Galactic Halfway House Discovered

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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