The wealth of data in the past decades, and especially in the past 15 years has transformed our picture of the gas around the Milky Way and other spiral galaxies. There is good evidence for extraplanar gas that is a few kpc in height and is seen in all gaseous phases: neutral; warm atomic; and hot, X-ray emitting gas. This medium is seen not only around the Milky Way, but other spiral galaxies and it is related to the star formation rate, so it is likely produced by the activity in the disk through a galactic fountain. More extended examples of halo gas are seen, such as the HVC around the Milky Way and around M 31. This gas is typically 10–20 kpc from the galaxy and is not seen beyond 50 kpc. This gas is most likely being accreted. A hot dilute halo (106 K) is present with a similar size, although its size is poorly determined.

An ongoing controversy surrounds the relative amounts of outflow from the disk and accretion onto galaxies such as the Milky Way. There is good evidence for accretion of cold material onto the Milky Way and other galaxies, but it is not clear if there is enough to modify the gas content and star formation properties in the disk. The reservoir of accretion material is as yet unidentified. Some of these findings may be related to the issue that galaxies are baryon-poor: their baryon to dark matter ratio is well below the cosmological value. The absence of baryons may be due to extremely violent outflow events in the early stages of galaxy formation. We may be able to understand this stage of galaxy evolution by applying our deepening understanding of our local disk-halo environment.

This paper gives an overview of the three basic constituents of the interstellar medium (ISM) of our Galaxy, namely, the ordinary matter (made of gas and dust), magnetic fields, and cosmic rays. Without going into any detail, it presents their observed physical characteristics as well as their spatial distributions at large and small scales.

The thermal pressure of gas in the Galaxy is relatively small compared to the total pressure, but the values and dispersions of this pressure in different phases of the medium can yield important diagnostics of turbulent and explosive processes, as well as the exchange of matter between the disk and halo. Diverse methods of measuring this pressure allow us probe conditions in cool, neutral clouds, the warm, ionized medium, and the hot gas in the halo, all of which are summarized here.

The interstellar medium (ISM) is subject, on one hand, to heating and cooling processes that tend to segregate it into distinct phases due to thermal instability (TI), and on the other, to turbulence-driving mechanisms that tend to produce strong nonlinear fluctuations in all the thermodynamic variables. In this regime, large-scale turbulent compressions in the stable warm neutral medium (WNM) dominate the clump-formation process rather than the linear developent of TI. Cold clumps formed by this mechanism are still often bounded by sharp density and temperature discontinuities, which however are not contact discontinuities as in the classical 2-phase model, but rather “phase transition fronts”, across which there is net mass and momentum flux from the WNM into the clumps. The clumps grow mainly by accretion through their boundaries, are in both thermal and ram pressure balance with their surroundings, and are internally turbulent as well, thus also having significant density fluctuations inside. The temperature and density of the cold and warm gas around the phase transition fronts fluctuate with time and location due to fluctuations in the turbulent pressure. Moreover, shock-compressed diffuse unstable gas can remain in the unstable regime for up to a few Myr before it undergoes a phase transition to the cold phase, and is furthermore constantly replenished by the turbulence. These processes populate the classically forbidden density and temperature ranges. Since gas at all temperatures appears to be present in bi- or tri-stable turbulence, we conclude that the word “phase” applies only locally, surrounding phase transition sites in the gas. Globally, the word “phase” must relax its meaning to simply denote a certain temperature or density range.

Radio synchrotron emission, its polarization and its Faraday rotation are powerful tools to study the strength and structure of interstellar magnetic fields. Total fields in gas-rich spiral arms and bars of nearby galaxies have strengths of 20–30 μG and are dynamically important. Ordered fields with spiral structure exist in grand-design, flocculent and even irregular galaxies. The strongest ordered fields of 5–10 μG are found in interarm regions, sometimes forming “magnetic spiral arms”. Faraday rotation sometimes reveals large-scale patterns which are signatures of coherent fields generated by dynamos, but in most galaxies the field has a complicated structure. – Magnetic fields with X-shaped patterns are observed in radio halos around edge-on galaxies. The synchrotron scale height allows to measure the mean outflow velocity, which seems to increase with the star-formation rate.

