HALOGAS: the properties of extraplanar HI in disc galaxies
  We present a systematic study of the extraplanar gas (EPG) in a sample of 15
nearby late-type galaxies at intermediate inclinations using publicly
available, deep interferometric HI data from the HALOGAS survey. For each
system we mask the HI emission coming from the regularly rotating disc and use
synthetic datacubes to model the leftover "anomalous" HI flux. Our model
consists of a smooth, axisymmetric thick component described by 3 structural
and 4 kinematical parameters, which are fit to the data via a Bayesian MCMC
approach. We find that extraplanar HI is nearly ubiquitous in disc galaxies, as
we fail to detect it in only two of the systems with the poorest spatial
resolution. The EPG component encloses ~5-25% of the total HI mass, with a mean
value of 14%, and has a typical thickness of a few kpc, incompatible with
expectations based on hydrostatic equilibrium models. The EPG kinematics is
remarkably similar throughout the sample, and consists of a lagging rotation
with typical vertical gradients of about -10 km/s/kpc, a velocity dispersion of
15-30 km/s and, for most galaxies, a global inflow in both the vertical and
radial directions with speeds of 20-30 km/s. The EPG HI masses are in excellent
agreement with predictions from simple models of the galactic fountain powered
by stellar feedback. The combined effect of photo-ionisation and interaction of
the fountain material with the circumgalactic medium can qualitatively explain
the kinematics of the EPG, but dynamical models of the galactic fountain are
required to fully test this framework.
