The monthly media for February is from Dr Tristan Reynolds at our UWA node. Tristan brings us his recent paper in which he and his colleagues investigate how galaxies lose neutral hydrogen gas as they “fall” into galaxy clusters (groups of galaxies), and what affect that has on the formation of new stars.

Using the WALLABY survey, Tristan was able to use galaxy cluster images 5x larger than previous surveys and images could provide, for the same level of detail, making it easier to understand what happens as the galaxies fall into clusters.

Previous surveys of galaxies’ neutral hydrogen gas content required choosing between either small numbers of galaxies that could distinguish the locations of gas within galaxies, or large numbers of galaxies that could observe the total gas content but not the details of where the gas is located. WALLABY bridges this gap and spatially resolves the gas for large samples of galaxies.

In order to quantify the removal of gas within galaxies, they compared the sizes of each galaxy’s disc of gas to their optical disc (full of bright stars). As the galaxies fall into a cluster, the gas disc gets smaller relative to the optical disc, as the gas gets stripped away.

The below figure shows a large sample of galaxies (when looking at the neutral hydrogen gas) from the WALLABY observations, and the three types of galaxies of interest: cluster galaxies, galaxy’s falling into the cluster (infalling), and galaxies not in the cluster (field).

Filled circles indicate galaxies detected by the WALLABY survey (empty squares are known galaxies that the survey did not detect due to the majority of their gas being depleted from forming stars and/or previously removed by the environment or are too faint to be detected). The dotted grey circle indicates the distance from the centre of the cluster (blue dots) where the agreed “edge” of the cluster lies – known as the virial radius. The pink dots are galaxies falling into the cluster, and orange are field galaxies that are unaffected by the cluster. The virial radius is about the field of view of previous surveys of the same image detail.

The amount of gas in a galaxy is known to impact on the rate it forms stars, as it is out of compressed clouds of hydrogen gas that stars form. However, the gas within a galaxy is not evenly distributed. Tristan and his colleagues found that the gas being stripped from infalling galaxies was taken from the outskirts of the galaxy only, and that the formation of stars did not seem to be immediately affected (as most star formation occurs within the optical disc). It’s not until the galaxies get further into the cluster that star formation gets affected by the reduction in gas.

In the below figure, we can see that galaxies in the cluster have smaller gas discs (purple) relative to their optical discs (red) and that their star forming discs (blue dashed) have also shrunk due to gas being stripped from the outskirts during the infalling stage.

Top row: Field galaxies that have not been affected by a galaxy cluster. You can see for the most part both the gas discs (purple) and the star forming regions (blue dashed) are larger than the optical discs (red). Middle row: Galaxies falling in towards a cluster. You can see the gas discs (purple) have shrunk but are still larger than the optical discs (red), and the star formation discs (blue dashed) have also shrunk a little. Bottom row: Galaxies inside clusters. You can see both the gas discs (purple) and star formation rates (blue dashed) have shrunk again, and are comparable or smaller than the optical discs (red), respectively.

The WALLABY survey is transforming our view and understanding of the gas in galaxies and the processes affecting this star forming fuel, by providing the largest galaxy sample for which we can observe where the gas is located within the galaxies. In the future WALLABY will provide this information for thousands of galaxies.