The vertical extent and kinematics of the HI in NGC 2403

TL;DR

This study investigates the vertical structure and kinematics of neutral hydrogen in NGC 2403, suggesting a two-component model with a thin disk and a thick, slowly rotating HI layer, impacting our understanding of galaxy disk-halo interactions.

Contribution

It introduces a two-component model for HI distribution in NGC 2403, highlighting the presence of a thick, slowly rotating HI layer and its implications for disk-halo connection.

Findings

A two-component HI structure fits observations better than a single thick disk.

The HI layer shows a decrease in rotational velocity with height.

A vertically extended HI layer may be common in star-forming spiral galaxies.

Abstract

The neutral hydrogen line profiles along the major axis of the nearby spiral galaxy NGC 2403 show a wing towards the sytemic velocity. This asymmetry can be explained with the presence of an abnormally thick HI disk (FWHM ~ 5 kpc) or with a two-component structure: a thin disk and a slowly rotating, thicker (1-3 kpc) HI layer. The latter model gives a better representation of the observations. These results throw a new light on the disk-halo connection. In particular, the decrease of rotational velocity with height above the plane may be the result of a galactic fountain flow. A vertically extended, slowly rotating HI layer may be common among spiral galaxies with high levels of star formation.