Bubbles and Superbubbles are among the largest and most energetic structures in the Interstellar Medium (ISM). It is thought that most bubbles are formed from the combined effects of stellar winds and supernovae, which plow through the ISM, sweeping up matter in to dense atomic hydrogen (H i) shell walls and heating and ionising the gas left behind. Although this explanation works clearly for some of the smaller objects, it is hard to understand how many superbubbles apparently require hundreds to thousands of massive stars to explain their sizes. In some galaxies, like the Large Magellanic Cloud, the H i structure of the disk is dominated by H i shells. Similarly, the ISM of the Milky Way is riddled with tens, perhaps hundreds, of H i shells. In the Milky Way, as well as in the nearest disk galaxies, there is clear evidence of superbubbles that have grown so large as to break out of the disk. It would seem, then, that not only do these objects have a significant impact on the disk, they provide an invaluable source of energy for the halo. In this review I present some of the latest results on bubbles and superbubbles in disk galaxies and their relationship to the halo.

We have used the Japanese VLBI array VERA to perform high-precision astrometry of an H2O maser source in the Galactic superbubble NGC 281. We successfully detected a trigonometric parallax of 0.355±0.030 mas, corresponding to a source distance of 2.82±0.24 kpc. The source distance as well as the absolute proper motions were used to demonstrate the 3D structure and expansion of the NGC 281 superbubble, ∼650 pc in size parallel to the Galactic disk, indicating the superbubble expansion may be confined to the disk. We estimate the expansion velocity of the superbubble as ∼20 km s−1 both perpendicular to and parallel to the Galactic disk with a consistent timescale of ∼20 Myr.

We present new parsec-scale resolution data from a multi-phase study of the ISM in the walls of Galactic supershells. Hi synthesis images and CO survey data reveal a wealth of substructure, including dense-tipped fingers and extended molecular clouds embedded in shell walls. We briefly consider formation scenarios for these features, and suggest that both the interaction of an expanding shell with pre-existing dense clouds, as well as in-situ formation of CNM and molecular gas, are likely to be relevant. Future work will also examine the role of instabilities in structure formation and breakup, and will investigate the presence of high-altitude gas associated with supershells and chimneys.

In this paper observations are presented of interstellar high-ion absorption-line profiles in three diverse environments: the extended halo of the Milky Way, the host galaxies of gamma-ray bursts at z > 2, and intervening damped Lyman-α absorbers at z ≈ 3. In the first two cases, outflow is directly detected, and in the remaining case outflow is suggested (but not strictly required) by the absorber kinematics. Together these observations demonstrate the ability of the high-ions to trace large-scale galactic outflows.

It has long been known that the starburst in M 82 drives a large-scale bipolar superwind, carrying large volumes of material (in the form of hot, warm and cool gas and dust) out of the disk plane. However, what is not known is exactly how the hundreds of catalogued super star clusters power the outflow, and how it becomes so structured and collimated.

Here I will present part of the most comprehensive set of spectroscopic integral field observations ever made of the starburst clumps and outflow in M 82. I will describe how we have mapped in detail the spatial variation of Hα kinematics, gas density and excitation from the wind roots out into the halo, and how these compare to features seen in recent high resolution imaging campaigns. These observations will be used to address questions such as: how does the 2D variation of gas kinematics evolve from the core out into the halo? Our spectra indicate evidence for the sites of mass-loading – where is it occurring within the wind?

We investigate the correlation of star formation quenching with internal galaxy properties and environment by comparing observation and theoretical models. We first classify galaxies as the most massive “central” or “satellite” in each halo in order to investigate the environmental effect. For observed central galaxies, we are unable to determine whether star formation quenching is primarily connected with halo mass or stellar mass, because these two quantities are strongly correlated. For satellite galaxies, a nearly equal dependence on halo mass and stellar mass is seen. We find that theoretical models with AGN feedback reproduce the dependence on colours and specific star formation rates for central galaxies reasonably. However, the same models seriously fail to reproduce the star formation rates of satellite galaxies by over-quenching star formation via strong strangulation: satellite over-quenching problem.

