Spiral
galaxies with bar-shaped arms may not have been born that way. Astronomers have
found that the number of bars has tripled in the past 7 billion years,
indicating that spiral galaxies evolve as they age.
Barred
galaxies are shaped like a tiger's eye, with two
starry arms trailing off either end of a long, dark stardust lane. They
take shape as stellar orbits in a disk become unstable and deviate from a
circular path.
"The
formation of a bar may be the final important act in the evolution of a spiral
galaxy," said study leader Kartik Sheth of NASA's Spitzer Science Center at Caltech. "Galaxies are thought to build themselves up through mergers
with other galaxies. After settling down, the only other dramatic way for
galaxies to evolve is through the action of bars."
More than
two-thirds of spiral galaxies, including our own Milky Way, have a bar-shaped
path through their middles.
According
to new observations of over 2,000 spiral galaxies, made as a part of the Hubble Space Telescope's
Cosmic Evolution Survey (COSMOS), the bar scene was dramatically different
seven billion years ago, when the universe was half as old as it is today.
The
astronomers involved in the study, detailed in a recent issue of The
Astrophysical Journal, discovered that while spiral galaxies were around in
the distant past, only around 20 percent of them possessed the bars that are so
common in their modern counterparts.
The
tripling rate does not proceed in a uniform way, either. "They are forming
mostly in the small, low-mass galaxies," Sheth said, adding that among the
most massive galaxies, the proportion of bars to no bars is the same as it is
today.
"We
know that evolution is generally faster for more massive galaxies — they form
their stars early and fast and then fade into red disks," Sheth explained.
"Low-mass galaxies were also known to form more slowly, but now we see
that they also made their bars slower."
Bars grow
in galaxies after stellar orbits in a spiral galaxy begin to deviate from a
circular path, said survey team member Bruce Elmegreen, an astrophysicist with
IBM's Research Division.
"It
locks more and more of these elongated orbits into place, making the bar even
stronger. Eventually a high fraction of the stars in the inner disk join the
bar," Elmegreen added.
Bars are
perhaps the most important catalysts for changing a galaxy, Sheth said. They
force a large amount of gas towards the galactic center, fueling new star
formation, building bulges —spheres in the centers of galaxies made only of
stars — and feeding massive black holes.
"They
pull stars and gas out of their normal circular orbits into the central
regions, perhaps even funneling gas to the central supermassive black
hole," said Nicholas Scoville of Caltech and a COSMOS principal
investigator. "Without this fueling, the black holes would be starved and
the central regions of galaxies devoid of young stars."
The Milky
Way, possibly the best-known barred galaxy, is a massive one whose bar
probably formed somewhat early, like the bars in other massive galaxies, Sheth
suggests.
"Understanding
how this occurred in the most distant galaxies will eventually shed light on
how it occurred here, in our own backyard," he adds.
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