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Main clues on galaxy evolution are derived independently from stellar masses in the K-band and from dust thermal emissions at 12µm, both for high-z populations of galaxies.

We use Ha measurements of complete sets of H II regions in disc galaxies to compute the fraction of ionizing photons which escape from them using the hypothesis of density bounding. The escaping flux is more than enough to ionize the observed diffuse ionized gas (DIG) and the balance implies that an important fraction of the flux escapes into the intergalactic medium (IGM).

Dust grains are a key component of the Universe. They play a crucial role in various astrophysical processes, such as the formation of stars and planets, the catalysis of molecular hydrogen... Several important questions about dust are still under debate. One of them deals with the site of dust production.

Interplanetary dust particles (IDPs) collected from the stratosphere are derived from asteroids and comets. The chondritic porous (CP) subset of IDPs, some of which are almost certainly cometary, are physically, chemically, and isotopically primitive objects. Large D/H and 15N isotopic anomalies provide evidence for survival of presolar interstellar components. Embedded silicate grains exhibit both crystallographic and trace element evidence of formation by (nebular) gas phase condensation. The fine-grained matrices of CP IDPs contain GEMS, submicrometer spheroids whose exotic properties resemble those of interstellar “amorphous silicate” grains which are ubiquitous throughout interstellar space. Some GEMS are embedded in isotopically non-solar carbonaceous matrix (with 15N ∼ 25 – 40%) (Messenger et al., 1996), providing a further hint of their presolar origins. It is possible, but as yet unproven, that some CP IDPs are composed exclusively of aggregates of inters tellar and circumstellar dust grains, and that they carry a detailed record of fundamental grain formation and destruction processes in space.

Nature and origin of the high-velocity clouds (HVCs) remain enigmatic after thirty years (Wakker & van Woerden 1997, ARA & A, 35, 217), owing to lack of distance information. Hypotheses range from supernova shells at 100 pc to intergalactic clouds at 1 Mpc. On statistical grounds, Blitz et al. (1996, BAAS, 28, 1349) claim that the HVCs are “remnants of Local Group formation, best explained as members of the Local Group of galaxies”. Reliable distances must come from the presence or absence of absorption at the HVCs velocity in spectra of stars at different distances. For Complex A, Mgll absorption is seen in HST spectra of the Seyfert galaxy Mrk 106, but not in the star PG0859+593 at 4 kpc (Wakker et al. 1996, ApJ, 473, 834). La Palma spectra of the RR Lyr star AD UMa at 11 kpc distance show Call absorption by Complex A at both K and H, which is lacking at 4 kpc. These absorptions are not confused with stellar metal lines. Our distance bracket 4 < d < 11 kpc places Complex A in the Galactic Halo, at 2.5 < z < 7.5 kpc above the plane; a distance similar to Local Group galaxies is excluded. The HI mass implied lies between 0.15 and 1.2 times 106 M⨀. Our result precludes local origins for this HVC. It allows an origin in a Galactic Fountain, or in interaction of infalling intergalactic material (from the Magellanic System or the Local Group) with the Galactic Halo.

We present new radio continuum images of the supermassive, peculiar galaxy NGC 1961 at 1.5, 4.9, 8.4 and 15 GHz. NGC 1961 (Arp 184) is a very massive (dynamical mass > 1012 M☉) Sb galaxy with a peculiar and asymmetric optical appearance. Furthermore, it exhibits an unusual, asymmetric HI distribution with an extensive wing of gas extending 30 kpc to the north-west and a sharp edge to the south-east (Shostak et al. 1982).

We have obtained HST snapshots observations using the WFPC2 through the F702W broad band red optical filter of nearly the complete 3CR sample of radio galaxies and quasars. Here we present results on the dust in the 3CR radio galaxies in the redshift range 0.0 < z < 0.5. The images have ~ 0.1″ resolution, corresponding to (6, 55, 170, 500) parsecs at redshifts of z = (0.003, 0.03, 0.1, 0.5) respectively.

It is fitting to conclude this volume never forgetting that sense of wonder, and that sense of awe, which observers feel time and time again when, perhaps on clear but cold nights, they step out onto a catwalk … or step outside the Observatory building … to gaze upon the Milky Way.

