Scientists
have spotted a rare class of galaxy that could represent a cosmic halfway house
between two stages of galaxy development.
Galaxies
usually fall into one of two categories: pinwheel-shaped
spirals that tend to be blue or reddish, egg-shaped ellipticals. But now
astronomers have discovered a group of red spiral galaxies that could be the
missing link between the two main types.
Scientists
think many galaxies begin their lives as spirals, when bursts
of star formation create young, hot, blue stars in clumps that create
spiral arms. As stars age they get cooler and redder, and without fresh star
formation in a galaxy, the lumpiness of spiral arms tends to settle down into a
smoother, rounder shape.
The
newly-discovered red spiral galaxies could present a
transition point between young blue spirals and old red ellipticals.
"We
want to establish the link between the different stages more firmly," said
Christian Wolf, an astrophysicist at the University of Oxford. "At the
moment what we see is that one type of galaxy gets replaced by the other. But
we have not been able to watch any individual galaxy for millions of years and
establish that it changes along this particular route."
Mounting
evidence
Though
some red spirals have been spotted before, there were so few of them that
researchers couldn't be sure if they were isolated phenomena or represented a
more common stage of galaxy evolution.
But
recently two teams independently observed significant populations of these
galaxies. The Galaxy Zoo project uses volunteers from the general public to
classify galaxies in images from the Sloan Digital Sky Survey of the local
universe. A separate project, the Space Telescope A901/902 Galaxy Evolution
Survey (STAGES), used the Hubble Space Telescope to observe a region of space
crowded with galaxies known as the A901/902 supercluster. Both teams were
surprised to find so many red spirals.
"We
have been used to just sorting galaxies by their color — red and blue,"
Wolf told SPACE.com. "We needed either the Hubble Space Telescope
to see their morphology more clearly and say that some of these red galaxies
are actually spirals. Or we needed a much larger sample, such as Sloan Digital
Sky Survey with Galaxy Zoo, to really spot them."
These
transitioning galaxies tend to be located in more crowded
areas near other galaxies, and their setting could play an important role
in shutting down star formation, allowing them to morph from spirals to
ellipticals. Scientists aren't sure exactly why the creation of new stars gets
turned off in galaxies, but one hypothesis is that in crowded regions, hot gas
drives out cold gas, which is necessary for the clumping of matter that births
new stars. If the cold gas were driven out of galaxies in congested areas,
causing them to stop forming stars, their existing stars would age and become
redder, making the whole galaxy appear red.
Big
red
Additionally,
most of the red spirals the researchers found are large galaxies, whose heavy
mass also appears to affect their transition. It appears that lower-mass
galaxies might make the changeover from spiral to elliptical more quickly,
which would make spotting red galaxies that are still in their spiral shape
tougher. Since larger galaxies could spend more time in this middle phase,
astronomers are more likely to find large red spirals than small red spirals.
"Just
as a heavyweight fighter can withstand a blow that would bring a normal person
to his knees; a big galaxy is more resistant to being messed around by its
local environment," said Galaxy Zoo team member Bob Nichol of Portsmouth
University. "Therefore, the red spirals that we see tend to be the larger
galaxies - presumably because the smaller ones are transformed more
quickly."
Wolf and
other members of the STAGES team found that the red spirals hadn't completely
clamped down on star formation. In fact, low levels were still going on, but
this activity was masked behind a shroud of dust.
"Blue
spirals have a similar amount of dust, but blue spirals have so much star
formation, so despite dust it's still clearly visible," Wolf said. In the
red spirals, the astronomers could only spot the star formation in
long-wavelength infrared light, which can pierce through the dust clouds into
the galaxies' hearts.
Both
teams plan to detail their findings in an upcoming issue of the Monthly
Notices of the Royal Astronomical Society.
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