Staveley-Smith et al. report pilot Widefield ASKAP L-band LegacyAll-sky Survey (WALLABY) neutral atomic hydrogen (HI) observations observations of the dwarf galaxy pair NGC 4532/DDO 137. NGC 4532 (which is listed in the New General Catalogue) and DDO 137 (which appears in a catalogue of dwarf galaxies compiled by the David Dunlop Observatory) are a pair of irregular/Magellanic-type galaxies lying in the Virgo cluster southern extension, one of a number of filaments which lead to the massive Virgo cluster of ~2000 galaxies. The WALLABY observations have revealed a huge (150,000 light year) bridge of gas between the two galaxies, as well as numerous arms and clouds which connect with the even longer (1.6 million light year) tail of gas previously discovered with the Arecibo telescope.

The team’s modelling suggests that a combination of ram pressure and tidal forces are responsible for the nature of the system. Ram pressure due to infall through an extensive envelope of hot gas around the cluster is most likely responsible for the HI tail. The ongoing tidal interaction with the Virgo cluster appears to have prevented a rapid merger of the binary pair, with the mutual tidal interaction between the galaxy pair being responsible for raising gas from the outer parts of the galaxy potential wells into the HI bridge and common envelope. The NGC 4532/DDO 137 system — including the bridge and tail — mirrors many of the physical features of the Magellanic System, and may lead to a better understanding of that system, as well as casting more light on the relative importance of interaction mechanisms in the outskirts of dynamically young galaxy clusters such as the Virgo cluster. The team’s results are also summarised in this article in The Conversation. The image above shows the contours of the WALLABY HI column density for the NGC 4532/DDO 137 system overlaid on a DESI Legacy DR9 grz cutout (Image credit: Legacy Surveys/D. Lang, Perimeter Institute).