In galaxies like the Milky Way, baryons cycle between the disk, the halo, and the intergalactic environment in ways that have profound consequences for galaxy growth and evolution. The vertical structure, support, and kinematics of gaseous, disk-halo interfaces are thus indicative of the processes driving galaxy growth at low redshift. We use observations of nearby disk galaxies viewed from a range of inclination angles to develop a three-dimensional picture of the kinematics of extraplanar diffuse ionized gas (eDIG) layers. These layers challenge our understanding of the dynamical state of disk-halo interfaces, as their observed exponential electron scale heights, h_z ∼ 1 kpc, exceed their thermal scale heights by factors of a few. For the edge-on galaxies NGC 891 and NGC 5775, we pair optical emission-line spectroscopy with radio continuum observations from Continuum Halos in Nearby Galaxies - an EVLA Survey to constrain the turbulent, magnetic field, and cosmic-ray pressure support at the disk-halo interface. A dynamical equilibrium model is only stably satisfied at large galactocentric radii where the gravitational field is relatively weak (R ≥ 8 kpc and R ≥ 10 kpc for NGC 891 and NGC 5775, respectively), suggesting that bulk flows are present in the warm ionized phase of disk-halo interfaces. We directly detect evidence of such flows in the nearby, low-inclination galaxy M83 by developing a Markov Chain Monte Carlo method to separate planar and extraplanar emission. We find indications of a galactic fountain near star-forming regions, as well as a vertical velocity dispersion ([small sigma]_z ∼ 100 km/s) that greatly exceeds the horizontal dispersions in edge-on galaxies ([small sigma]_y = 40 − 60 km/s). This suggests that a disk-halo circulation as well as anisotropic, random motions play a role in supporting the eDIG layer at its observed scale height. This work favors a non-hydrostatic, disk-halo flow model for eDIG layers in which the gas both originates in and returns to the disk. It also raises new questions about the dependence of eDIG kinematics on star-formation rate and the multiphase nature of disk-halo flows, motivating future work to further illuminate the disk-halo connection in the present-day universe.

All three of these studies suggest that OB stars, both in H II regions and in the field, play a major role in creating and maintaining the DIG, and that other mechanisms, such as shocks, may also contribute to the ionization of the DIG.

On the occasion of the retirement of Ulrich Schwarz, a symposium was held in Groningen in May of 1996, celebrating his contributions to the study of the int- stellar medium, including his work on the high-velocity clouds. The coming together of many specialists in the latter ?eld prompted the idea of compiling a book c- taining their contributions, and summarizing the status of our understanding of the high-velocity cloud phenomenon. This seemed especially worthwhile at the time, since many exciting developments were taking place. After the discovery of some H i clouds with high velocities, about 40 years ago, the subject had been dominated by 21-cm observations of H i emission. Starting in the mid-1980s much progress was being made because of the availability of new instruments, such as large ground-based optical telescopes and UV observatories in space. The connections between the work on high-velocity clouds and other studies of the properties of the (hot) interstellar medium also became clearer.

This book consists of invited reviews written by world-renowned experts on the subject of the outskirts of galaxies, an upcoming field which has been understudied so far. These regions are faint and hard to observe, yet hide a tremendous amount of information on the origin and early evolution of galaxies. They thus allow astronomers to address some of the most topical problems, such as gaseous and satellite accretion, radial migration, and merging. The book is published in conjunction with the celebration of the end of the four-year DAGAL project, an EU-funded initial training network, and with a major international conference on the topic held in March 2016 in Toledo. It thus reflects not only the views of the experts, but also the scientific discussions and progress achieved during the project and the meeting. The reviews in the book describe the most modern observations of the outer regions of our own Galaxy, and of galaxies in the local and high-redshift Universe. They tackle disks, haloes, streams, and accretion as observed through deep imaging and spectroscopy, and guide the reader through the various formation and evolution scenarios for galaxies. The reviews focus on the major open questions in the field, and explore how they can be tackled in the future. This book provides a unique entry point into the field for graduate students and non-specialists, and serves as a reference work for researchers in this exciting new field.

Gaseous nebulae offer outstanding opportunities to atomic physicists, spectroscopists, plasma experts, and to observers and theoreticians alike for the study of attenuated ionized gases. These nebulae are often dusty, heated by radiation fields and by shocks. They are short-lived phenomena on the scale of a stellar lifetime, but their chemical compositions and internal kinematics may give important clues to advanced stages of stellar evolution. The material herein presented is based on lectures given at the University of Michigan, University of Queensland, University of California, Los Angeles, and in more abbreviated form at the Raman Institute, at the Scuola Internazionale di Trieste, and elsewhere. Much of it is derived origionally from the series "Physical Processes in Gaseous Nebulae" initiated at the Harvard College Observatory in the late 1930s. I have tried to emphasize the basic physics of the mechanisms involved and mention some of the uncertainties that underlie calculations of many basic parameters. Emphasis is placed on ionized plasmas with electron temperatures typically in the neighborhood of 10,OOOoK. Dust and other ingredients of the cold component of the interstellar medium are treated briefly from the point of view of their relation to hot plasmas of H II regions and planetaries. Chemical composition determinations for nebulae are discussed in some detail while the last section deals with interpretations of elemental abundances in the framework of stellar evolution and nucleogenesis. Gaseous nebulae offer some particularly engaging opportunities for studies of stellar evolution.