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

## TL;DR

This study investigates the properties of the most HI-rich galaxies, revealing that their high gas content is due to continuous gas accretion and high angular momentum, which leads to inefficient star formation despite similarities to average galaxies.

## Contribution

The paper introduces the HIX galaxy survey focusing on the most HI-rich galaxies and compares their properties to typical galaxies, highlighting the role of angular momentum in star formation efficiency.

## Key findings

- HIX galaxies have similar stellar properties to control galaxies.
- HIX galaxies are less efficient at forming stars.
- High angular momentum explains the HI excess and star formation inefficiency.

## Abstract

We present the HI eXtreme (HIX) galaxy survey targeting some of the most HI rich galaxies in the southern hemisphere. The 13 HIX galaxies have been selected to host the most massive HI discs at a given stellar luminosity. We compare these galaxies to a control sample of average galaxies detected in the HI Parkes All Sky Survey (HIPASS, Barnes et al. 2001). 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 HI massive galaxy in our sample (ESO075-G006, $\rm log\ M_{HI}\ [M_{\odot}] = 10.8$) the kinematic properties are the reason for inefficient star formation and HI excess. Examining the Australian Telescope Compact Array (ATCA) HI 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 fit to the HI disc together with the gas-phase oxygen abundance distribution supports the scenario that gas has been constantly accreted onto ESO07-G006 but the high specific angular momentum makes ESO075-G006 very inefficient in forming stars. Thus a massive HI disc has been built up.