The Draco nebula, the faint reflection nebulosity LBN 406/412/415 (Lynds 1965) at 1≈91°, b≈38°, z≥500 pc (Goerigk and Mebold, 1986), contains delicate filaments at its low-1, low-b (SW) boundary (Goerigk et al. 1983, Mebold et al. 1985). The density in the boundary is characterized by the presence of molecules like CO and H2CO (Mebold et al. 1985) and NH3 (Mebold et al. 1987). Johnson (1986) found an excess of unidentified IRAS point sources in the area of the Draco nebula, a large fraction of which is located close to the SW boundary.

After reviewing the history of the early research on gas in the Milky way halo, observational methods to investigate the halo are described. The meaning of the concepts Halo and HVC is discussed and selectional bias in databases is pointed out. The essentials of the interpretation of observed velocities are given. The scanty information on distances of HVCs is reviewed and an outlook is given on the possibilities to firmly settle the distance of at least a few HVCs.

We present preliminary results from a new high resolution optical study of halo gas at the coudé focus of the Canada - France - Hawaii Telescope. Our work is still in progress so two general results are presented here: significant absorption is produced in interstellar gas beyond 500 pc from the galactic plane, and well-resolved halo clouds are identified.

In the sequel we will be discussing that - apart from all other halo flows - it is highly probable that our Galaxy has a systematic mass loss; most other galaxies should behave in a similar way. This mass loss should occur in the form of a supersonic galactic wind provided the intergalactic pressure is low enough. We will investigate this possibility mainly for our Galaxy. We shall argue [1] that the energetic particle component of the interstellar medium, the cosmic rays (CRs), is basically responsible for driving a wind for typical gas temperatures below 106 K. Thus for asymptotic wind velocities of the order of the escape velocity, the mass flux is essentially determined by the observable CR energy loss from the Galaxy. Galaxies with very hot gas on the other hand hardly need the CR component and drive a wind almost exclusively through their thermal pressure.

The analysis of the vertical motion of dust grains inside the disk of spiral galaxies shows that grains at high latitudes (z/H > 0.3 — 0.5, where H is the scale height of the gaseous disk) can be removed from the gaseous disk by radiation pressure from field stars. Once the dust particles are located at the base of the galactic corona, and neglecting the presence of magnetic fields, their motion is determined by the global light and mass distributions. We follow the dynamical evolution of grains embedded in a hot galactic corona (T ≥ 105 K and n ≤ 10−2 cm−3), including drag and sputtering. We find that typical sputtering time scales are larger than 108 yrs, and, in these times scales, the grains can travel large radial and vertical distances from their original positions. This process can remove grains from the inner part of a galaxy and send them to large galactocentric distances. The evolution resembles a dusty galactic fountain which may pollute the outer galaxy with grains and heavy elements. Similarly, for some cases the dust particles can be detached from the galactic gravitational field and they are expelled to the intergalactic medium. In these cases, the chemical pollution is not restricted to the host galaxy and may reach material which is otherwise primordial.

Examining the high velocity cloud complex C and its possible interaction with gas of the galactic disk we became aware of a dust cloud located at galactic longitude l = 86° and latitude b = 38°.5 seen in the l00µm image of the IRAS. This cloud consists of a cucumber shaped core surounded by a horseshoe shaped halo opening towards larger galactic coordinates. At very low 100µm intensities dust lanes starting at the core or the ends of the horseshoe can be traced for about 5° towards high galactic latitude and longitude.

We present high resolution (FWHM≈7 km/sec) and high signal-to-noise observations of the bright Seyfert galaxy NGC3783 obtained with the CES spectrograph linked by fibres to the 3.6m telescope at La Silla, Chile.

We report on fully three-dimensional hydrodynamic and radiative transfer simulations of collisions between high latitude clouds. Our model uses the smoothed particle hydrodynamics code described by Lattanzio and Henriksen (1988) to compute the velocity, temperature, and density fields in the impacted clouds and a recently developed radiative transfer code to compute 13 CO line radiation from the simulated source. By including the instrumental effects involved in a particular observations we can make detailed comparisons with the observations. The model shows that: 1) The previously unexplained energy source for the broad CO line wings reported by Blitz, Magnani, and Wandel (1988) derives from the collisions. 2) Collisions can induce rapid gravitational collapse and star formation in these clouds which are otherwise supported against gravitational contraction via their internal energy content. 3) The external pressure due to intercloud HI, firts proposed for these objects by Keto and Myers (1986), plays a significant role in the stability and evolution of the high latitude clouds.

The present observational and theoretical status of multiple-supernova remnants is briefly reviewed, and evolutionary models of the remnants around OB associations are presented. The remnants are followed for up to almost 30 Myr in various galactic environments (Gaussian or composite disks combined with cold or hot halos). It is found that the presence of a hot halo profoundly modifies the evolution of the remnants, promoting the evaporation of their shells and leading to noneruptive merging of the ejecta with the halo gas. The results also suggest an explanation for the observed deficiency of evolved multiple-supernova remnants.

The intercloud medium (ICM) is kept hot and supplied with gas by supernova remnants of Type II. The rate at which mass is added balances the rate at which gas flows away from the ICM into the galactic halo. A fountain is thus formed, since the gas only has enough energy to escape from the disk, but not from the Galaxy as a whole. The upward flow must be followed by the descent of the gas, in the form of a shocked sheet: the return flow seems to be observed in the form of intermediate velocity clouds.

