The H I cloud population in the lower halo of the Milky Way

To constrain the physical properties and distribution of the population of H I clouds in the lower halo of the MilkyWay galaxy, and to provide insight into their origin and nature, the Galactic All-Sky Survey was performed and used to identify and measure properties of H I halo clouds. Two regions of the Galaxy that are symmetricallylocated with respect to the line of sight to the Galactic centre were searched for H I halo clouds located near tangent points. The number of clouds detected within these regions are strikingly different. The physical properties of the clouds are similar, however, suggesting that they belong to the same population and may have originated from similar environments. The cloud-to-cloud velocity dispersions are also similar, despite a factor of two difference in their vertical scale heights. This suggests that the kinematics of the clouds are driven by the same physical processes in each quadrant and that the cloud-to-cloud velocity dispersions are not responsible for the heights the clouds reach. This large, homogeneously selected sample of halo clouds has allowed their spatial distribution to be determined for the first time and has revealed that they are strongly correlated with the spiral structure of the Galaxy. We propose a scenario where the H I halo clouds are related to areas of star formation in the form of superbubbles and gas that has been swept into the halo due to stellar feedback. This proposal was tested by performing three-dimensional hydrodynamic simulations of a superbubble within a realistic clumpy medium. The simulations revealed that H I clouds may form in the disk-halo interface due to the evolution of a superbubble, in the form of disk gas that has been swept into the halo from the walls of chimneys. The resulting clouds have diameters and heights similar to those of observed clouds, but are less dense. The large number of clouds detected in both regions suggest that the clouds are a major component of the Galaxy and would likely be detected throughout it. The H I halo clouds therefore play an important role in the circulation of gas between the disk and halo, and these features likely exist in many external galaxies.