A 50-million-light-year-long flow of hydrogen gas has been observed linking two tiny galaxies 50 million light-years away, giving astronomers a spectacular view of how galaxies attract, warp, and change each other over billions of years.
The enormous structure stretches around 185,000 light-years between galaxies NGC 4532 and DDO 137. A newly seen tail of gas extends even further—one massive 1.6 million light-years—an all-time high. Both were found by researchers at The University of Western Australia node of the International Centre for Radio Astronomy Research, using one of the world’s most sensitive radio telescopes.
The discovery is part of the Widefield ASKAP L-band Legacy All-sky Survey, or WALLABY, which uses CSIRO’s ASKAP radio telescope in Western Australia. The project is intended to map hydrogen, the universe’s most abundant element, and study how it interacts within galaxies.
Hydrogen is not only star material but also raw fuel that decides whether galaxies expand or perish. If it is pulled, stripped, or heated, the destinies of star formation can be rewritten in whole.
It’s like capturing galaxies during the act of talking with each other,” said Professor Lister Staveley-Smith. “Our modeling suggested tidal forces between these galaxies and their nearness to the giant Virgo cluster were a key element of gas dynamics we were seeing.”
So what type of forces could construct such enormous buildings? The team blames two. The first are tidal forces—gravity’s constant pull as the galaxies swing around each other. The second are ram pressure, which happens when galaxies collide with hot, thin gas that fills space within and around galaxy clusters.
The conditions in the Virgo cluster are extreme. The gas is about 200 times hotter than the surface of the Sun. When NGC 4532 and DDO 137 entered here, their own gas began to strip off, like a storm stripping paint.
“The process resembles atmospheric burn-up when a satellite enters the Earth’s atmosphere, but over a billion years,” Staveley-Smith said.
By comparing the electron density throughout the area and the velocity of the galaxies, scientists made sure enough material was being stripped off to explain both the bridge and the enormous tail.
The photo is distant, but the implication is closer to home. Astronomers point out that the Milky Way also possesses its own satellite galaxies—the Magellanic Clouds—bridged by just such hydrogen gas bridges.
These kinds of structures tell us about how our galaxy is interacting with those around it,” said co-author Professor Kenji Bekki. “Neutral hydrogen plays a central part in starbirth, so seeing how it’s remixed is crucial to understanding how galaxies are formed.”.
The new system provides a natural laboratory to test theories of gas flowing into and out of galaxies, how gas is heated or churning into the intergalactic medium, and how these contribute to star formation.
Astronomers are currently utilizing WALLABY’s large scale to look for other, more subtle connections between galaxies. Each find adds another piece to the cosmic jigsaw puzzle of how galaxies come together and die.
In crowded regions like galaxy clusters, life is particularly turbulent. Galaxies are not solitary islands but dancers in an eternal cosmic waltz. They draw gas from each other, throw it out into the surrounding environment, and sometimes recover it. This “breathing” of galaxies over cosmic time scales dictates whether new generations of stars shine or galaxies fade into obscurity over time.
The researchers’ observation highlights the importance of surveys that combine radio and optical data over large areas. Mapping the way hydrogen flows across millions of light-years constructs a history of collisions between galaxies inscribed in the record of the most diffuse gas.
As Staveley-Smith portrayed, “Understanding these bridges and their dynamics gives us an important insight into how galaxies evolve over time, how galactic gas is cycled and the conditions under which stars do or don’t form.”
By following the trails of hydrogen gas on these scales, astronomers are better able to understand how galaxies exist, change over time, and eventually die. This data is incorporated in computer simulations of how the universe is developing so that astronomers can test theories about the creation of stars and how galactic structures form.
Closer to home, such observations can refine our understanding of the past and future of the Milky Way. Because our galaxy also has encounters with small neighbors, insights from distant systems provide a template for what might occur in our own galactic neighborhood.
In a more general sense, knowing how material for stars is distributed makes possible the existence of planets—and of life itself.
Research findings are available online in the journal Monthly Notices of the Royal Astronomical Society.
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