We review mm/submm observations of dust in galaxies and show that the optically thin dust emission is a reliable tracer for the total gas mass. We discuss the various problems related to this method, in particular the determination of the dust temperature and the gas-to-dust ratio. We compare the dust emission from three complete FIR flux limited samples of normal spirals, active Mkn galaxies and radio-quiet QSOs. The temperature of the coldest dust component turns out to be a function of both, the luminosity and the activity of a galaxy; it varies from 15 K in non-active galaxies to 80 K in highly redshifted radio galaxies. We propose that the ratio of IR luminosity over gas mass is a unique signature for the global activity of a galaxy.

The majority of interstellar grain models are based on a dust population composed of carbon and silicate grains. Abundance studies using high-resolution absorption line spectra, however, suggest that other grain types, perhaps oxides or metal grains, are prevalent in interstellar clouds. The data further suggest that not enough carbon is depleted from the gas-phase interstellar medium to account for the carbon-based grains often used to explain the measured optical and UV dust opacity. Silicon atoms are likely to be more populous in grain mantles than in silicate grain cores.

The spatial correlation function of Ly-α clouds should be similar to that of galaxies if both populations have the same mass distributions and if both formed by the process of gravitational instability. The 1-D correlation function (along the line-of-sight) is equal to the 3-D function, so long as the spatial separation of clouds is much greater than their size. Observations of close quasar pairs suggest that this may not be the case. We present an explanation for why the spatial correlation of these clouds may be negligibly small. This explanation takes into account the large cloud size to cloud separation ratio, the redshift evolution of the correlations and mixing in the same catalog objects located at a very wide range of redshift. The implications for observing strategies are discussed.

An overview is presented of the methods of probing for the geometry, and strength of intergalactic magnetic fields. Recent results are briefly surveyed for galaxy halos, galaxy clusters, and the intergalactic medium on various scales, and some relevant physical processes and radiation processes are mentioned, as well as the “coupling” between intergalactic magnetic fields and cosmic rays.

The past decade has heralded a revolution in our understanding of dust in galaxies due largely to the extraordinary success of the Infrared Astronomical Satellite (IRAS). The satellite, launched in 1983, surveyed 96% of the sky yielding continuum flux measurements at 12, 25, 60 and 100μm for literally thousands of galaxies (Cataloged Galaxies & Quasars in the IRAS Survey 1985). Complimentary 60, 100 and 160μm observations have also been obtained for a limited number of galaxies with the Kuiper Airborne Observatory (KAO). The past decade has also witnessed a dramatic development in the field of submillimeter astronomy with the commissioning of several new submillimeter telescopes including the JCMT, CSO, IRAM and SEST, all of which have yielded new results on the 300 to 1300μm continuum emission from galaxies. The goal of this review is to discuss the new observations within the context of the origin of the far infrared luminosity, the temperature of dust in galaxies and the implications of the new data for the opacity of spiral galaxy disks.

Intergalactic gas and dust are very difficult to detect. Several methods to detect intergalactic gas have been developed. The results of these methods have provided qualitative arguments and quantitative limits on the amount of intergalactic matter in the intergalactic environment. We present here arguments as to why the density of intergalactic matter is higher than previously thought possible.

After discussion of the solar and stellar magnetic fields we devote this chapter to the magnetism in the immense universe: in the Milky Way, extragalactic nebulae and intergalactic space. The main difference between magnetic fields of these two categories lies in the following. The Sun and magnetic stars have relatively strong fields in comparatively small volumes, while galaxies (including our Milky Way) and the intergalactic medium possess extremely faint fields in quite vast scopes. Therefore, the methods of measurements as well as the structure and characteristics of these two kinds of magnetic fields are to a great extent different.

The basic observational facts known about the intergalactic medium in both smoothly distributed (IGM) and clumped forms (Lyα clouds) are reviewed. Six candidate theories of Lyα clouds and the IGM are compared with the essential non-controversial observational facts. Deficiencies of the theories and inconsistencies in the existing data sets are stressed. The three most plausible candidate theories successfully explain some aspects of the IGM, but fail to explain others. A new approach, employing hydrodynamic N-body simulations is presented. In this approach elements of several theories are combined in an effort to study the evolution of the IGM and the formation and properties of Lyα clouds. Preliminary results of such simulations are presented and compared to observations.

