HALOGAS: the properties of extraplanar HI in disc galaxies

¹ ASTRON, Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
² Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV Groningen, The Netherlands
e-mail: marasco@astro.rug.nl
³ Department of Physics and Astronomy, University of Bologna, Via P. Gobetti 93/2, 40129 Bologna, Italy
⁴ CSIRO Astronomy and Space Science, PO Box 1130, Bentley, WA 6102, Australia
⁵ Dept. of Astronomy, Univ. of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
⁶ Ruhr-University Bochum, Faculty of Physics and Astronomy, Astronomical Institute, 44780 Bochum, Germany
⁷ Department of Physics and Electronics, Rhodes University, PO Box 94, Makhanda 6140, South Africa
⁸ South African Radio Astronomy Observatory, 2 Fir Street, Black River, Park Observatory, Cape Town 7405, South Africa
⁹ Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
¹⁰ Department of Astronomy and Steward Observatory, University of Arizona, Tucson, AZ, USA
¹¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹² Department of Astronomy, New Mexico State University, Las Cruces, NM 88001, USA

Received: 17 July 2019
Accepted: 6 September 2019

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.