HALOGAS: The properties of extraplanar HI in disc galaxies

Abstract

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 H I data from the Hydrogen Accretion in LOcal GAlaxieS (HALOGAS) survey. For each system we masked the H I emission coming from the regularly rotating disc and used synthetic datacubes to model the leftover "anomalous" H I flux. Our model consists of a smooth, axisymmetric thick component described by three structural and four kinematical parameters, which are fit to the data via a Markov chain Monte Carlo (MCMC) based Bayesian method. We find that extraplanar H I 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 H I mass with a mean value of 14%, and has a typical thickness of a few kpc which is 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 ∼ - 10 km s-1 kpc-1, a velocity dispersion of 15-30 km s-1, and, for most galaxies, a global inflow in both the vertical and radial directions with speeds of 20-30 km s-1. The EPG H I masses are in excellent agreement with predictions from simple models of the galactic fountain that are 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.