We construct a chemo-dynamical model for galaxy formation using a three dimensional SPH method. We simulate the formation of two spheroidal systems, i.e., the elliptical galaxy and the Galactic bulge, based on the collapse scenario for protogalaxies. We obtain the chemodynamical formation and evolution models for the two systems during the first ∼ 1 Gyr. The relative ratio of kinetic to thermal energy of supernovae is found to heavily determine the outcome. By giving the explosion energy of supernovae to the interstellar gas with a physically meaningful relative ratio of kinetic to thermal energy, the elliptical galaxy model has the hot halo and the galactic wind, but it is not the case for the Galactic bulge model.

The metallicity-sensitive (U – G) colors from the new homogeneous catalog of photographic RGU data in seven high-latitude fields have been used to determine the larger-scale metallicity distributions of the Galactic population components. For the thick disk, preliminary analysis based on our best structural models provides a mean metallicity 〈[M/H]〉 = −0.6 ± 0.3dex and a marginal vertical metallicity gradient ≈ −0.1dex/kpc. The observed color distributions are further consistent with the (old) thin disk having mean abundance 〈[M/H]〉 = −0.3 ± 0.2dex and abundance gradient ∂[M/H]/∂z = −0.6dex/kpc.

The question is adressed whether the problem of the stellar metallicity distribution and the dynamics of the stellar components in the bulge as found out by refined observations during recent years can be understood within the context of the evolution of the whole Galaxy. A selfconsistent galaxy model has to explain the apparent differences in the effective yields of the enrichment of bulge, disk and halo. Moreover, it has to account for the observed age-metallicity distribution of the stellar components. It appears that chemo-dynamical infall models provide a consistent description of the bulge, in particular the metallicity distribution of the K giants. It should be emphasized that simple closed-box models are not appropriate, because during the bulge formation there is infall of cloudy medium (CM), as well as outflow of hot, ionized gas ejected by supernovae type II (SNII). Therefore dynamical processes have to be taken into account. For details of the chemo-dynamical description we refer to Samland & Hensler (1994) and references therein.

It is well known that the velocity dispersions of the stars in the solar neighbourhood increase with their ages (Wielen 1977). In Fig.1 we show |W| weighted velocity dispersions (cf. Wielen 1977) of the stars in the Third Catalog of Nearby Stars (Gliese and Jahreiß 1994). Open symbols indicate main sequence stars and crosses indicate McCormick stars, a kinematically unbiased subset of the CNS3, respectively, whereas the filled symbols are the Edvardsson et al. (1993) data. Stars older than 14 Gyr are not shown because they are probably thick disk stars (Freeman 1991). We have assumed a maximum age of the old thin disk stars of 12 Gyr as suggested by the Edvardsson et al. data. As can be seen from Fig.1 both data sets fit ideally together. The solid line indicates a σ ∝ τ1/2 law.

Observations show that in the solar neighborhood the velocity dispersions of disk stars increase with their age. In this work we present the results of a critical analysis of the existing interpretations of the data, as well as of previous theoretical explanations of the heating phenomenon. It is shown that different relaxation mechanisms based on star-cloud collisions can result in a wide set of age–velocity dispersion relations (AVDR). Thus the observed differing power laws of the heating of the stellar component can be a consequence of the different relaxation mechanisms.

AM-2 is a sparse cluster located at low galactic latitude. It has been suspected to be a globular cluster. We present a study of AM-2 based on the deep UBVI CCD photometry obtained using the Las Campanas duPont 2.5m telescope. The color-magnitude diagrams of AM-2 show (a) a main-sequence extending up to V ≈ 19 mag at (B – V) ≈ 1.1 mag, (b) a small number of red giant clump giants, (c) the brightest red giant at V ≈ 16.1 mag and (B – V) ≈ 1.9 mag, and (d) a small group of mysterious blue stars at V ≈ 16.6 mag and (B – V) ≈ 0.9 mag. We have estimated the reddening using the color-color diagram, E(B – V) = 0.56 ± 0.04. The metallicity of the main-sequence stars has been estimated from the ultraviolet excess, δ(U – B)0.6 = 0.09 ± 0.04, to be [Fe/H] = −0.4 ± 0.2 dex. The distance to the cluster has been measured using the Zero-Age-Main-Sequence fitting method, (m – M)0 = 14.8 ± 0.3 (d = 9.1 ± 1.4 kpc). Finally we have estimated the age of the cluster using the Revised Yale isochrones and the Morphological Age Ratio (MAR) method, obtaining a value of 5 ± 1 Gyrs (Fig. 1). This shows that AM-2 is not a globular cluster, but an old open cluster.

