Abstract
ABSTRACT We present a kinematic study of ionized extraplanar gas in two low-inclination late-type galaxies (NGC 3982 and NGC 4152) using integral field spectroscopy data from the DiskMass H α sample. We first isolate the extraplanar gas emission by masking the H α flux from the regularly rotating disc. The extraplanar gas emission is then modelled in the 3D position–velocity domain using a parametric model described by three structural and four kinematic parameters. Best-fitting values for the model are determined via a Bayesian MCMC approach. The reliability and accuracy of our modelling method are carefully determined via tests using mock data. We detect ionized extraplanar gas in both galaxies, with scale heights $0.83^{+0.27}_{-0.40}\, \mathrm{kpc}$ (NGC 3982) and $1.87^{+0.43}_{-0.56}\, \mathrm{kpc}$ (NGC 4152) and flux fraction between the extraplanar gas and the regularly rotating gas within the disc of 27 and 15 per cent, respectively, consistent with previous determinations in other systems. We find lagging rotation of the ionized extraplanar gas in both galaxies, with vertical rotational gradients $-22.24^{+6.60}_{-13.13} \, \mathrm{km\, s^{-1}\, kpc^{-1}}$ and $-11.18^{+3.49}_{-4.06}\, \mathrm{km\, s^{-1}\, kpc^{-1}}$, respectively, and weak evidence for vertical and radial inflow in both galaxies. The above results are similar to the kinematics of the neutral extraplanar gas found in several galaxies, though this is the first time that 3D kinematic modelling of ionized extraplanar gas has been carried out. Our results are broadly consistent with a galactic fountain origin combined with gas accretion. However, a dynamical model is required to better understand the formation of ionized extraplanar gas.
Highlights
Spiralgalaxies are surrounded by multiphase gas layers extending up to several kpc from their disc planes, which are detected both in emission (H I and H α) and in low- and highions absorption against bright background sources (Lehner & Howk 2011; Zheng et al 2017)
We use the extraplanar gas (EPG) modelling code written by Marasco et al (2019) to study the EPG in NGC 3982 and NGC 4152, which was originally written with the aim of studying the neutral H I EPG in the HALOGAS survey (Marasco et al 2019) but in principle is capable of characterizing structure and kinematics of generic lineemitting medium outside the disc
Not comparable with the velocity resolution in the HALOGAS sample, we argue that an full width at halfmaximum (FWHM) of 30 km s−1 is sufficient for kinematic study of ionized gas, and mock galaxy tests in Section 4 verify that the code can model the EPG successfully under such velocity resolution
Summary
Spiralgalaxies are surrounded by multiphase gas layers extending up to several kpc from their disc planes (see Reynolds, Scherb & Roesler 1973; Lockman 2002; Marasco & Fraternali 2011 for observations of the Milky Way’s gas layers and Dettmar 1990; Rand, Kulkarni & Hester 1990; Swaters, Sancisi & van der Hulst 1997; Oosterloo, Fraternali & Sancisi 2007 for observations of other galaxies), which are detected both in emission (H I and H α) and in low- and highions absorption against bright background sources (Lehner & Howk 2011; Zheng et al 2017). A possible scenario is fountain-driven accretion, where interaction with the fountain’s neutral clouds decreases the cooling time of the hot coronal gas to a time-scale shorter than a cloud’s orbit time, and part of the hot gas is condensed and accreted on to the disc (Fraternali 2017) Models based on this scenario explain the EPG kinematics observed in external galaxies (Fraternali & Binney 2008) and the phase-space distribution of the cold (neutral) and warm (ionized) EPG in the Milky Way (Marasco, Fraternali & Binney 2012; Fraternali et al 2013; Marasco, Marinacci & Fraternali 2013).
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