The Physical Properties of Intermediate-Velocity Gas at High Galactic Latitudes

Abstract

The accretion of matter onto galactic disks, the formation of molecular clouds, and the formation of stars are of major importance for the evolution of galaxies over cosmic times. During their lifetimes, high-mass stars inject large amounts of energy and momentum into the interstellar medium (ISM) of galaxies, driving galactic fountains and outflows of material into their halos. Such extra-planar gas clouds, the intermediate-velocity clouds (IVCs), have been discovered in spectroscopic surveys of atomic hydrogen (HI) in our Galaxy. IVCs are thought to be related to a Galactic fountain process. <br /> In this thesis we present an observational study of Galactic IVCs and their molecular content. By the combination of HI and far-infrared (FIR) dust emission, we infer the distribution of molecular material within IVCs. A detailed study of two individual IVCs and their transition from atomic to molecular gas is conducted and the first global analysis of molecular gas within IVCs is performed. New all-sky HI and FIR surveys constitute an exceptional database for such a study of the Galactic ISM. <br /> The two particular IVCs of interest approach the Galactic disk and are indistinguishable as measured by the Effelsberg-Bonn HI Survey (EBHIS). However, the one is a rare molecular IVC, while the other is a purely atomic IVC. Because of their similarities and differences, we suggest that the two objects are at different stages in the transition from atomic to molecular clouds at the disk-halo interface. This transition is thought to be driven by the bulk motion of the clouds through the surrounding medium, which causes ram pressure that perturbs and condenses the infalling IVCs. At the leading front the pressure is enhanced, which accelerates the formation of molecular hydrogen. These processes are not apparent in EBHIS data suggesting that the accumulation of gas and the H₂ formation occur on angular scales well below the angular resolution limit of EBHIS.<br /> We conducted dedicated high-resolution observations of the two IVCs using the Westerbork-Synthesis Radio Telescope (WSRT) for the HI and the IRAM 30m telescope for measurements of carbon monoxide ¹²CO(1-0) emission, the main tracer of molecular hydrogen. The atomic IVC is not detected in ¹²CO(1-0) emission and the HI gas appears to be diffusely and smoothly distributed, lacking high-column density cores. The molecular IVC harbours a rich atomic and molecular clumpy substructure. The atomic and molecular small-scale structures appear to be connected. Most of the ¹²CO(1-0) emission originates from the eastern edge of the molecular cloud, which we propose to be the leading front, where the strongest ram-pressure interactions are expected to occur.<br /> An analogy between infalling IVCs and turbulent colliding flows of atomic gas is put forward. The action of dynamical and thermal instabilities causes the formation of dense and cold cores, in which the formation of H₂ is much faster. Ram pressure is thought to move the warm neutral medium into the thermally unstable regime, from which it rapidly cools down to the thermally stable cold medium. In this sense the HI-H₂ transition within IVCs appears to be a natural step during the late phases of the Galactic fountain cycle in the disk-halo interface region.<br /> The first global analysis of the molecular content within the high-Galactic IVC sky is conducted towards the northern and southern Galactic hemispheres. In total 206 molecular IVC (MIVC) candidates are identified that include the previously known objects except for a single one. While on the northern Galactic hemisphere there are large IVC complexes, no similar IVC structures are detected south. The almost complete lack of MIVC candidates with positive radial velocities in the local standard of rest suggests different physical processes within out- and inflowing gas.<br /> The derived physical and kinematical properties of the IVC samples are consistent with the expectations from a Galactic fountain process. A large fraction of fountain ejecta within our Galaxy may not be observable as atomic IVC gas. Extrapolating from the local IVC population, the global Galactic fountain inflows may be an important contribution to the required accretion rate of the Milky Way.