Spiral galaxies like our Milky Way continuously form new stars. These form from the hydrogen gas that is also present in the galaxy. Small gas clouds can collapse due to their own gravity and eventually ignite as new stars. However, if there is no new gas coming into the galaxies, star formation will stop and they will fade to become “dead” galaxies. By measuring how many stars are formed per year and how much gas there still is, we can work out that for nearby spiral galaxies the gas supply will run out in one or two billion years. But spiral galaxies are not “dead”: we know that they have been forming stars for almost the entire lifetime of the universe, often at a much higher rate, so they must have an external supply of gas.
It turns out that there is a large amount of hot gas between galaxies that could be accreted by the galaxies if it cools enough to be captured. The hot gas is difficult to observe, but as the gas cools it should become visible as neutral hydrogen and be observable with radio telescopes. Many attempts have been made over the years to detect this accreting neutral hydrogen, but no unambiguous detection has ever been made.
Does this mean our understanding of how galaxies evolve is wrong? Computer simulations suggest that the gas is in extremely diffuse filaments surrounding the target galaxies and could not have been observed with the previous generation of radio telescopes. The new MeerKAT radio telescope in South Africa will change this. It is much more sensitive and can produce radio images of a much higher quality.
We are observing 30 nearby galaxies with MeerKAT and will produce radio images that are almost a hundred times more sensitive than any previous observations of these galaxies.
Over the last two years we have been processing our data. We also have been working on computer algorithms to analyse these data as the extreme sensitivity of the new observations demands novel high-precision algorithms. In the last few months we have produced our first full-sensitivity images and we are now studying them, looking for signs of the very diffuse gas that is predicted to be present around the galaxies. In the next phase of this project we will confirm or rule out the existence of these accreting gas filaments. This will eventually lead to a much better understanding of how galaxies get their gas, and form a fundamental component in improving our understanding of how galaxies and the universe evolve.
