Researchers studying a filament of hydrogen between the Andromeda and Triangulum galaxies found rotating clumps of gas the size of dwarf galaxies. But questions remain.
This combined graphic shows new, high-resolution GBT imaging of recently discovered hydrogen clouds between M31 (upper r.) and M33.
Bill Saxton, NRAO/AUI/NSF
EnlargeNew observations of a bridge of tenuous hydrogen gas stretching between two nearby galaxies may help solve a longstanding puzzle: Billions of years after star formation peaked in the universe, what continues to fuel the formation of new stars in spiral galaxies like the Milky Way?
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Newly published radiotelescope observations of this segment of what researchers have dubbed the ?cosmic web? reveal that about half of the neutral hydrogen gas in the bridge is contained in rotating clumps the size of dwarf galaxies. Neutral hydrogen ? atoms with one proton and one electron ? represents the raw material for new stars.
?If this gas is being accreted by the galaxies, then we need to understand how they're doing that. That information could, in principle, help us understand how galaxies like Andromeda, like our own Milky Way, can acquire gas to form new stars,? says Spencer Wolfe, a PhD candidate in astronomy at West Virginia University and the lead scientist on the project.
Over the past decade, astronomers have come to appreciate the potential of gas between galaxies to provide fresh fuel for making stars in spiral galaxies.
Star formation in the universe appears to have peaked some 10 billion to 11 billion years ago. Stellar birthrates these days are less than 10 percent of what they were then, notes Robert Braun, an astronomer at the Australia Telescope National Facility in Epping, New South Wales.
Left to their own devices, galaxies have on average about 1 billion to 2 billion years worth of gas in the cosmic tank, a condition that has existed throughout most of the universe's history, Dr. Braun writes in an e-mail. Many of them, therefore, should have stopped forming stars billions of years ago.?Moreover, the total mass of stars in the universe today is about five times higher than the amount of neutral hydrogen available 12 billion years ago, suggesting that the universe's larger inventory of ionized hydrogen kept star formation going in some way.
Researchers have identified other mechanisms for the galactic equivalent of in-flight refueling. For instance, gas gets recycled for a time through successive generations of stars. Collisions, mergers, and even near-misses between galaxies can trigger bursts of star formation.?But filaments of ionized hydrogen appear to be the only features persistent enough to keep galaxies stocked with stars over billions of years of cosmic history. Somehow, within those filaments, enough of the ionized gas condenses into the neutral form to serve as new stellar nurseries.
The filament or bridge Mr. Wolfe and his team studied appears between the Milky Way's nearest neighbor, the Andromeda Galaxy, and the Triangulum Galaxy. Andromeda is some 2.5 million light-years from Earth, while the Triangulum is roughly 3 million light-years away.
The presence of neutral hydrogen in the bridge was first reported in 2004 and confirmed in follow-up observations published last year. But it's fiendishly difficult to detect. One way neutral hydrogen betrays its presence is via radio waves, with a tell-tale signal at about the same frequency that a typical cell-phone uses. But the clumps are so wispy that their radio emissions were too faint for detailed studies with the radio telescopes used in the early work.
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