Based on XMM-Newton observations of M 87 and the Centaurus cluster, abundance profiles of various elements of the intracluster medium (ICM) are derived. The abundances of Si and Fe show strong decreasing gradients. In contrast, the O and Mg abundances are about half of the Si abundance at the center.

From the gas mass to stellar mass ratio and the comparison of Mg abundance with the stellar metallicity, the stellar mass loss from the central galaxies is indicated to be the main source of gas in the very central region of the clusters.

The observed O, Si and Fe abundance pattern determines the contribution of supernova (SN) Ia and SN II, with the abundance pattern of ejecta of SN Ia. Most of the Si and Fe of the ICM in the central region of the clusters comes from SN Ia which occured in the central galaxies. In order to explain the observed O/Si ratio of a half solar, SN Ia products should have similar abundances of Si and Fe, which may reflect dimmer SN Ia observed in old stellar systems.

Far-infrared (FIR) observations with ISOPHOT aboard the Infrared Space Observatory were carried out to observe the thermal FIR emission from diffuse intracluster dust (ICD) in galaxy clusters and outlying dust structures in the elliptical galaxies M86 and NGC 5128 (Centaurus A). Extended FIR emission from ICD has only been detected in the Coma cluster. It is likely caused by tidal interactions, since Coma is a dynamically young cluster with on-going gravitational interactions in the centre and infalling galaxy groups. M86 has a complex FIR morphology with a compact double source near its centre and several off-centre sources. The brightest FIR source lies between M86 and the nearby spiral NGC 4402 and represents the first direct detection of a localized intracluster dust cloud not associated with optical or neutral hydrogen emission. Overall, the FIR data do not support the ram pressure dust stripping scenario suggested on the basis of IRAS data, but indicate a significant influence of tidal interactions. NGC 5128 shows obvious optical signs of interaction, most notably a central dust lane and stellar shells. The FIR emission of outlying dust 15 kpc from the centre has been discovered where previously HI and molecular gas closely associated with the shells had been found. NGC 5128 is the first case of a shell galaxy where all components of the interstellar medium (ISM) are detected far off the nucleus, indicating an inner disk origin of the outlying material.

Recent observations at low and high redshift seem to confirm the presence of dust at very low abundances in the InterGalactic Medium (IGM) and especially in the IntraCluster Medium (ICM). We have studied the impact of this dust on the IGM, in terms of heating and cooling. on one hand, with an analytical model of dust emission, we have proved that the dust can be considered as the dominant cooling agent of the ICM at large scale, when the temperature is greater than T = 107 K. on the other hand, with a strong UV Background and a low temperature (Te ≤ 105 K), dust grains become an efficient heating agent of the IGM. These two opposite effects may have played an important role regarding structure formation of the Universe at large and small scales.

The statistical analysis of six nearby groups of galaxies, connected with the Milky Way, M31, M81, M101, NGC5128 and NGC5236 shows an isotropic distribution of galaxy planes, similarly to galaxies within the Local Group. Also planes of galaxies in the nearby (Vr < 500 km s−1) region of the Local Supercluster exhibit random distribution, disregarding their membership to groups. The result shows that strong environmental effects observed among dwarf galaxies do not change generally random distribution of galaxy planes in this region.

For 20 hot luminous Beppo-SAX galaxy clusters the occurrence of substrucures has been investigated in 2-D optical data, mainly through the application of the wavelet analysis. Our results are compared with others, coming from 3-D optical data, as well as X-rays. In this sample of galaxy clusters a higher percentage of substructure is observed among non-cooling flow clusters than cooling flow clusters.

Studies of the HI in galaxies have clearly shown that subtle details of the HI distribution and kinematics often harbour key information for understanding the structure and evolution of galaxies. Evidence for the accretion of material has grown over the past many years and clear signatures can be found in HI observations of galaxies. We have obtained new detailed and sensitive HI synthesis observations of three nearby galaxies which are suspected of capturing small amounts of HI and show that indeed accretion of small amounts of gas is taking place in these galaxies. This could be the same kind of phenomenon of material infall as observed in the stellar streams in the halo and outer parts of our galaxy and M 31.

