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Observations of dust grains in the intergalactic medium (IGM) allow us to study an important aspect in the evolution of galaxies. Although its existence had been previously speculated upon, direct evidence of the presence of dust in the intergalactic space has only been available recently. We discuss various issues regarding the presence of dust in the IGM—its sources, transport mechanisms from galaxies into the IGM, its effect on reddening and on cosmological studies.

The concept of a Universe undergoing an acceleration in its expansion rate and predicating the existence of dark energy is based on observed deficits in brightness of Type 1a supernovae at high redshifts, amounting to Δ m∼0.3–0.5. We show that the effect of intergalactic graphite whiskers of radii in the general range 0.03–0.07 μm and lengths in excess of ∼5 μm will be to mimic the effects of dark energy in the redshift magnitude relation for Type 1a supernovae. The mean intergalactic density of whiskers required for such an effect is ∼3×10−34 g cm−3, about 10−5 of the critical closure density.

Two mechanisms of mass loss: galactic wind accompanied by the expulsion of supershell remnants and dust expelling from galaxies are briefly considered. Theoretically estimated parameters of the supershell fragments are similar to those of observed objects in vicinity of the Milky Way: small HI clouds and OIV structures observed with FUSE. Dust expelling brings dust in the IGM, and thus the analysis of intergalactic extinction curve will reveal a nature of the intergalactic dust. Evaporation of circumgalactic dust will change the chemical composition of gaseous galactic coronae. Ultraviolet observatories will have a significant impact on studies of these processes. Some prospects of such observations are outlined.

Galactic winds are important in recycling energy and metals in galaxies. They are also suspected to be the primary mechanism in the enrichment of the intergalactic medium. New observations are revealing the ubiquity of galactic winds, particularly at high redshift. We review the observational evidence for them in nearby star-forming and active galaxies and in the high-redshift universe.

Comparison of recent spectra by V. Motta et al. (V. Motta et al.: Astrophys. J. 574, 719–725) of two images of a distant gravitationally lensed quasar (z = 1.38) leads to an extinction curve of the lens galaxy (z = 0.83) showing clear evidence of a 2175A absorption feature. We compare these data with measured extinction curves from the Galaxy, LMC and SMC and with predictions from models. As it has been found by other authors in the Local Group galaxies we found that the SBS 0909 + 532 curve is compatible with three component models of classical spherical grains and PAHs. We note that apart from this standard point of view, the fits also admit an interpretation in terms of products of a biological origin involving three grain components.

Here we present a summary of the first discussion session on the interstellar and intergalactic medium.

The problem of influence of the ambiental intergalactic pressure on extended gas associated with normal field galaxies is briefly discussed. An empirically established characteristic radius of absorption can not be explained in the context of simple two-phase halo model as a consequence of pressure equilibrium of the hot halo with the smooth intergalactic medium. This conclusion is based upon the stringent constraints on the temperature and density of the intergalactic plasma obtained through the CMBR measurements. On the other hand, ram-pressure stripping caused by peculiar motions of galaxies does present a viable alternative for the hot halo truncation. It is shown that for a particular set of chosen parameters, a simple model is capable of producing the absolute upper limit on the extent of gas associated with the galaxy. The values obtained are compatible with the results of the recent QSO absorption-line studies, and are significantly higher than the radius at which the cooling timescale of the halo gas equates the dynamical timescale.

Chains ofnormal galaxies dominate small-scale clustering. This filamentary structure has nothing to do with large-scale filaments between clusters; and the revelation of short galaxy chains among normal galaxies was not expected by Arp, who investigated chains ofpeculiar galaxies. The nature of revealed chains cannot be explained in terms of so-called filaments observed in a scale of 100 Mpc: there is no continuation between 100 kpc and 100 Mpc chains of galaxies.

Extinction is treated through the optical thickness τ. Some important characters of dust, such as the composition and the size of grains, are reviewed in the light of observational evidence. Extinction is investigated through three kind of parameters: (1) those affecting it directly such as the geometrical cross sections of the grains (through the radius) and the wavelength of radiation interacting with them; (2) those connected witht the solid state constitution of grains, such as the dielectric constant and the conductivity; and (3) those connected with the distribution of dust in the intergalactic medium (IGM), such as the dust density and the geometrical thickness of dust slabs. The results are discussed and compared with observations.

