2014MNRAS.437.3736E

A study is presented of Hi line observations of the nearby spiral galaxy NGC 3521 observed with the Very Large Array as part of The Hi Nearby Galaxy Survey. Clearly evident in the Hi data cube is the presence of an anomalous Hi component that is both diffuse and slow rotating. The data cube is dynamically decomposed into regular and anomalous Hi components. A mass of M_H I=1.5×10^9M_☉ is estimated for the anomalous Hi - 20 percent of the total Hi mass. Standard Hi data products and rotation curves are produced for each dynamical component. In terms of circular rotation speed, the anomalous Hi is found to lag the regular Hi by ∼ 25-125 km/s. Three-dimensional models are generated and used to determine the possible location of the anomalous Hi. The results strongly suggest it to be distributed in a thick disc with a scale-height of a few kpc ( ∼ 3.5kpc). It is concluded that the anomalous Hi in NGC 3521 constitutes a slow-rotating halo gas component, consistent with similar findings for other nearby galaxies. A study of the radial distribution of the anomalous Hi shows it to be spatially coincident with the inner regions of the stellar disc where the star formation rate is highest. It is most likely a galactic fountain that has deposited gas from the disc of the galaxy into the halo.