Two compact H I clouds which seem to belong to the Ophiuchus superbubble were studied at ∼30′′ resolution using the Very Large Array (VLA) in C and D configurations together with the Green Bank Telescope (GBT) providing the short-spacing flux. Here we present preliminary results of the data analysis.

We present a dynamical scenario of instantaneous dense gas entrainment by stellar winds in a wind-dominated HII region. Stellar winds and radiation pressure will become more important in an HII region as the central star or cluster is sufficiently luminous, therefore the region is windswept instead of photoevaporation-dominated. In our model, a cloud is smaller than the region as a whole, hence its mass injection occurs through either ordinary photoevaporation or wind ablation. We predict that the instantaneous wind ablation will cause strong mixing between hot winds and warm gas, and the mass injection rate of the wind-confined photoevaporated flow is higher than that of the ionizing source.

The emission properties of the components of the multiphase interstellar medium (ISM) require observations at very different wavelengths covering the range from X-rays for the hot phase to radio emission for the non-thermal component. This contribution gives an overview of the observational evidence for the presence of a multiphase ISM in halos of spiral galaxies. It is shown that the presence of halo gas correlates with the starformation rate in the disk. Recent radiocontinuum observations for NGC 253 are discussed with regard to the propagation of cosmic rays into the halos.

Many phenomena first detected in Galactic H i, such as high velocity clouds and gaseous warps, have now been detected and studied in nearby galaxies. Given this valuable perspective I examine three aspects of Galactic extra-planar gas that appear somewhat puzzling from our vantage in the Milky Way disk. I. Spiral galaxies have a rotation curve that decreases with distance above their mid-plane; where is the lagging halo in the Milky Way? II. Other systems show clear evidence for accretion of neutral gas; where is this gas in the Milky Way? III. Warps of the H i layer are common in the outskirts of disk galaxies; are we confident that we’ve correctly parameterized our own warp? The answers appear to be that lagging halo gas could well be present in the Galaxy but would be difficult to detect; that there is now solid evidence for the accretion of high-velocity H i clouds by the disk, though the details are still mysterious, and that the warp continues to baffle us, as it exhibits a puzzling morphology and kinematics.

Multiphase gas has been observed at large vertical distances above the star forming disk in several nearby spirals. Much of this gas is likely a by-product of star formation processes in the disk, but some may trace gas accretion from the intergalactic medium. High quality observations of the distribution and kinematics of this extraplanar gas shed light on the relative importance of these critical processes, and their contribution to the redistribution and replenishment of the galaxy’s reservoir of star forming material. We present recent observations of the kinematics of ionized and neutral extraplanar gas in several nearby galaxies. The balance between infall and outflow implied by the observations is discussed.

We derive the 3-D volume density distribution for the Milky Way H i gas out to a galactocentric distance R = 60 kpc and separate the extra-planar component from the disk gas. The main part of the Milky Way H i distribution is significantly warped but shows a coherent structure out to R ∼ 35 kpc. Extra-planar gas can be distinguished from this disk component. From profile wings observed at the Galactic poles we find evidence for a highly turbulent H i distribution. For a velocity dispersion of 74 km s−1 the turbulent pressure gradient of this extra-planar gas is on average in balance with the gravitational forces. Such a component with a radial scale length of 7.5 kpc describes also the H i gas at the outskirts (40 < R < 60 kpc). About 10% of the H i gas is in this state. This component resembles the anomalous gas discovered in some galaxies.

Using HI absorption spectra taken from the recent surveys of λ 21-cm line and continuum emission in the Galactic plane, the distribution of cool atomic clouds in the outer disk of the Milky Way is revealed. The warp of the midplane is clearly seen in absorption, as it is in emission, and the cool, neutral medium also shows flaring or increase in scale height with radius similar to that of the warm atomic hydrogen. The mixture of phases, as measured by the fraction of HI in the cool clouds relative to the total atomic hydrogen, stays nearly constant from the solar circle out to about 25 kpc radius. Assuming cool phase temperature ∼50 K this indicates a mixing ratio of 15% to 20% cool HI, with the rest warm.

