Cold and hot gas in the most HI deficient compact groups

Dense groups of galaxies should be prime sites for galaxy interactions and mergers, and their evolution has been the subject of controversy for 25 years. In order to explain the HI deficiency found in compact groups of galaxies we previously proposed an evolutionary scenario in which the amount of HI decreases with the secular evolutionary state of a group. But where does the HI go? One possibility is that the HI is heated or even shocked by the frequent galaxy interactions/collisions that occur in these dense environments. Here we present a comparison of the neutral gas distribution with the hot gas traced in X-rays. Surprisingly some of these groups do not show HI emission down to column densities as low as 10¹⁹ cm^-2. In some cases ram pressure stripping by a hot intragroup medim can explain the observed HI deficiency, while in others no extended X-ray emission is found. The star formation activity is found to be similar to isolated galaxies, hence gas consumption via current star formation would neither be a general cause of the HI deficiency in compact groups.