In a recent attempt to explain the cosmic microwave background without the big bang, a thermalization mechanism involving intergalactic whisker grains of graphite was proposed. The effect of absorption by the intergalactic medium in general, and of the above type in particular, on the measurement of the deceleration parameterq 0 of the expanding universe is discussed. Its effect is shown to be comparable in magnitude but opposite to that of the luminosity evolution in galaxies. A consequential selection effect is also discussed.

An analysis is made of the implications of assuming that suprathermal dust grains (a≃3×108 cm) of intergalactic origin may acquire cosmic ray energies as high as 1020 eV. These dust grains may attain ‘suprathermal energy’ (vg≃3×108 cm s−1) by the Fermi process. Initially the dust grains are accelerated by the radiation pressure against the drag of the ambient gas of the medium, but once these dust grains attain a terminal velocity (U≃105 cm s−1), then they may be expelled out of the galactic region into the intergalactic medium and finally acquire high energy

We used theIRAS All Sky Maps in order to search for infrared emission in the direction of the Okroy Cloud (R.A.=12h50m, δ=22°). An enhancement of 100 μm diffuse emission is evident in such a region, with an anomalous value of the ratioIv (100 μm)/Av; hydrogen 21 cm emission is also present with low radial speed, thus suggesting that the cloud could be a satellite of our Galaxy.

The observations of the reddening of the distant galaxies and the weak diffuse radiation in the clusters of galaxies can be interpreted as a consequence of the presence of dust grains in the intergalactic medium. When allowance is made for the destruction of the grains in collision with particles of the hot gas, its lifetime is about 107–108 yr at a gas concentrationng≈10−3 cm−3. The detection of the infrared (IR) emission from the galaxy clusters might be the test for the proof of the presence of dust grains in the intergalactic medium. In this paper the estimates of the expected intensities and fluxes of IR emission for the spectral region 50–300μ are presented for two galaxy clusters in Coma and Perseus. The parameters of the hot gas spatial distribution are chosen from X-ray observations. Having assumed that intergalactic dust can be ejected only from the galaxies, we used such a model for intergalactic dust grains which explains very well the interstellar dust effects. It is shown that the dust temperature, which is determined from the general energetic balance of the dust grains, can achieve some scores of degrees of Kelvin. Two models of the dust spatial distribution are considered. It is found that the maximum of IR flux for the Coma cluster lies near λ=100μ and the same for the Perseus cluster near λ≈50–70μ. The total fluxes of IR emission from these clusters are about 105–106 Jy and can be detected by modern observational methods.

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

The observations of the Zel'dovich effect in galaxy clusters are reviewed. The failure to detect the effect at short wavelengths is interpreted as proof of the existence of intergalactic dust. Small and fast spinning grains can emit the power needed to compensate the decrease in temperature expected from the Zel'dovich effect.

In 1934 Bok argued that the observed phenomenon of clustering of galaxies could not be caused by intergalactic dust matter distributed like the interstellar clouds of dust in the Milky Way. Bok considered background clusters between foreground clusters of galaxies, while Zwicky investigated distant concentrations of galaxies through foreground clusters. Regarding the most distant chains of galaxies observed in the blue band, we investigate the foreground obscuration caused by dust. We indicate the case that galaxies between long chains of galaxies may be considered as obscured by solid grains, familiar of early investigations in statistical astronomy; probably the extinction law is similar to interstellar extinction. We agree with Zwicky's view that intergalactic matter affects the clustering of galaxies: moreover, we extend his research programme to the space between clusters of galaxies. Contrary to half-century tradition, the large clouds of grains similar to the grains occurring in the atmosphere of some stars must be taken as a real factor in the coil of complicated problems connected with the clustering of galaxies.

