IC 4200: a gas-rich early-type galaxy formed via a major merger

We present the result of radio and optical observations of the S0 galaxy IC 4200. We observed the galaxy at the 21 cm wavelength with the Australian Telescope Compact Array, and we obtained optical spectroscopy and V- and R-band images with ESO/NTT/EMMI. Our aim is to determine the link between Hi and stellar content of IC 4200 and derive a coherent picture of its formation. We find that the galaxy hosts 8.5 x 10(9) M-circle dot of Hi rotating on a similar to 90 deg 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. Ionised gas is observed within the stellar body and is kinematically decoupled from the stars and characterised by LINER-like line ratios. 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.