The abundance of the elements in clouds of halo gas, as determined from observations, is an important parameter for the test of the validity of models explaining the existence of, e.g., the high latitutude high-velocity clouds (HVCs) of the Milky Way. Individual HVCs have been detected in absorption only on very few lines of sight so that the distance of the HVCs, another important parameter for the models, stays ill determined as well. We will follow here the more or less established convention by calling HVCs those with /v/ >100 km s−1 and IVCs (intermediate-velocity clouds) those with 50 </v/ <100 km s−1. We will define halo as the space with /z/> 1 kpc, although for /b/> 45 also /z/> 0.5 kpc is used.

We have found and cataloged over 100 vertical structures in H I, infrared, and radio continuum emission. These correspond to the H I worms detected by Heiles (1984). The infrared and the radio continuum properties of worms suggest that some worms have associated ionized gas. The area filling factor of superbubbles in the inner Galaxy is estimated to be greater than ~0.1.

A review is given of large-scale magnetic fields in disks and halos of spiral galaxies. A particular attention is given to vertical field structures, and we discuss their origin and implication on their interaction with halo gas. We point out that the disk-halo magnetic interface plays an important role in circulation of interstellar gas in galaxies, in particular a large-scale circulartion from the galactic center to outer disk regions.

The physical processes that determine the size of the galactic cosmic-ray halo are discussed and observational information on the cosmic-ray halo is summarized. Based on theoretical investigations as well as observations, we conclude that the halo surrounding the galactic disk is extensive and that its extent in the direction perpendicular to the galactic plane is more than 10 kpc.

Changes of the cosmic ray electron spectrum throughout the Galaxy have been found, based on the comparison of large-scale radio continuum surveys. These observations are not compatible with the assumption of a static Galactic halo, but indicate the existence of a Galactic wind. Galactic plane surveys reveal sources of cosmic ray electrons in the Galactic disk. Recent studies of the population of radio sources show no evidence for a large number of compact Galactic non-thermal sources. Most of the extended sources are probably HII-regions. Relatively few new supernova remnants (SNRs) with low surface brightness could be identified. Most of the non-thermal emission in the disk-halo interface seems diffuse or unresolved, even at arcmin angular resolution.

Recent observations of the pulsar dispersion measures show a large scale height and relatively high density for the diffuse, ionized gas in HI regions. The maintenance of the ionization requires a pervasive ionizing agent. This agent could hardly be the UV photons from O stars or the extragalactic background UV photons due to their large optical depth; nor could it be the soft X–rays from the hot, ionized gas due to a low flux indicated by observations. Studies of the Galactic diffuse low energy γ-rays provides a clue to this problem. The extraordinarily high intensity of MeV electrons, which is derived from the γ–ray flux, could account for the ionization. In this scenario the electrons do both ionization and heating in HI regions, accordingly the atomic hydrogen gas is warm (~ 104 K). The ionized gas density has a radial gradient with a scale length 4 kpc, there being more ionized gas in the inner Galaxy, due to a gradient in the electron intensity.

The imminent launch of NASA's Gamma Ray Observatory (GRO) has focussed on the need to provide up to date estimates of the energy density of interstellar radiation away from the Galactic Plane, this parameter being a prerequisite for calculation of the flux of gamma rays coming from the inverse Compton scatterings of cosmic ray electrons. In this work we use recent information on the stellar distributions and on the extinction properties of dust in the interstellar medium in the calculation. An important feature of the radiation field is that the field energy density is still substantial in the halo, as was found in our previous work. The differences between groups are due to the total energy input and the dust distribution. Our ensuing energy densities are probably accurate to about 30%, the uncertainty being due to lack of precise knowledge of the input parameters rather than approximations used in the calculation.

In this paper we discuss the large-scale structures (> 500 pc) of the neutral atomic hydrogen in the disk of nearby galaxies, with an emphasis on finding evidence for neutral gas features in the disk - halo interface. Most nearby galaxies studied in detail appear to have hole and shell type structures in their HI disks and several show evidence for gas at peculiar velocities, probably examples of gas streaming out of the disk into the halo. Not all of the observed phenomena can be explained by stellar winds and supernova explosions and other explanations will briefly be discussed.

The instrumental requirements and current observational results are outlined for the study of resolved neutral hydrogen structures in both emission and absorption in external galaxies. Neutral super-shell structures exterior to ionized shells around OB associations seem to be a common phenomenon. Further analysis of existing data should allow quantification of the degree and rate of energy deposition in the ISM by massive stars. HI absorption studies offer great promise in constraining the physical properties of the neutral ISM, including an estimate of the gas pressure. Together these data will provide hard constraints on the existence and prevalence of chimney formation, and hence disk – halo interactions in a variety of galactic environments.

We discuss the distribution and spectral characteristics of diffuse ionized gas in nearby galaxies. The existence of this elusive component of the interstellar medium (ISM), also referred to as the Warm Ionized Medium, is by now well established from deep imaging and spectroscopic surveys in several emission lines in external galaxies. Diffuse ionized gas is characterized by a relatively high ratio of [SII] over Hα intensities, typically twice as high as for discrete HII regions. The diffuse gas has been mapped in both edge-on and more face-on galaxies providing information on the radial and vertical distribitions. Emission from diffuse ionized gas is strongest around star forming regions. The vertical distribution appears related to the radio continuum thick-disk emission. We also briefly discuss ionization mechanisms, and the connection between star formation characteristics and morphology of the interstellar medium.