Recent developments, both observational and theoretical, require a reevaluation of the effects of clustered supernovae on the two-dimensional porosity parameter Q2D and the rates of mass injection into the halo Ṁ of both cold and hot gas. Clustered supernovae produce two types of bubble. Most clusters produce breakthrough bubbles, which do no more than break through the dense gas disk. But large clusters produce enough energy to make blowout bubbles, which blow gas up into the halo. We calculate area filling factors and mass injection rates into the halo for different types of galaxy. We relate our calculations to two observables, the area covered by H I ‘holes’ and the area covered by giant H II regions. We also reiterate the difficulty of producing the very largest supershells by clusered supernovae.

As an illustration of what the next generation of γ-ray telescopes may show us, an up-to-date COS-B ‘finding chart’ of potential γ-ray point sources and unexplained extended γ-ray features is presented. The latter, in particular, may be related to energetic phenomena in the interstellar medium, capable of enhancing the local cosmic-ray density. As an example, a prominent feature, extending over at least 10° — 15° almost perpendicular to the Galactic disk, is discussed in some detail, linking it to the giant HII region S54 and Complex C of high-velocity clouds.

The interstellar medium has a thick disk structure (|z | ≲ 1500 pc, the ECL or extracloud layer) with a thin zone of cloud contamination at |z | ≲ 100 pc. The properties of all components other than clouds (e.g. cosmic rays, magnetic field, extracloud matter, pressure) drop slowly with |z| across the thick distribution, giving this layer a very important influence in the evolution of superbubbles. It seems likely to quench blowout or breakout and virtually all fountain activity. The weight of this layer stabilizes the clouds at low z; its high pressure and low density provide a cushion for impacts of infailing clouds.

The abundance of 21-cm absorption lines seen in surveys at high latitudes can be translated into a line of sight abundance of clouds vs. column density using an empirical relationship between temperature and optical depth. As VLA surveys of 21-cm absorption at low latitudes are now becoming available, it is possible to study the variation of this function with galactic radius. It is interesting to compare the abundance of these diffuse atomic clouds (with temperatures of 50 to 100 K and masses of 1 to 10 M⊙) to the abundance of molecular clouds. To do the latter we must make assumptions about cloud cross-sections in order to convert the line of sight abundance of diffuse clouds into a number per unit volume, and to convert from cloud column density to mass. The spectrum of diffuse clouds matches fairly well the spectrum of molecular clouds, although observationally there is a gap of several orders of magnitude in cloud mass. Optical absorption studies also agree well with the 21-cm results for clouds of column density a few times 1020 M⊙.

The evolution of large scale expanding structures in differentially rotating disks is studied. High column densities in some places may eventually lead to molecular cloud formation and initiate also star-formation. After some time, multi-structured arms evolve, where regions of intensive star-formation are separated from each other by regions of atomic gas or molecular clouds. This is due to the deterministic nature and to the coherence of this process. A simple model of galactic evolution is introduced and the different behaviour of Sa, Sb, and Sc galaxies is shown.

TAURUS observations in the line of Hα and VLA HI mapping of the HII complex No. 722 in M31, reveal what seems to be a spherical cavity 330 pc in diameter blown out by a stellar association of over 20 × 106 year old. Evidence of induced star formation which was initiated less than 5 × 106 years ago is present in the form of bright HII emission and numerous O, B and Wolf-Rayet stars which are found within the shell surrounding the cavity. The energy necessary to create the HI shell is estimated to be about 5 × 1051 erg.

Supernovae of Types Ia, Ib, and II contribute, on average, 1051 ergs of turbulent energy to the interstellar medium for each 2 Mo of new heavy elements. This permits a dynamical extension of the homogeneous one-zone model (with constant IMF and instantaneous recycling) that is familiar from studies of galactic chemical evolution. Chemically possible scenarios predict current kinetic energy inputs ranging from 1039 to 1040 erg yr−1 per solar mass of interstellar gas. Dynamical studies might narrow this.

The 1/4 keV diffuse X-ray background (SXRB) is discussed in relation to the local interstellar medium (LISM). The most likely source for these soft X-rays is thermal emission from a hot diffuse plasma. The existence of a non-zero flux from all directions and the short ISM mean free path of these X-rays (1020HI cm-2), coupled with ISM pressure constraints, imply that the plasma has a local component and that it must, at least locally (nearest hundred parsecs), have a large filling factor. Our understanding of the geometry and physical parameters of the LISM is therefore directly tied to our understanding of the SXRB.

The radio flux measurement of the compact extragalactic objects are known to show two types of variablity i.e. intrinsic and extrinsic. Some of the unusual minima in the flux measurement seem to involve large structures in the interstellar medium. In this article we outline a simple ray approach for simulation of these minima in the flux measurement. Simulation will help in estimating the physical parameters of these large scale irregularities or structures in the interstellar medium. Some preliminary results of the simulation will also be described.

A warped disk in our own galaxy is evident by means of HI, HII, γ-rays and dust observations, but unexistent when star distributions are considered, specially those of late type stars. This fact is in disagreement with the theories which assume a gravitational origin of warps, for instance a tidal interaction with the Magellanic Clouds. We tried to find the z-distribution of open clusters of different ages, for which a warp distribution was neither found nor rejected. Assuming an intergalactic magnetic field origin of the warp, we obtain a direction of the field in the Milky Way neighborhood given by (b,1) = (45°, 74°).