HALOGAS: HI Observations and Modeling of the Nearby Edge-on Spiral Galaxy NGC 4565

TL;DR

This study presents detailed 21-cm HI observations and models of NGC 4565, revealing a warped, asymmetric disk with a vertical rotational lag, but no significant extended HI halo, and discusses implications for galaxy dynamics and halo presence.

Contribution

First detailed HI modeling of NGC 4565 showing a warped disk, radial motions, and a vertical rotational lag without evidence of a massive HI halo.

Findings

NGC 4565 has a warped, asymmetric, and flaring HI disk.

A vertical rotational lag of -40 to -30 km/s/kpc is observed within inner regions.

No evidence found for a massive, extended HI halo around NGC 4565.

Abstract

We present 21-cm observations and models of the neutral hydrogen in NGC 4565, a nearby, edge-on spiral galaxy, as part of the Westerbork Hydrogen Accretion in LOcal GAlaxieS (HALOGAS) survey. These models provide insight concerning both the morphology and kinematics of HI above, as well as within, the disk. NGC 4565 exhibits a distinctly warped and asymmetric disk with a flaring layer. Our modeling provides no evidence for a massive, extended HI halo. We see evidence for a bar and associated radial motions. Additionally, there are indications of radial motions within the disk, possibly associated with a ring of higher density. We see a substantial decrease in rotational velocity with height above the plane of the disk (a lag) of -40 +5/-20 km/s/kpc and -30 +5/-30 km s/kpc in the approaching and receding halves, respectively. This lag is only seen within the inner ~4.75' (14.9 kpc) on the approaching half and ~4.25' (13.4 kpc) on the receding, making this a radially shallowing lag, which is now seen in the HI layers of several galaxies. When comparing results for NGC 4565 and those for other galaxies, there are tentative indications of high star formation rate per unit area being associated with the presence of a halo. Finally, HI is found in two companion galaxies, one of which is clearly interacting with NGC 4565.