We give a review of cosmic ray propagation models. It is shown that the development of the theory of cosmic ray origin leads inevitably to the conclusion that cosmic ray propagation in the Galaxy is determined by effective particle scattering, which is described by spatial diffusion. The Galactic Disk is surrounded by an extended halo, in which cosmic rays are confined before escaping into intergalactic space. For a long time cosmic ray convective outflow from the Galaxy (galactic wind) was believed to be insignificant. However, investigations of hydrodynamic stability and an analysis of ISM dynamics (including cosmic rays) showed that a galactic wind was emanating near the disk, and accelerating towards the halo, reaching its maximum velocity far away from the disk. Therefore convective cosmic ray transport should be important in galactic halos. Recent analysis of the gamma-ray emissivity in the Galactic disk of EGRET data, which showed that cosmic rays are more or less uniformly distributed in the radial direction of the disk, as well as the interpretation of soft X-ray emission in galactic halos, give convincing evidence of the existence of a galactic wind in star forming galaxies.

We show that the X-ray emission observed towards the center of our Milky Way Galaxy is consistent with a strong (2.1 M⊙/yr) outflow powered by both cosmic-ray pressure and thermal-gas pressure. In addition, the inferred launch parameters of such an outflow seem consistent with conditions inferred in the central Milky Way and other galaxies (although it is not clear if a significant vertical magnetic field exists in the center of the Galaxy). We also show that in galaxies with cosmic-ray pressure, gas pressure, and a vertical magnetic field component, cosmic-ray pressure can yield outflows over a wider range of conditions.

Recent studies at optical and radio wavelengths have shown that interstellar clouds have density structures on scales smaller than 1 pc. These have been identified in form of temporal variations or differences in column density within small angular separations. Despite the number of detections with various methods, the nature and ubiquity of the small-scale structure in the ISM is not yet fully understood. Using UV absorption data from FUSE of six LMC stars with small angular separations we study these small-scale structures in the Galactic disk and IV gas at spatial scales less than 1 pc. Based on H2 and O I we find that the differences in column densities represent small density structures in the disk and halo gas.

Spiral arm shapes, locations, and velocities can be well determined, when model pitfalls are avoided (Vallée 2008a). Similarly, circularly orbiting gas and magnetic field lines can cross the spiral arms, when observational pitfalls are avoided (Vallée 2008b).

The interstellar medium (ISM) is a key ingredient in galaxy formation and evolution as it provides the molecular gas reservoir which fuels star formation and supermassive black hole accretion. Yet the ISM is one of the least studied aspects of distant galaxies. Molecular and atomic transitions at (sub)millimetre wavelengths hold great promise in measuring macroscopic properties (e.g. masses, morphologies, star formation laws), as well as microscopic properties (e.g. gas densities, temperatures, cooling) of high-z galaxies. In this overview I summarize the growing number of high-z molecular line detections, highlighting some of the most intriguing results along the way. I end by discussing a few areas where future facilities (e.g. ALMA, EVLA, CCAT, LMT) will drastically improve on the current state of affairs.

The luminous components of galaxies can act as powerful probes of halo gas in foreground galaxies along the sightline. We use Keck/LRIS absorption-line spectroscopy of a luminous, blue background galaxy at z = 0.69 to study Mg II halo gas in the outskirts (ρ = 11.1h−1kpc) of a massive, poststarburst galaxy in the foreground at z = 0.47. The foreground absorber shows signs of recent merger activity and is host to a low-luminosity AGN. The halo absorption we observe is extremely strong (Wr(2796) = 4.0 ± 0.1Å) and is indicative of a large Mg II gas velocity width (> 400kms−1). We briefly discuss the possible origins of this absorption, including multiphase cooling of hot halo gas and galactic winds/outflows.

