As part of the Widefield ASKAP L-band Legacy All-sky Survey (WALLABY), an international team of astronomers has observed the dwarf galaxy pair NGC 4532 / DDO 137 (WALLABY J123424+062511), which is located around 53 million light-years from Earth. The observations have revealed a huge (185,000 light-years) bridge of neutral hydrogen between the two galaxies, as well as numerous arms and clouds that connect with the even longer (1.6 million light-years) tail of gas previously discovered with the Arecibo telescope.
Radio (left) and optical (right) images of NGC 4532 / DDO 137. Image credit: ICRAR / D. Lang, Perimeter Institute.
“The discovery marked a significant step forward in understanding how galaxies interact,” said Professor Lister Staveley-Smith, an astronomer with the University of Western Australia node at the International Centre for Radio Astronomy Research (ICRAR).
“Our modeling showed that the tidal forces acting between these galaxies, alongside their proximity to the massive Virgo Cluster of galaxies, played a crucial role in the gas dynamics we observed.”
“As the galaxies rotated around each other and moved toward the hot gas cloud surrounding the Virgo Cluster, which was 200 times hotter than the Sun’s surface, they experienced what is known as ram pressure, which stripped and heated the gas from the galaxies.”
“The process is akin to atmospheric burn-up when a satellite re-enters the Earth’s upper atmosphere, but has extended over a period of a billion years.”
“The density of electrons and the speed at which galaxies are falling into the hot gas cloud are enough to explain why so much gas has been pulled away from the galaxies and into the bridge and surrounding areas.”
The observations were part of the WALLABY survey, a large-scale project that maps the sky and studies the distribution of hydrogen gas in galaxies, using CSIRO’s ASKAP radio telescope.
“Neutral hydrogen plays a crucial role in the formation of stars, making this finding fundamental to understanding how galaxies interact and evolve, particularly in dense environments,” said ICRAR Professor Kenji Bekki.
“The system had strong similarities with our own Milky Way and Magellanic System, providing a unique opportunity to study such interactions in detail,” Professor Staveley-Smith said.
“Understanding these gas bridges and their dynamics provides critical insights into how galaxies evolve over time, how galactic gas is redistributed, and the varying conditions under which galaxies may or may not form stars.”
“This contributes to our broader understanding of the most massive structures in the Universe and their life cycles, which helps us grasp more about their vast complexities and history of star formation.”
The results appear in the Monthly Notices of the Royal Astronomical Society.
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L. Staveley-Smith et al. 2025. WALLABY pilot survey: the extensive interaction of NGC 4532 and DDO 137 with the Virgo cluster. MNRAS 543 (1): 526-539; doi: 10.1093/mnras/staf1443