New 21cm observations with the Green Bank Telescope show that a significant fraction of the HI in the inner Galaxy's halo ~ 1 kpc from the midplane exists in the form of discrete clouds. Some look very much like a Spitzer (1968) “standard” diffuse cloud but with their HI in two phases. They mark the transition between the neutral disk and the highly ionized halo. The dominant motion of the clouds is Galactic rotation, but some have random velocities of as much as 50 km s−1. They are part of the Galaxy and are not related to high-velocity clouds, yet their origin is obscure.

Recent, high sensitivity, HI observations of nearby spiral galaxies show that their thin ‘cold’ disks are surrounded by thick layers (halos) of neutral gas with anomalous kinematics. We present results for three galaxies viewed at different inclination angles: NGC 891 (edge-on), NGC 2403 (i=60°), and NGC 6946 (almost face-on). These studies show the presence of halo gas up to distances of 10-15 kpc from the plane. Such gas has a mean rotation 25-50 km s−1 lower than that of the gas in the plane, and some complexes are detected at very high velocities, up to 200-300 km s−1. The nature and origin of this halo gas are poorly understood. It can either be the result of a galactic fountain or of accretion from the intergalactic medium. It is probably analogous to some of the High Velocity Clouds (HVCs) of the Milky Way.

We are studying the properties of the holes and the high velocity gas in NGC 6946. Here we present some puzzling results.

We present VLA HI imaging of the blue compact dwarf (BCD) galaxy Haro 2 and a map of its CO(1-0) distribution obtained with the OVRO millimeter array. In addition we obtained NIR JH images with the 2.1m telescope of Mexico's National Optical Observatory (OAN) and derived surface brightness profiles. Combining all these results we find an intriguing picture: the HI reveals that the kinematical major axis lies perpendicular to the photometric major axis. Our observations support the suggestion that this configuration is due to recent capture of an important quantity of gas of external origin.

We summarize the results of the Far-Ultraviolet Spectroscopic Explorer (FUSE) program to study O VI in the Milky Way halo. Spectra of 100 extragalactic objects and two distant halo stars are analyzed to obtain measures of O VI absorption along paths through the Milky Way thick disk/halo and beyond. Strong O VI absorption over the velocity range from −100 to 100 km s−1 reveals a widespread but highly irregular distribution of O VI, implying the existence of substantial amounts of hot gas with T~3×105 K in the Milky Way thick disk/halo. The overall distribution of O VI can be described by a plane-parallel patchy absorbing layer with an average O VI mid-plane density of no(O VI) = 1.7×10−8 cm−3, an exponential scale height of ~2.3 kpc, and a ~0.25 dex excess of O VI in the northern Galactic polar region. Approximately 60 percent of the sky is covered by high velocity O VI with |vLSR|>100 km s−1. This high velocity O VI traces a variety of phenomena in and near the Milky Way including outflowing material from the Milky Way, tidal interactions with the Magellanic Clouds, accretion of gas onto the Milky Way, and warm/hot gas interactions in a highly extended (>70 kpc) Galactic corona or with hot intergalactic gas in the Local Group.

We report on the results of XMM-Newton observations of nearby starburst galaxies that form part of a multi-wavelength study of all phases of extraplanar gas in external galaxies, which is conducted in order to assess the importance of halos as repositories of a metal-enriched medium and their significance in terms of galactic chemical evolution and possible metal enrichment of the intergalactic medium (IGM). XMM-Newton observations of the starburst galaxy NGC 1511 revealed e.g. the presence of a previously unknown extended hot gaseous phase of its interstellar medium (ISM), which partly extends out of the disk plane. We also present preliminary results based on XMM-Newton observations of NGC 1808, NGC 4666 and NGC 3628.

To address, among other questions, puzzling observations about star forming in the extreme outer HI disk of M31 (Cuillandre et al. 2001), a scenario of interstellar gas cycling between the visible and a very cold invisible phase is investigated. The key new element sketched here, allowing to maintain the bulk of the gas out of sight, is that molecular hydrogen becomes liquid or solid below 33 K at sufficiently high pressure, allowing AU-sized spheres of very cold gas to be stabilised by incompressible cores of condensed H2 (Pfenniger 2004). These predicted cold gas globules are relatively weakly bound (~ 10−3eV/nucleon), such that their lifetime depends directly on the ambient UV/CR excitation level. At galactic scale the globules behave as collisionless bodies, and evaporate and become the usual visible ISM gas through heating. Much of the ISM gas can thus spend a long time in this cold condensed phase in low excitation regions. N-body simulations of galactic disks modeling such effects have been run, and some of their features are described in more detail in this volume and elsewhere (Revaz & Pfenniger 2004).