21-cm studies of the Galactic extra-planar gas layer can help us understand the origin of the gaseous Galactic halo and its interaction with the Milky Way disk. The 100-m Effelsberg radio telescope was utilized to detect and analyze a population of Hi halo clouds belonging to this layer, at Galactocentric radii R > 8.5kpc, following Galactic rotation. This sample of clouds has similar properties to the population detected by Lockman with the GBT at R < 8.5kpc. High resolution follow-up observations resolve them into a number of small, dense, cold arc-minute sized Hi cores. Their study provides evidence for a thermal equilibrium with the extended warmer envelopes. Single dish observations with the 300-m Arecibo telescope disclosed a sample of Hi clouds with similar properties. A power spectrum analysis of the Arecibo maps probes the characteristics of turbulence as a dominant physical process in the halo region.

Thick layers of warm, low density ionized hydrogen (i.e., the warm ionized medium or WIM) in spiral galaxies provide direct evidence for an interaction between the disk and halo. The wide-spread ionization implies that a significant fraction of the Lyman continuum photons from O stars, produced primarily in isolated star forming regions near the midplane and often surrounded by opaque clouds of neutral hydrogen, is somehow able to propagate large distances through the disk and into the halo. Moreover, even though O stars are the source of the ionization, the temperature and ionization state of the WIM differ significantly from what is observed in the classical O star H ii regions. Therefore, the existence of the WIM and observations of its properties provide information about the structure of the interstellar medium and the transport of energy away from the midplane as well as place significant constraints on models.

We present new results on extraplanar DIG (eDIG), based on high spatial resolution narrowband imaging observations of four late-type, actively star-forming edge-on spirals, obtained with ACS on-board HST. Our Hα observations reveal a multitude of structures on both small and large scales. Whereas all four galaxies have been studied with ground-based telescopes before, here the small scale structure of the extended emission line gas is presented for the very first time at a spatial resolution of 0.05′′, corresponding to 5 pc at the mean distance to our galaxies. The eDIG morphology is very different for all four targets, as a result of their different star formation activity and galaxy mass. There is a very smooth DIG morphology observed in two of the galaxies (NGC 4634 and NGC 5775), whereas the other two (NGC 4700 and NGC 7090) show a much more complex morphology with intricate filaments, and bubbles and supershells. We find that the morphology of the eDIG, in particular the break-up of diffuse emission into filaments in galaxy halos, shows a strong dependence on the level of star formation activity per unit area, and eDIG can be arranged into a morphological sequence.

The analysis of hydrogen and metal absorption lines in the spectra of distant quasars (QSOs) has become a powerful method to study the distribution and physical properties of low-density gas situated in the extended halos and circumgalactic environment of galaxies at low and high redshift. We here review the overall properties of halo absorption-line systems seen in QSO spectra and discuss their connection to the Milky Way’s population of intermediate- and high-velocity clouds.

The X-ray band provides a very powerful and unique way to explore the hot phase of the ISM in external galaxies. This phase has been known to exist in galaxies since the early Einstein observations, but only with recent detailed observations of objects in the local universe are we beginning to study it in detail and appreciate the importance of the hot ISM as a vehicle of chemical enrichment in different astrophysical environments. This knowledge can then be used to understand the properties of galaxies at the epoch of formation and their subsequent evolution, both in the field and in clusters.

We investigate the unresolved X-ray emission originating from the bulge of M 31. We show that a part of this emission is due to a large number of faint sources – mainly accreting white dwarfs and active binaries, associated with the old stellar population, similar to the Galactic ridge X-ray emission of the Milky Way. We also detect soft X-ray emission from ionized gas. It has a temperature of about ∼300 eV and total mass of ∼2 × 106Msun. The gas distribution is extended along the minor axis of the galaxy suggesting that it may be outflowing in the direction perpendicular to the disk of M 31. The outflow can be maintained from the mass and energy supply from evolved stars and type Ia supernovae respectively.

The probability distribution functions (PDFs) of the average densities of the diffuse ionized gas (DIG) and the diffuse atomic gas are close to lognormal, especially when lines of sight at |b| < 5∘ and |b|≥ 5∘ are considered separately. Our results provide strong support for the existence of a lognormal density PDF in the diffuse ISM, consistent with a turbulent origin of density structure in the diffuse gas.