There is some evidence for disk-halo connections in M31, i.e.: (1) The central area seems inclined w.r.t. the main disk. (2) In several regions in the southern half the magnetic field has a significant component perpendicular to the disk. However, (3) any general emission from a thick disk at λ75 cm is ~100x weaker than that from our Galaxy. The uniform disk field seems to inhibit cosmic-ray diffusion perpendicular to the plane, and/or the field at high z is exceptionally weak.

The properties of hot gaseous haloes in massive early-type galaxies are briefly reviewed. Gas flows in such haloes are complex yet so large-scale that they may guide us in the understanding of flows around disk galaxies. The intracluster medium is discussed as a further illustration of the properties of diffuse hot gas trapped in a gravtational well. Finally, the possibility of the existence of a significant diffuse medium in the Local Group, and in groups in general, is revived. Such a medium would generate a substantial disk-halo interaction with our Galaxy.

A recent analysis of the surface brightness profiles of a complete sample of 9381 spiral galaxies extracted from ESO-LV (Lauberts and Valentijn, 1989) showed that many galaxy disks of especially Sb–Sc galaxies are opaque (Valentijn, 1990). This paper studied how the observed surface brightness μobs varies as a function of the observed axial ratio a/b, by fitting the data of samples of spirals with (1) assuming the a/b to give the inclination angle of the disks to the line of sight. A sample of galaxies that were fully transparent at the particular radius used for the test would have C ≃ 1, while C <~ 0.25 signifies opaque conditions, the transition value being heavily dependent on the spatial distribution of the absorbing material and the effect of multiple scatterings (Bruzual et al. 1988). The most frequently-used C values range from 0.5–0.9 (Holmberg 1975, de Vaucouleurs et al. 1976, Sandage and Tammann 1981). However, Valentijn (1990) derived C values well below 0.25 for large samples of spirals throughout the galaxy disks; this result is not significantly affected by selection effects in either magnitude, angular diameter or axial ratio, nor by the presence of bulges (at least for types Sb and later).

The constraints on cosmic ray electron sources and propagation as derived from radio continuum observations of galaxies are reviewed. Special attention is paid to the inferences which can be obtained from the radio continuum properties of spiral galaxies seen edge-on.

Spiral galaxies host interstellar magnetic fields of 4-15 μG total strength. A significant fraction of the field lines shows large-scale structures. At face-on or moderately inclined view, the field lines run generally parallel to the spiral arms, either with uniform direction with respect to azimuthal angle (axisymmetric spiral, ASS), with one reversal along azimuthal angle (bisymmetric spiral, BSS), or with spiral orientation without dominating direction.

At edge-on view, the field is concentrated in a thin disk, often surrounded by a thick radio disk with field lines mostly parallel to the plane, similar to the quadrupole-type dynamo field. Radio polarization data from NGC891 indicate that the thermal gas seen in Hα is responsible for Faraday depolarization. The required scaleheight of the field of ~4 kpc is comparable to the value expected in case of energy equipartition between magnetic fields and cosmic rays. The interacting edge-on galaxy NGC 4631 shows a much larger radio halo with field lines perpendicular to the disk, possibly driven by a strong galactic wind or the result of a dipole-type halo field.

Field lines bending out of the plane are also visible in face-on galaxies as regions with high rotation measures and low star-formation activity. The resemblance to the phenomenon of the solar corona suggests to call them “galactic coronal holes”.

We report the discovery of radio synchrotron emission from the outflow in M82. The brightness morphology and radio spectral index distribution add new insights into the physical processes and origin of the wind, which are briefly discussed in this paper.

Three questions are posed, the answers to which are relevant to our understanding of the physical processes which shape the radio continuum morphology of normal spiral galaxies. Observations of the edge-on galaxy NGC 891 have been made for many years by several groups with the intention of contributing at least partial answers to these questions. We review here the work which we have recently done on this subject.

Hα images and long-slit spectra of NGC 891 show the presence of a thick layer of diffuse ionized gas (DIG) in this edge-on galaxy. Hα emission originating in the DIG of NGC 891 is detected out to several kpc above the midplane with a typical scale height for the electron density ne(z) of he ~ 600 pc. The distribution of the gas is very inhomogeneous with a large scale asymmetry and filamentary structure. Individual Hα emitting structures can be traced out to 4-5 kpc above the midplane. The emission line ratios of [NII] and [SII] to Hα vary with height above the disk. We compare these properties with the analogous gas phase in our Galaxy.

The distribution of the DIG is related to star formation in the underlying galactic disk and we conclude that in NGC 891 the interstellar medium is heavily effected by processes related to star formation.

We report 12CO J=2-1 line observations of the edge-on galaxy NGC 891, made with the IRAM 30 m telescope. These observations show that the molecular gas probably extends to large distances from the galactic plane.

Spiral galaxies whose radio continuum emission is dominated by their disks show a flattening of the radio continuum spectrum at frequencies well below 1 GHz. The effect appears to be stronger for edge-on than for face-on galaxies The most feasible explanation appears to be that the flattening reflects free-free absorption of nonthermal emission by a very low temperature ionized gas. This gas is probably highly clumped, and must be well-mixed with the nonthermally emitting plasma.