Hydrogen gas

Neutral hydrogen gas (HI for short) is an essential component of a galaxy as it is the raw fuel for star formation. If you have a galaxy with no HI gas, it can’t form any new stars and will become “red and dead.”

This page discusses my PhD research on HI (2009 – 2013). To read about my current HI work, see Galaxy evolution with MeerKAT.

Above: ‘Three Minute Thesis’ video explaining my PhD work for a general audience.

One challenge is to understand whether the total amount of HI in galaxies has changed over the history of the Universe and whether or not this can explain the drastic changes that have been observed in the star formation rates of galaxies.

HI gas emits light at a wavelength of 21cm, meaning we can detect it with radio telescopes. However, this signal is very weak, so we have a lot of difficulty detecting it much beyond the nearby Universe.

During my PhD, I worked on a strategy to detect this gas in galaxies that are much more distant – up to 1 billion light years away! I helped to develop a statistical detection technique called spectral stacking.

Usually you point a radio telescope at one galaxy for a very long time, in order to eventually pick up a faint detection of HI gas. However, we pointed the telescope at many different galaxies for a very short time each and didn’t actually detect HI in any of these galaxies. We then combined, or ‘stacked’ these undetected signals together in a way that gives a strong ‘average’ detection. So from seemingly nothing, you can actually get a lot of information about how much HI gas exists in distant populations of galaxies!

The telescope I used for most of this work was the Parkes 64m radio telescope – nicknamed ‘The Dish’ in New South Wales, Australia. During my PhD I proposed my own observations with The Dish and won 379 hours of time to use it for my project! So several weeks of my PhD were spent at Parkes learning how to use the gigantic telescope and then conducting more than 200 hours of observations. It was my tradition to watch the movie ‘The Dish’ on the first night of each observing run!

You can find my publication on HI stacking here.

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