# HALOGAS Observations of NGC 4559: Anomalous and Extra-planar HI and its Relation to Star Formation

## TL;DR

This study uses deep HI observations and modeling to analyze extra-planar gas in NGC 4559, revealing a thick disk with a velocity lag, its association with star formation, and discovering a merging dwarf galaxy.

## Contribution

It provides detailed modeling of the extra-planar HI in NGC 4559, linking it to star formation processes and identifying a new merging dwarf galaxy.

## Key findings

- Approximately 10-20% of HI is extra-planar.
- Extra-planar gas correlates with star-forming regions.
- Discovered a merging dwarf galaxy near NGC 4559.

## Abstract

We use new deep 21 cm HI observations of the moderately inclined galaxy NGC 4559 in the HALOGAS survey to investigate the properties of extra-planar gas. We use TiRiFiC to construct simulated data cubes to match the HI observations. We find that a thick disk component of scale height $\sim\,2\,\mathrm{kpc}$, characterized by a negative vertical gradient in its rotation velocity (lag) of $\sim13 \pm 5$ km s$^{-1}$ kpc$^{-1}$ is an adequate fit to extra-planar gas features. The tilted ring models also present evidence for a decrease in the magnitude of the lag outside of $R_{25}$, and a radial inflow of $\sim 10$ km s$^{-1}$. We extracted lagging extra-planar gas through Gaussian velocity profile fitting. From both the 3D models and and extraction analyses we conclude that $\sim10-20\%$ of the total {\HI} mass is extra-planar. Most of the extra-planar gas is spatially coincident with regions of star formation in spiral arms, as traced by H$\alpha$ and GALEX FUV images, so it is likely due to star formation processes driving a galactic fountain. We also find the signature of a filament of a kinematically "forbidden" HI, containing $\sim 1.4\times 10^{6}$ M$_{\odot}$ of HI, and discuss its potential relationship to a nearby HI hole. We discover a previously undetected dwarf galaxy in HI located $\sim 0.4^{\circ}$ ($\sim 58$ kpc) from the center of NGC 4559, containing $\sim 4\times10^{5}$ M$_{\odot}$. This dwarf has counterpart sources in SDSS with spectra typical of HII regions, and we conclude it is two merging blue compact dwarf galaxies.