Interstellar dust is not a phenomenon in any way peculiar to our own galaxy. Photographs of external galaxies show striking evidence for dust, particularly in spiral and irregular systems (Sandage, 1961). Dust lanes often serve to delineate spiral arms, young stars and HII regions that are present in these galaxies. One of the most dramatic examples of extragalactic dust is to be seen in NGC 4594 (the Sombrero Hat, Fig. 10.1) where the galaxy is divided through its central plane by an opaque dust layer. Scarrott et al. (1987) dicovered linear polarisations of a few percent perpendicular to the dust layer near the extremities of the disk, and attributed this to scattering by grains. A galaxy such as ours viewed edge on would look like this, with a dust layer some 150 pc or so thick along its central plane. Fig. 10.2 shows the dust lanes in M51, which is a spiral galaxy viewed almost face-on. The integrated light from such galaxies shows linear polarisation, indicating the presence of aligned grains. The galaxy NGC 891 shown in Fig. 10.3a is interesting in having protruberences in the dust layer extending normal to its plane, to angular distances of about 30 sec of arc, corresponding to heights of ~ 100 pc. It can be argued that such filaments are evidence of dust being lifted by radiation pressure against the gravitational potential energy of the galaxy. There is other evidence of entire galaxies being shrouded in dust, showing that dust generated within the galaxy is somehow expelled.

The formation of the first stars in gas of primordial composition depended upon the complex interplay of the hydrodynamics of gravitational collapse with nonequilibrium chemistry and its effect on the thermal evolution of the gas. The initial conditions and boundary values for the clouds which collapsed gravitationally to form galaxies and stars were determined by the thermal and ionization history of the general intergalactic medium. We describe new detailed numerical calculations of the evolution of the intergalactic medium in a post-recombination Friedmann universe, including a solution of the nonequilibrium rate equations for the ionization and recombination of H and He, the energy equation, and the equation of radiative transfer. The implications of this study for the characteristic mass scale and epoch of gravitational collapse for the average “first” star forming clouds will be discussed. The star formation rate and associated metallicity generation implied if the intergalactic medium was fully ionized by starlight by redshift z > 4 as suggested by recent interpretations of the Gunn-Peterson effect will also be discussed. Finally, we show that the relative strengths of metal lines from a quasar absorption-line Lyman limit system cloud at z = 3 photoionized by our intergalactic uv radiation background are compatible with the observations, at least for background sources of either AGN-type or star-forming galaxy spectra.

We review some exciting new observational developments in the infrared background (1 – 1000 µm). The Nagoya-Berkeley collaboration have made accurate measurements of the infrared background intensity between 100 and 1000 µm and have detected excess radiation, compared to a 2.74 K blackbody, peaking at 700 µm. New detailed studies of the IRAS background data show the clear possibility of a cosmological background at 100 µm, though there is still not a concensus on the correct model for the foreground zodiacal emission. Finally, the Nagoya group claim a significant background intensity at 2 µm. Possible interpretations of the far infrared background as radiation from starburst galaxies undergoing strong cosmological evolution are discussed. The spectrum of the 700 µm excess does not agree well with such a model. More promising are models involving a pregalactic generation of stars which make dust and light. The expected infrared background from a variety of pregalactic sources (primeval galaxies, Population III stars, accreting black holes, large-scale structure formation, decaying elementary particles) are discussed in detail. These backgrounds would generally appear in the near infrared unless they have been absorbed by galactic or intergalactic dust, in which case they should have been reprocessed to the far infrared.

About 25 years ago Zwicky (1957), Holmberg (1958) and Hoffmeister (1962) found intergalactic dust by observations. Zwicky (1957) explained the deficiency of distant clusters of galaxies by intergalactic dust inside the near clusters. Holmberg (1958) made extensive observations of the Virgo cluster. He found systematic differences of the colour indexes between cluster and field galaxies. Intergalactic dust inside of the Virgo cluster should be the reason of these differences. Hoffmeister (1962) found a clear deficiency of galaxies in the region of Microscopicum. Again the reason should be a large cloud of intergalactic dust.

