The HIX galaxy survey investigated galaxies that host at least 2.5 times more atomic hydrogen than expected from their optical properties. By comparing the properties of HIX galaxies to a control sample and semi-analytic models, we found that these galaxies are able to host their gigantic HI discs thanks to their large specific angular momentum. They likely inherited their kinematic properties from their high spin host halos. Results are published in Lutz et al. ( 2017, 2018, 2020).
IntroductionGalaxies 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:
- What determines the amount of gas and dust in galaxies?
- Why are some galaxies outliers to scaling relations?
- What role do (gas) kinematics play in galaxy evolution?
- How do galaxies replenish their gas reservoir to be able to sustain star formation in the future?
- What mechanisms influence the star formation process?