Counter-Rotating and Lagging Extra-planar HI in NGC 4559

We use new deep 21 cm HI observations of NGC 4559 in the Westerbork Hydrogen Accretion in LOcal GAlaxies (HALOGAS) survey to investigate the properties of extra-planar gas associated with this moderately inclined galaxy building upon a previous study by Barbieri et al. (2005). We use the Tilted Ring Fitting Code (TiRiFiC) to construct high-resolution simulated data cubes to match the HI observations. We find that the data cannot be adequately reproduced without a thick disk component of scale height ˜2 kpc, characterized by a negative vertical gradient in its rotation velocity (lag), suggesting the presence of a non-trivial amount of extra-planar gas. The values of lag in the approaching and receding halves of the galaxy were fit independently and are -13±5 and -7± 3 km s-1 kpc-1, respectively. We quantify the amount of extra-planar gas using the HI line profile in the data cube, then extract that gas from each HI line profile. We find that of the ˜4.5×109 M⊙ total HI mass, ˜4.0×108 M⊙ of that HI is not a part of the traditionally rotating disk, and is likely extra-planar. The spatial extent of the extra-planar gas is coincident upon the star-forming disk of the galaxy and is likely of galactic fountain mechanism origin. We use ancillary Hα imaging to estimate the star formation properties in quadrants of the galaxy, finding remarkably uniform star formation rates. An emission feature located in the kinematically 'forbidden' region of the position-velocity diagram along the major axis is found to contain ˜1.4×106 M⊙ of HI. We explore the possible origins of this counter-rotating feature and its potential connection to a large nearby HI hole, which would require ˜107 M⊙ to fill