The HIX galaxy survey I: Study of the most gas-rich galaxies from HIPASS

We present the H I eXtreme (HIX) galaxy survey targeting some of the most H I rich galaxies in the Southern hemisphere. The 13 HIX galaxies have been selected to host the most massive H I discs at a given stellar luminosity. We compare these galaxies to a control sample of average galaxies detected in the H I Parkes All Sky Survey (HIPASS). As the control sample is matched in stellar luminosity, we find that the stellar properties of HIX galaxies are similar to the control sample. Furthermore, the specific star formation rate and optical morphology do not differ between HIX and control galaxies. We find, however, the HIX galaxies to be less efficient in forming stars. For the most H I massive galaxy in our sample (ESO075-G006, log MH I [M☉] = (10.8 ± 0.1)), the kinematic properties are the reason for inefficient star formation and H I excess. Examining the Australian Telescope Compact Array (ATCA) H I imaging and Wide Field Spectrograph (WiFeS) optical spectra of ESO075-G006 reveals an undisturbed galaxy without evidence for recent major, violent accretion events. A tilted ring fitted to the H I disc together with the gas-phase oxygen abundance distribution supports the scenario that gas has been constantly accreted on to ESO075-G006 but the high specific angular momentum makes ESO075-G006 very inefficient in forming stars. Thus, a massive H I disc has been built up.