HALOGAS: Extraplanar gas in NGC 3198

TL;DR

This paper reports the first detection of extraplanar gas in NGC 3198, modeling its properties and kinematics, and analyzing the galaxy's rotation curve within the MOND framework, revealing a thick HI disk with a lagging rotation.

Contribution

First detection and modeling of extraplanar HI gas in NGC 3198, including its morphology, kinematics, and implications for galaxy dynamics.

Findings

Extraplanar gas constitutes about 15% of total HI mass.

The thick disk has a scale height of ~3 kpc and a rotation lag of 7-15 km/s/kpc.

Rotation curve extended to larger radii and modeled with MOND.

Abstract

We present the analysis of new, deep HI observations of the spiral galaxy NGC 3198, as part of the HALOGAS (Westerbork Hydrogen Accretion in LOcal GAlaxieS) survey, with the main aim of investigating the presence, amount, morphology and kinematics of extraplanar gas. We present models of the HI observations of NGC 3198: the model that matches best the observed data cube features a thick disk with a scale height of ~3 kpc and an HI mass of about 15% of the total HI mass; this thick disk also has a decrease in rotation velocity as a function of height (lag) of 7-15 km/s/kpc (though with large uncertainties). This extraplanar gas is detected for the first time in NGC 3198. Radially, this gas appears to extend slightly beyond the actively star-forming body of the galaxy (as traced by the Halpha emission), but it is not more radially extended than the outer, fainter parts of the stellar disk. Compared to previous studies, thanks to the improved sensitivity we trace the rotation curve out to larger radii. We model the rotation curve in the framework of MOND (Modified Newtonian Dynamics) and we confirm that, with the allowed distance range we assumed, fit quality is modest in this galaxy, but the new outer parts are explained in a satisfactory way.