Gamma-ray astronomy has become a rich field of research and matured significantly since the launch of NASA's Compton Gamma Ray Observatory in April 1991. Studies of the diffuse γ-ray emission of the Galaxy can now be performed in far more detail and extended into the MeV regime, including both continuum and line emission. These studies provide unique insight into various aspects of the interstellar medium, in particular of the cosmic-ray component. This paper gives a brief review on the diffuse Galactic γ-ray emission and summarizes early results and prospects from the Compton Observatory.

Some of the principal problems which can be confronted with HI observations of our Galaxy are illustrated here by a selection of maps showing the disposition on the sky of HI emitting in particular velocity ranges. The data are from the new Leiden/Dwingeloo survey of the entire sky north of δ ≥ −30°, which has occupied the 25–meter radio telescope of the Netherlands Foundation for Research in Astronomy for a period of five years. The tendency of the general gas/dust correlation to involve some HI structures with quite anomalous velocities is illustrated by comparing some λ21–cm data taken on a finer grid using the Arecibo 1000–foot telescope with IRAS data in the 100–micron band.

In virtually all of the dense interstellar medium, H2 is the most abundant form of hydrogen, but it is not directly observable in the bulk of the gas. As a result, we are forced to use trace constituents of the gas as surrogates when we want to know the distribution of material in the dense ISM. Most commonly, we employ the lowest few rotational transitions of CO and its isotopes as the trace species. One of the most hotly debated issues in the study of the molecular ISM is the extent to which one can trust CO or isotopic CO lines to reflect reliably the underlying H2 distribution (see Shier, Rieke, & Rieke (1994), Sodroski et al. (1995) for recent comments on the I(12CO)/N(H2) ratio and Lada et al. (1994) for a recent analysis of the relationship between isotopic CO and total cloud column densities). CO becomes increasingly unreliable as a tracer of H2 as the average column density between cloud surfaces exposed to ultraviolet photons and the shielded centers of clouds becomes smaller. Young stars in the galactic plane perfuse atomic and molecular clouds with far–UV (λ > 91 nm) radiation. This radiation tends to dissociate CO more readily than it dissociates H2 (Van Dishoeck & Black 1988). The differences in susceptibility of H2 and CO to photodissociation may lead to the existence of significant portions of the molecular medium where the usual trace species are underabundant or even absent. In addition, there is dense H I at the cloud boundaries, immediately outside the molecular material. In the UV–illuminated cloud surfaces, the gas-phase carbon is in the form of C I or C II. It is important, therefore, to determine the amount and location of large-scale C I emission if one hopes to know how much molecular and dense atomic gas is missing from studies using CO as a tracer and to what extent photodissociation is responsible for the absence of this CO. We discuss here some of the relevant theoretical and observational work on the relationship between C II, C I, CO and H2. Our principal aim is to see if and how observations of C I might help us to improve our knowledge of the distribution of dense neutral gas in the Milky Way.

The vertical distribution of neutral hydrogen has been studied over the last three decades (Schmidt 1957; Baker & Burton 1979; Lockman 1984; Savage & Massa 1987). The best fit curve to the HI z-distribution in the solar neighborhood has a FWHM of about 230 pc, and it extends appreciably beyond 500 pc (Dickey & Lockman 1990).

The velocity dispersion of the interstellar gas measured perpendicular to the disk in face-on spiral galaxies shows a remarkable consistency. Typically the width of the velocity distribution decreases monotonically with increasing galactic radius through the luminous, stellar disk; but in the outer disk where there is gas but no stars the velocity dispersion is συ ≃ 7 km s−1 with a scatter of 2 km s–1 at most from one position to another and from one galaxy to another. Trying to understand this number raises questions about the thermal state of the H I in outer, gaseous disks. The most important issue is to understand the dominant heating and cooling processes for the atomic gas.

