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 $\sim\,2\,kpc$,
characterized by a negative vertical gradient in its rotation velocity (lag) of
$\sim13 5$ 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 $10$ 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 $\sim
1.410^6$ M$_ødot$ of HI, and discuss its potential relationship to
a nearby HI hole. We discover a previously undetected dwarf galaxy in HI
located $0.4^\circ$ ($58$ kpc) from the center of NGC 4559,
containing $4\times10^5$ M$_ødot$. This dwarf has counterpart sources
in SDSS with spectra typical of HII regions, and we conclude it is two merging
blue compact dwarf galaxies.
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