Hi gas and stellar content of early-type galaxies

We present Australian telescope compact array observations of the neutral-hydrogen gas in 42 early-type galaxies detected by the HIPASS survey. We confirm the detection of 18 galaxies with Hi mass between a few times 109 and ∼1010M⊙ and column density below a few times 1020 cm−2. Most of the confirmed detections show disk- or ring-like Hi distribution with regular morphology and rotation. These Hi-rich galaxies could have formed via either a Hi-rich merger event or smooth accretion of gas from the intergalactic medium. As a part of a larger project, we present the results of optical spectroscopy and imaging and deeper 21 cm wavelength observations of one of the Hi-rich galaxies, IC 4200, performed in order to distinguish between the two formation processes. We find that the galaxy hosts 8.5 × 109M⊙ of Hi rotating on a ∼90° warped disk extended out to 60 kpc from the centre of the galaxy. Optical spectroscopy reveals a simple-stellar-population-equivalent age of 1.5 Gyr in the centre of the galaxy, and V- and R-band images show stellar shells. We interpret these observational results as evidence for a major merger origin of IC 4200, and date the merger back to 1-3 Gyr ago.