The physical state of optically thin, primordial gas in intergalactic space at redshift about Z=2.5 is re-examined, assuming the gas is in ionzation and thermal equilibrium by the integrated UV flux from QSOs. For densities in the range 10−5–10−2 cm−3, the structure of the gas can be well described by an isothermal polytrope of infinite index. The Lα absorbing clouds observed in the spectra of distant QSOs may arise in such self-gravitating intergalactic clouds. Some properties of the clouds are discussed with reference to recent observations and equilibrium models.

Some results concerning the intergalactic dust matter are presented. The results of Tarraro, considered as the first undoubted revelation of intergalactic extinction; and of Zabierowski, treated by him as a trial, seem not to be equivalent. Not experimental data, but their theoretical interpretation is responsible for this non-equivalence.

The Estonian School results (Joeveeret al., 1977) concerning the galaxy cell structure were partly interpreted in the spirit of Zwicky's idea on intergalactic dust, although it remained a speculation. The same view was proposed also by Rudnicki to explain the evident deficit of galaxies in the central region (Central Void) of the Jagellonian field. (Zieba (1974) tried to explain the effect of the central void in terms of an interstellar obscuration.) This explanation of the central void (CV) in terms of a possible intergalactic dust was opened up again inNuovo Cimento, 1984 and 1985; and its merits considered by Rudnickiet al. inComments on Astrophysics, 1989. Now we present a new solution of the problem of the CV mystery.

Charged dust grains of radiia≃3×10−6∼3×10−5 cm may be driven out of the galaxy due to radiation pressure of starlight. Once clear of the main gas-dust layer, dust grains may then escape into intergalactic space. Such grains are virtually indestructible-being evaporated only during galaxy formation. The dust grains, once injected into the intergalactic medium, may acquire suprathermal energy, thus ‘suprathermal grains’ in collision with magnetized cloud by the Fermi process. In order to attain relativistic energy, suprathermal grains have to move in and out (‘scattering’) of the magnetic field of the medium. It is now well established that high energy cosmic rays are of the order 1020 eV or more. We have speculated that these high energy (>-1018 eV) cosmic ray particles are charged dust grains, of intergalactic origin. This is possible only if there exists a magnetic field in the intergalactic medium.

As the origin of the soft X-ray background, emission of soft X-rays from shocks occurred in the accretion of intergalactic gas onto the Galaxy is studied. Infall of discrete gas clouds cannot explain the diffuse component of soft X-rays. If intergalactic gas rich in heavy elements as the cosmic abundance continously flows into the Galaxy and forms a standing shock surrounding the Galaxy, the line emissions by heavy elements from the shocked gas explain the soft X-ray background. Formation of the high velocity cloud by thermal instability in the shocked gas is also discussed briefly.

Computer simulations are performed which suggest that narrow intergalactic filaments can be produced by tidal forces. A fixed potential perturber passes a self-gravitating disk of stars in a nearly parabolic orbit. Due to their large velocity dispersion, the stars which are pulled out by the tide form only an amorphous cloud. The gas clouds are simulated by a disk of points of zero mass, zero velocity dispersion superimposed on the stellar disk. The ‘gas clouds’ form remarkably long, narrow filaments; but not so narrow as the narrowest filaments observed.

The explicit expressions for the orientation distribution function of interstellar and interplanetary dust grains in the anisotropic corpuscular or radiation fluxes, with consideration for the magnetic field influence, are obtained. An orientation is shown to be possible in a medium having an anisotropic temperature, which is usually the case for a non-equilibrium plasma in a magnetic field. It is noted that the small inhomogeneous dust grains should possess a specific rotation of polarization. The orientation of these dust grains is considered. The time required for the orientation is estimated. A possibility of explaining the interstellar polarization and polarization of the cometary radiation is discussed.

Transition radiation is emitted when a charged particle traverses a boundary separating two media. Such radiation must therefore be emitted as X-rays and ultraviolet radiation, from a medium where cosmic-ray electrons pass through dust grains. Calculation shows that transition radiation emitted in this way might account for soft X-rays emission from relativistic charged dust grains in the intergalactic medium (IGM). This could be a help in understanding the IGM as well as an indirect method of measuring the energies of high energy particles.