The hypothesis that photoionization of the IGM by high redshift radiation sources explains the absence of a detectable Gunn-Peterson effect in the spectra of all known quasars is investigated. This hypothesis has strong implications for the epoch and nature of galaxy formation as it relates to the origin of the required ionizing radiation background. New calculations are described of the ionization and thermal history of the IGM which determine the minimum required ionizing photon emissivity of the universe at z ≳ 4. The inability of the observed high redshift quasars to produce this emissivity is discussed, as are the consequences for primordial galaxy luminosity and metal production if early-type stars provide the emissivity instead. Other alternatives, such as radiation from protogalactic shocks and the radiative decay of unstable’inos, are also discussed.

An operational definition of the absorption in intergalactic space is presented. It is shown that in the case of the Central Void-discussed previously in literature- the parameter of extragalactic extinction is not zero i.e. is not neutral. Extragalactic extinction has been operationally defined.

A very faint intergalactic background light component (to be called IBL) was photographically discovered by Zwicky (1957) in the centre of the Coma Cluster. More recently Welch & Sastry (1971,1972) have mapped the area using isodensity tracings of photographic plates. In order to provide an accurate calibration of the intensity and to determine the colour of the IBL in the Coma Cluster, photoelectrical observations have been made of a region located ~2′ east of IC 3949 and ~12′ south-west of the cluster centre. The observations were made using a 60-cm Ritchey-Chretien telescope at the Metsahovi Observatory near Helsinki. The diaphragm size was 115″. In Table I the results of these observations are given together with three other available photoelectrical values for the IBL intensity in clusters. For comparison, the surface brightness due to visible galaxies has been estimated. The resulting values are: IV = 11.6 S10 (≙ 25.1 mag sec−2), IB = 4.6 S10 (≙ 26.1 mag sec−2), and thus the IBL intensity amounts to 25 percent and 39 percent in V and B, respectively, of the light of visible galaxies. For the colour index of the IBL one obtains using the values in Table I: B-V = 0.54 ± 0.18. Two mechanisms have been hitherto proposed in the literature to explain the IBL in the Coma Cluster: (1) light from dwarf galaxies, intergalactic globular clusters or individual intergalactic stars, and (2) thermal bremsstrahlung from a hot intergalactic gas with Te = 0.5−10 x 105 K. A third possible mechanism can still be mentioned, namely the scattering of the light of galaxies by intergalactic dust grains.

The existence of intergalactic dust has been proved by the following observational facts: the decrease of the numbers of distant galaxies and clusters of galaxies behind the central regions of near clusters of galaxies; the different distributions of RR Lyrae stars and galaxies near ι Microscopii (Hoffmeister's cloud); the dependence of colour excesses of galaxies on supergalactic coordinates as well as on the surface density of bright galaxies; the colour index vs redshift correlation of quasistellar objects. The densities of intergalactic dust are estimated to be between 5×10−30 g cm−3 (near the centers of clusters of galaxies) and 2×10−34 g cm−3 (in general intergalactic space). The grains may be formed either in the early phases of the Universe (25

Models of chemical evolution of elliptical galaxies, where galactic winds powered by SN explosions (of type I and II), as well as detailed nucleosynthesis prescriptions for single elements, are taken into account, predict the following results: i) There are different mass-metallicity relations for different heavy elements (O, Mg, Si and Fe), the one for iron being the steepest one. ii) In spite of the fact that star formation stops at early epochs (< 1 Gyr), explosions of SNe of type I at the present time are predicted, in agreement with observations. Type I SN rates higher by a factor of ~ 20 with respect to the current ones are predicted for ellipticals at high redshift (≥ 3). iii) The contribution to the enrichment of Fe and Si of the intergalactic medium from ellipticals is computed. Good agreement is obtained with the observed abundances from X-rays, and it is claimed that the majority of the gas in clusters should be of primordial origin.

Recent observations of interstellar extinction and diffuse galactic light in the ultraviolet are interpreted in terms of a bi-modal size distribution of interstellar grains.

An up-dated review is given of the evidence for the presence of intergalactic matter and radiation in the Universe. It is concluded that the only important constituents which may make a sizable contribution to the total mass-energy are intergalactic gas and condensed objects with a very high mass-to-light ratio. If the QSOs are not at cosmological distances, cold atomic hydrogen may still be the most important constituent and may contribute much more mass than do the galaxies. The X-ray observations still do not unambiguously show that very hot gas is present, though it is very likely on general grounds that some hot gas is present in clusters of galaxies.