| Description: | The WSRT Hydrogen Accretion in LOcal GAlaxieS (HALOGAS) Survey is a long-running project with the primary aim of searching for extraplanar neutral hydrogen (HI) gas in nearby spiral galaxies. Using very deep WSRT observations, we are able to produce extremely detailed models of the structure and kinematics of the HI gas in the HALOGAS galaxies. In some cases, like NGC 4244 and NGC 4565, no evidence is found for significant amounts of extraplanar HI. These cases are very important because they help us understand the conditions under which galaxies do or do not support a thick gas disk.
On the other hand, in some HALOGAS galaxies we do find evidence for substantial extraplanar gas. In a recently accepted paper led by Gianfranco Gentile (University of Ghent) and including several collaborators (many from ASTRON), we have presented a new and very detailed investigation of NGC 3198. This galaxy is literally the textbook example of a galaxy with a flat rotation curve - one of the key pieces of evidence for the presence of dark matter. Our HALOGAS observations are shown in today's image, as white contours on top of high-quality optical images obtained by our team at the Kitt Peak observatory in Arizona. The HI data reveal the presence of a vertically thick, slowly rotating HI disk. This is illustrated in the bottom panels of the image. These are "position-velocity" diagrams, which show the change of the HI spectrum along the major axis of the galaxy. A comparison between the kinematic structure of the data (black contours) and two models is shown. The red contours represent a thin, normally rotating disk, and clearly do not match the data. The green contours show how well the data are reproduced by adding a thickened, slowly rotating HI disk to the model. This extraplanar gas contributes 15% of the total HI mass of the galaxy! |
