In the last two decades, deep HI observations of nearby late-type galaxies have revealed the presence of extra-planar HI layers extending up to a few kpc above the galaxy midplane and accounting for ~10% of the total HI content. In the few cases studied in detail, these HI layers were found to be characterised by a slow-rotating, globally inflowing kinematics, which is expected by gas in a galactic fountain cycle triggered by stellar feedback.
We now present a homogenous and detailed analysis for a sample of 13 late-type galaxies with deep HI observations from the HALOGAS project. For each system we have masked out the HI emission coming from the rotating thin disk and produced synthetic data-cubes to model the leftover extra-planar emission. Our model features 3 structural and 4 kinematical global parameters, which are fit to the data via a Bayesian MCMC method.
We found that extra-planar HI layers are ubiquitous in disc galaxies, with HI masses that are in excellent agreement with predictions from simple models of galactic fountain powered by stellar feedback. In most cases, the kinematics show a global inflow with speed of 20-30 km/s in the vertical and radial directions, along with a vertical rotational lag of 5-20 km/s/kpc, suggesting an interaction between the material outflowing from the disc and the circumgalactic medium.