Instead of using background QSO’s, we suggest using the light from galaxies to probe cool gas flows within their own halos. We highlight two DEEP projects based on this idea, one by Sato et al. (2009) that uses NaD absorption lines of galaxies at low redshifts z < 0.6 and another by Weiner et al. (2009) that uses MgII lines seen in galaxies at higher redshifts z ∼ 1.4.

In a recent study we have used a large sample of extragalactic radio sources to investigate the redshift evolution of the Rotation Measure (RM) of polarized quasars up to z ≈ 3.0. We found that the dispersion in the RM distribution of quasars increases at higher redshifts and hypothesized that MgII intervening systems were responsible for the observed trend. To test this hypothesis, we have recently obtained high-resolution UVES/VLT spectra for 76 quasars in our sample and in the redshift range 0.6 < z < 2.0. We found a clear correlation between the presence of strong MgII systems and large RMs. This implies that normal galaxies at z ≈ 1 already had large-scale magnetic fields comparable to those seen today.

Galaxies in dense environments of clusters and groups are objects to ram pressure and hydrodynamical effects of the intra-cluster medium (ICM) that can significantly affect their interstellar content (ISM). We perform numerical simulations of the ram pressure stripping using tree/SPH code GADGET with an adapted method for calculating ISM-ICM interactions. We study the stripping effects of various environments of clusters, with compact or large distributions of the ICM, and the role of the disk tilt angle. Our results indicate that the total amount of the momentum transferred by the encountered ICM to the galaxy’s gas may provide the main features of the stripping. To reproduce our numerical results we propose a fitting formula relating the stripped mass fraction to the encountered ICM column density and the tilt angle. This dependence is superior to that on the peak ram pressure seen in Gunn and Gott criterion.

It is shown that mass distribution of baryonic matter accounts for the rotation curve of galaxy NGC 4736.

The environment of high-redshift galaxies is characterized by both wind-driven outflowing gas and gravitationally infalling streams. To investigate such galaxy-IGM interplay we have generated synthetic optical absorption line spectra piercing the volume surrounding a starbursting analog of a Lyman Break Galaxy selected in a z ≈ 3 output from a SPH simulation, including a detailed treatment of mechanical feedback from winds. Distributions for several observable species (Hi, Ciii, Civ, Siii, Siiii, Siiv, Ovi, Ovii, and Oviii) have been derived by post-processing the simulation outputs. The hot wind material is characterized by the presence of high-ionization species such as Ovi, Ovii, and Oviii (the latter two observable only in X-ray bands); the colder (T < 105.5 K) infalling streams can be instead identified by the combined presence of Siii, Siiii, and Ciii optical absorption together with Ovi that surrounds the cooler gas clumps. Considering both line profile and Pixel Optical Depth analysis of the synthetic spectra. We conclude that it may be difficult to clearly identify wind-blowing galaxies and their complex gaseous environment at high redshift in optical QSO absorption-line spectra based solely on the observed ion absorption patterns.

We investigate the radial column density distribution in a sample of 9 galaxies taken from the THINGS survey. The HI data have been used to probe the extent of the neutral (atomic) gas disks down to column densities of less than 0.1 ℳ⊙ pc−2. The size of the disks might be set simply by the amount of available gas that has contributed to the formation of the galaxy or truncated by ionisation by the extragalactic radiation field once the HI column density has dropped below a certain threshold.

The cosmic star formation history is known to evolve strongly, by an order of magnitude, out to a redshift about unity. Over the same time period, the average neutral gas density in galaxies evolves only marginally, by a factor of about two. The star formation rate density is high enough that the observed neutral gas in galaxies would be exhausted within about a gigayear in the absence of replenishment, and the question becomes one of what form the replenishment takes. For the observed neutral hydrogen mass density to remain constant or only slowly evolving, the replenishment must balance the consumption. Observations of gas infall and outflows in nearby galaxies suggest there may be insufficient infalling material to balance the consumption, though, raising the question of whether a new mechanism for replenishment may be required. We present a mechanism for replenishment that is associated with star formation, supershells within galaxies, and discuss the advantages and limitations of this mechanism.