An extensive survey of [C II] line emission at 158 microns using the balloon borne telescope (BICE) has provided a complete map of the emission intensity distribution in the first and the fourth quadrants of the galactic plane (280° < l < 80°, −5° < b < 5°: Okuda et al. 1993). The emission is very extended throughout the galactic plane in which three intensity maxima are seen towards the tangential directions of the Scutum and the Norma arms as well as in the Galactic center region. However the Galactic center maximum is much less prominent compared with the two other distributions, unlike the case of far infrared continuum and CO emissions.

The analysis of vertical distribution and kinematics of disks of galaxies can yield the local mass density in the disk, distinct from the integrated mass inside a radius, derived from a rotation curve. HI is a particularly good tracer because of its ubiquity, ease of observation, and near isothermal nature. In the simplest case one considers the turbulent pressure gradient of the gas balancing the gravitational force in the z-direction. But the atomic gas may be subject to other pressures, for example, magnetic, cosmic ray or radiation pressure. The relative contributions of these are essentially unconstrained. The mass densities obtained from the analysis of HI vertical equilibrium can be verified with a similar analysis only in the solar neighborhood, where the vertical distribution and kinematics of stars can give an independent measure of the total midplane mass density (ρ0).

Physical conditions of molecular gas are key parameters to the formation rate and initial mass function of stars formed in molecular clouds. The ongoing Tokyo-NRO survey has been observing the Galactic CO (J=2–1) emission with a beamsize matched to the Columbia CO (J=1–0) survey. Intensities of the two lines should reflect physical conditions of the CO-emitting gas. An out-of-plane survey of the inner Galaxy which covers from 20° to 60° in galactic longitude and from −1° to +1° in galactic latitude with grid spacings of 0.25° has already been made (Sakamoto et al. 1994). Its coverage is large enough to draw conclusions on global properties of molecular gas in the inner Galaxy.

We investigate a stability of the gaseous disk in the Milky Way. The rotation curve with smooth two kinks in the Solar vicinity (Fig.1) was adopted in accordance with the observations (Fridman et al., 1994a). Such a gaseous disk is unstable. The instability can generate the four-armed spiral (Fig.2) and four-vortex structure (Fig.3). The method of the analysis is described by Fridman et al., 1994b. The generation mechanism is similar to that described by Glatsel, 1987.

Most studies of spiral structure in galaxies use 2D models, since the motion of the stars is supposed to be confined very close to the plane of symmetry. However, galactic disks have a finite thickness and the vertical variation of spiral perturbations should be considered. The effect of 3D orbits in the dynamics of normal spiral galaxies is examined in this paper. We present the basic orbital behavior of the families of periodic orbits, which support a thick spiral pattern.

Poisson's equation, ∇2ψ = 4πGρ, can be solved approximately using basis sets of potential-density pairs. A given density is approximated by a truncated expansion; the computed expansion coefficients immediately yield the corresponding potential since each basis density function is paired with a basis potential function, and Poisson's equation is linear.

The dynamical modeling of various tracer populations in our galaxy is an important tool in the study of its formation and evolution. Planetary Nebulae (PNe) seem to be particularly useful for such a study. In this contribution we attempt to link the dynamics of PNe and OH/IR stars, and confirm on dynamical grounds that both classes are indeed related by stellar evolution. Moreover, we show that 2 integrals of motion are probably not sufficient to characterize the dynamical state of the PNe: the models produce a velocity dispersion which is too low, pointing at the likely presence of a third integral.

A powerful moving group-finding algorithm has been developed and applied to a large sample of B, A and F main sequence stars from the Hipparcos Input Catalogue. Four moving groups near the Sun (Pleiades, Sirius, IC2391 and Hyades) have been identified without assuming any a priori knowledge of the properties of moving groups. This is the most convincing objective evidence for their existence. After the members of the moving groups are removed from the sample, we investigate the disk heating problem. The results show that the velocity dispersion of the local disk stars increases with age, roughly as ≃ ∞ τ1/5.

We have investigated the force of dynamical friction on a softened point mass orbiting inside a disc galaxy on a planar circular orbit both analytically and by means of numerical simulations. Including a velocity dispersion large enough to stabilise the disc markedly changes the physical picture of the mechanisms producing friction, whereas the numerical value of the friction is almost unaffected. Self-gravity of the disc enhances the friction only by a moderate amount.