellar probe (BD+38 2182), giving a distance D<5kpc for complex M. Other results are controversial (BT Dra), uncertain(HD 135485) or atypical (4 Lac). Non-detections reported by e.g. Lilienthal et al. (1990, A&A 240, 287), Danly et al. (1992, ApJS 81, 125) and de Boer et al. (1994, A&A 286, 925), yield lower limits up to 2 kpc for complexes A, C and M. Absorption by heavy elements in HVCs is reported for twelve extra-galactic probes. All absorptions reported in HVCs with |v LSR| > 90km/s are given in the table (abundances are from low-resolution HI data and thus uncertain).

The extent and nature of molecular material in the outer Galaxy (R > R⊙) has been an open, interesting question since the discovery of molecular material in the outer Galaxy (Kutner & Mead, 1981). From large scale CO surveys, two estimates of the total molecular mass have been made with the same result, M(H2) = 6 × 108 M⊙ (Wouterloot et al., 1990), (Digel et al., 1990). Both estimates have a strong bias towards large, star forming (warm) clouds. Wouterloot et al. used the IRAS point source catalog as a finder chart for their observations and the inner Galaxy mass distribution to scale their result. Digel et al. conducted an unbiased survey with a large (8.8 arcminute) beam which underestimated the amount of material in small clouds due to the effects of beam dilution.

We propose a physical model of molecular clouds which is based on the idea that the back reaction of substructures of a cloud on the ambient medium maintains and stabilizes the cloud on larger scales: clumps, which are assumed to carry a magnetic moment, are coupled to the ambient medium by magnetic forces, hence continually inducing velocity fluctuations due to their random motion. The energy source is then the gravitational binding energy of the clumps in the global potential of the whole cloud.

The Southern Galactic Plane surveyed by the Molonglo Observatory Synthesis Telescope (MOST) gives an unprecedented view of fine-scale structure in low frequency radio continuum emission over the area 245° ≤ l ≤ 355°, |b| ≤ 1.5°. The telescope operates at 843 MHz with a resolution of 43″ and a sensitivity of approximately 2 mJy/beam for the sample region (5° × 3°), centred on G317.5+0.0, presented here to illustrate significant features in the sources. This is a mosaic of 35 fields, each representing a 12 hr observation.