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 HALOGAS survey. For each system we mask the H I emission coming from the regularly rotating disc and use synthetic datacubes to model the leftover 'anomalous' H I 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 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, 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 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.

Tracing the Dynamical Mass in Galaxy Disks Using H i Velocity Dispersion and Its Implications for the Dark Matter Distribution in Galaxies

“…Extended low luminosity or dark matter disks are also important for the inside out growth of galaxy disks driven by gas accreted via galaxy tidal interactions, major mergers, minor mergers (Marasco et al 2019;Heald et al 2011), or from the intergalactic medium (IGM) from the filaments of the cosmic web (Noguchi 2018;Kleiner et al 2017;Dekel et al 2009;Kereš et al 2005). Whatever the accretion process, star formation can proceed only if the disk mass surface density Σ is high enough to allow local or large scale instabilities to develop.…”

The haloes and environments of nearby galaxies (HERON) – II. The outer structure of edge-on galaxies

“…Recently, Stein et al (2019) studied the radio halo of the edge-on spiral galaxy NGC 4013, along with radio scaleheights and the linear polarization. Other features, such as tidal streams and tails (Shang et al 1998;Martínez-Delgado et al 2009;Miskolczi et al 2011), galactic fountains (Bregman 1980;Bregman & Houck 1997;Putman et al 2012), and extraplanar gas (Lee & Irwin 1997;Sancisi et al 2008;Marasco et al 2019) and dust structures (Howk & Savage 1999;Shinn 2018) can be studied as well. In summary, edge-on galaxies come in a variety of flavours (Kautsch et al 2006) and this wealth of information, which can be obtained from an analysis of the vertical structure of edge-on galaxies, is extremely important for probing existing models of galaxy formation and evolution within the dominant ΛCDM paradigm.…”

MIGHTEE-HI: The H I emission project of the MeerKAT MIGHTEE survey

“…However, sufficiently sensitive H i data that allow us to study gas accretion, and galactic fountains in detail, are available only for a small, not entirely representative sample of galaxies and there are still several important open questions. The HALOGAS sample from Marasco et al (2019) only contains 15 galaxies and is the largest collection of homogeneously studied objects. The MIGHTEE-HI survey has the required sensitivity and resolution to increase the number of galaxies for which data are available.…”

“…The hatched region corresponds to the redshift range 0.09 < z < 0.22 most affected by RFI.ers (e.g. Di Teodoro & Fraternali 2014), hot-mode accretion (e.g Heald et al 2011;Marasco et al 2019),. ram-pressure stripping (e.g Jaffé et al 2015;Yoon et al 2017),.…”