High Velocity Clouds

The Galactic halo is teeming with cold clouds of neutral hydrogen, which are known as high velocity clouds (HVCs) because of their inability to fit under any standard model of Galactic rotation. The origin and role of HVCs constitutes one of the major unsolved astrophysical puzzles. HVCs are suspected to provide the gaseous fuel necessary to keep the star formation going in our Galaxy, and they have long been sought after as the probable solution to the G-dwarf problem, or the fact that the metallicity distribution of the long-lived stars in our Galaxy cannot be easily explained without a continuous source of infalling low metallicity fuel.
HVCs are moving through the more diffuse, hot halo medium, as evident from their structure, and from detections of OVI absorption indicating an interaction between HVCs and this medium. The clouds are disrupted by this interaction and form a head-tail structure, i.e. the head of the cloud is compressed around a cold core, and a warmer, diffuse tail extends behind the cloud (see Figure below). It is a matter of debate whether HVCs survive the disruptive instabilities long enough to impinge on the Galactic disk as a ``cold cosmic rain'', implying a very localized fuel source, or whether they are being disrupted before they reach the disk, resulting in a ``warm cosmic rain''. In the latter case, the cloud would be integrated in the warm ionized thick disk of our Galaxy. While also available for feeding star formation, they would be much more easily mixed with higher-metallicity material.
Model HVC: column, centroid velocity and velocity dispersion.
Various measures of a model HVC, approaching the "observer" at an angle of 45 degrees. Top left to right: Column density, centroid velocity, and velocity dispersion. Bottom row: position-velocity plots for angles of 0, 45 and 90 degrees. The pv-plots are constructed from spectra taking along the long axis of the cloud (Heitsch+ 16).
Column density evolution of a HVC.
 

Star Formation and Magnetic Fields

Do magnetic fields play any role in star formation? The question (and its wide variety of answers) is highly contentious. Traditional models of star formation ascribe a dominant role to magnetic fields -- they are thought to prevent the wholesale collapse of parental molecular clouds, with stars forming due to the effects of ambipolar diffusion, or the decoupling between neutrals and charged particles. Observationally (see here) there is strong evidence that magnetic fields dominate the gas flows in the diffuse interstellar gas, while the dense gas is mostly dominated by gravity. That doesn't mean that magnetic fields don't affect the gas dynamics at all: see the two animations. The upper one shows the fragmentation and collapse of a molecular cloud without magnetic fields, and the lower one is exactly the same simulation, except that we add a magnetic field at 20% of the gravitational energy. The red dots appearing after a while are "sink particles", or structures that are collapsed so far that the code cannot resolve them any more. Once that threshold is reached, the gas is replaced by a particle, which can further accrete.
One key problem is to estimate the strength of magnetic fields. Line-of-sight components can be measured via the Zeeman effect (see above reference), while plane-of-sky components can be estimated by the Davis-Chandrasekhar-Fermi method (see here). The DCF method suffers from a few systematic effects, which can be assessed via simulations.
Column density animation of collapsing cloud. Without magnetic fields.
Column density animation of collapsing cloud. With magnetic fields.