A vast river of neutral hydrogen bridges two galaxies that are beginning to enter the monstrous Virgo Cluster, and this structure is helping astronomers better understand our Milky Way’s relationship with the nearby Magellanic Clouds.
The two galaxies, named NGC 4532 and DDO 137, are in fact falling into the Virgo Cluster at 547 miles (880 kilometers) per second, and in doing so are plowing through a vast cloud of gas that surrounds the cluster. For the galaxies, this is like wading through hot treacle that scours their leading edges, heating and tearing out hydrogen gas that then forms a 1.6-million-light-year-long tail.
This tail was previously identified by the Arecibo radio telescope in Puerto Rico. Now, astronomers targeting the two galaxies with the Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope have discovered a colossal bridge of gas crossing the 185,000 light-years between the two galaxies, as well as additional spurs and clouds of hydrogen around the pair and connecting to the huge tail.
According to astronomer Lister Staveley-Smith, who led the research from the University of Western Australia node of the International Centre for Radio Astronomy Research, the vast structures are the consequence of two phenomena.
“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,” said Staveley-Smith in a statement.
The 1.6-million-light-year-long tail is the product of ram-pressure stripping as the pair of galaxies, which are 53 million light-years away from Earth, are drawn towards the Virgo Cluster.
“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,” said Staveley-Smith.
For the pair of galaxies, the hot gas cloud of ionized gas, filled with free electrons and which stretches for millions of light years around the cluster, is like tumbling into a vat of treacle that then begins to scour the gas on the leading edge of the infalling galaxies, ablating it.
“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,” said Staveley-Smith.
The bridge between the two galaxies has a different origin. Here, gravitational tides between the galaxies pull gas out of each other, forming the bridge and associated spurs. We see a very similar phenomenon much closer to home, in the form of the Magellanic Stream of neutral hydrogen gas that stretches between the Large and Small Magellanic Clouds and onto the Milky Way.
These two dwarf galaxies orbit the Milky Way at distances of 163,000 and 200,000 light-years, respectively, moving through our galaxy’s outer halo of hot gas not too dissimilar, but on a much smaller scale, from the Virgo Cluster’s halo of hot gas. Stars are seen to be forming within the Magellanic Stream, and similar bridges and tidal tails are known to be hot-spots of star formation elsewhere in the universe.
“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,” said Staveley-Smith. “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 observations of NGC 4532 and DDO 137 were made as part of a pilot study for a new survey, the Widefield ASKAP L-Band Legacy All-sky Survey (WALLABY), which aims to study neutral hydrogen in galaxies all across the sky to better understand how the cosmic forces of gravity and pressure shape that gas, and what role it plays in star formation.
