Galaxies are vast systems of stars, gas, dust and dark matter. When studying their evolution, one of the important objects of investigation is the galactic gas - star formation cycle. In a simple picture of this cycle, the gas reservoir is made up of atomic hydrogen (HI). In particular in regions of high density and/or pressure, the HI condensates to form molecular hydrogen (H2), which occurs in giant molecular clouds (GMCs). These GMCs are the birthplaces of stars. After their formation, the stars go through their life cycle, towards the end of which they return part of their material back to the interstellar medium (ISM). The material expelled by stars, either in stellar winds or through supernovae, contains both hydrogen and metals. They cool and settle into the galaxy’s gas reservoir. The metals enrich the ISM and allow for dust growth. In addition to this internal cycle of gas and stars, galaxies need to accrete gas from their environments to be able to sustain star formation in the long term. While the overall mechanisms are understood, there are still open questions:
The HIX galaxy survey
The HIX galaxy survey is examining HI eXtreme galaxies, which contain at least 2.5 times more atomic hydrogen (HI) than expected from their optical properties. We then observed these galaxies with the Australian Telescope Compact Array. In addition we compiled a control sample from archival observations with the ATCA and enriched the data set with photometry and images from WISE, 2MASS and GALEX. On the left side you see for all HIX galaxies, the HI contours overlaid on top of SuperCOSMOS Bj-band images.
Based on this data set we analysed the star formation activity and the gas kinematics of the HIX and control galaxies. We found that HIX galaxies form stars less efficiently from their HI reservoir than control galaxies. In other words their star formation rate is similar to that of the control galaxies despite the fact that they host a lot more HI. When studying the resolved HI maps of HIX galaxies the reason for this behaviour becomes clear: a large amount of the HI is located well outside the visible stellar disc. At these locations the gas can not form stars because the pressure and density is too small to form molecular gas and subsequently stars. The gas is further stabilised against star formation by the high specific angular momentum. When comparing to the Dark Sage semi analytic model, we find that there are modelled galaxies, which behave similar to our observed HIX galaxies. These modelled galaxies preferentially live in dark matter halos with higher than average spin (i.e. angular momentum). So to summarise our current picture of HIX galaxies is the following: they live in high spin dark matter halos, thus they are able to better support their HI disc against star formation and maintain gigantic HI discs.
We are now in the process of studying the metallicity, dust and molecular gas content of HIX galaxies. Our ALMA observations just got queued So stay tuned for more results.
The SHREG survey
For the Southern HI REsolved Galaxies survey, we used the programmatic
interface of the ATCA data archive to collect as many HI observations
of nearby galaxies as possible. We then reduced the data of these
observations and modelled the kinematics of galaxies detected within
the resulting data cubes. We are now in the process of investigating
an HI mass-angular momentum relation, how often HI discs are warped
and how we can model HI kinematics more efficiently (currently the
process needs a lot of human supervision).