Our OVRO observations at 300 pc resolution of the molecular gas disk in the edge-on spiral galaxy NGC 4013 show no evidence for extraplanar material at our sensitivity limit. The observed molecular gas kinematics are in agreement with gas motion in a barred potential.

We present wide-field optical imaging covering the entire neutral hydrogen disc of the Local Group dwarf galaxy NGC 6822. These observations reveal the presence of numerous young stars at large galactocentric radii. The extended distribution of young stars implies that stars do form in the outer disks of dwarf galaxies; the HI is not necessarily much more extended than the stellar population. This finding has important consequences for the chemical enrichment of the interstellar medium throughout dwarf galaxies.

The amount of gas shed by red giant stars in the halo is assessed to find its contribution to the long known infall of high-b gas. The infall rate due to RG star mass loss equals 3 · 10−6 M⊙ kpc−1 y−1.

NGC 2915 is a blue compact dwarf galaxy embedded in an extended, low surface brightness HI disk with a bar and two-armed spiral structure. Common mechanisms are unable to explain those patterns and disk dark matter or a rotating triaxial dark halo were proposed as alternatives. Hydrodynamical simulations were run for each case and compared to observations using customized column density and kinematic constraints. The spiral structure can be accounted for by an unseen bar or triaxial halo, but the large bar mass or halo pattern frequency required make it unlikely that the spiral is driven by an external perturber. In particular, the spin parameter lambda is much higher than predicted by current CDM structure formation scenarios. Massive disk models show that when the gas surface density is scaled up by a factor of about 10, the disk develops a spiral structure matching the observed one in perturbed density as well as velocity. This suggests that the disk of NGC 2915 contains much more mass than is visible tightly linked to the neutral hydrogen. A classic (quasi-)spherical halo is nevertheless still required, as increasing the disk mass further to fit the circular velocity curve would make the disk violently unstable

We carried out global three-dimensional magnetohydrody-namical (MHD) simulations of galactic gaseous disks re-accreting intergalactic plasma. As the initial condition, we assume that a rotating slender torus is formed at 10kpc from the galactic center. We assume a gravitational potential generated by bulge stars, disk stars and dark matters. Numerical results indicate that magnetorotational instability (MRI) growing in the torus amplifies magnetic fields and generates turbulence. The Maxwell stress enhanced by turbulent magnetic fields drives mass accretion of the disk gas. The amplification of magnetic fields in the accreting gas disk drives magnetic activities such as flares and plasma heating due to magnetic reconnection. The magnetic activity is maintained for time scales longer than the accretion time scale, typically 5Gyr.

Dwarf galaxies are ideal laboratories to study influential effects on galaxy evolution. In particular, their gas-rich variant with very active star formation, starbursting dwarf irregulars, shows chemical and structural signatures that lead unambiguously to the conclusion that they are standing in a vital contact with their surroundings. Gas infall cannot only trigger star formation but also allows for a reduction of the metal content. on the other hand, active star formation ignites numerous supernovae type II which accumulate and can produce a galactic wind. This again depletes the metals pushing them into a gas mixing cycle with different timescales, locally of about 10 Myrs, but an galactic scales of at least 1 Gyr. This paper illuminates the different processes like gas infall and outflow and their effects on the chemical evolution, the star formation, and the gas mixing in dwarf irregular galaxies.

We study the ionization equilibrium of Fe using photoionization models that incorporate improved values for the relevant atomic data. The previously available photoionization models predict concentrations of Fe3+ which are a factor of 3-8 higher than the values inferred from emission lines of [Fe III] and [Fe IV]. Our new models reduce these discrepancies to factors of 2-5. We comment on the possible reasons behind the remaining discrepancies, and present an updated ionization correction factor for obtaining the Fe abundance from the Fe++ abundance.