New 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?
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
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. …