A newly discovered class of galaxies could represent the awkward teenage phase of galaxy evolution, providing a bridge between the two most common types of galaxies.

Most galaxies are either blue spiral galaxies (left column in image) like the Milky Way or Andromeda, and red elliptical galaxies (right column) shaped more like a football than pinwheel. The former are generally thought of as the rambunctious young 'uns, while the latter are more like staid old fogeys.

But now astronomers have spotted some red spiral galaxies (middle column) that could be the missing link between the two classes.

Because blue stars are generally younger and hotter, while red stars tend to be older and cooler, most bluish spiral galaxies are thought to be young compared to the reddish ellipticals. Astronomers think the spirals are undergoing an active bout of star formation, with lots of newly born, blue stars giving the galaxies their hue.

The newfound red spiral galaxies, spotted by scientists from the Space Telescope A901/902 Galaxy Evolution Survey and the Galaxy Zoo project could represent a transition point between the two main types of galaxies. The discovery will be published in the Monthly Notices of the Royal Astronomical Society.

"In order to have spiral arms, they must have been normal, blue, spiral galaxies up until fairly recently," Steven Bamford, a postdoctoral researcher at the University of Nottingham in England, said in a press release. "But for some reason their star formation has been stopped, and they have turned red. Whatever caused them to stop forming stars can't have been particularly violent, or it would have destroyed the delicate spiral pattern."

The red spirals tend to be found near locations crowded with other galaxies, and the scientists suspect their environment may play a role in slowing down star formation in these galaxies. But more research is needed to find out exactly what shuts off this process.

A galaxy's mass also affects how it transitions to old age. The red spirals tend to be more massive galaxies, and the scientists suggest that smaller galaxies can't retain their spiral arms for long after star formation shuts down, and transform more quickly into smooth, lens-shaped galaxies. To complete the changeover into full-blown ellipticals, a violent event such as a collision between galaxies is probably also needed.

"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 England's University of Portsmouth in a press release. "Therefore, the red spirals that we see tend to be the larger galaxies — presumably because the smaller ones are transformed more quickly."

See Also:

Image: Marco Barden, Christian Wolf, Meghan Gray, the STAGES survey / Sloan Digital Sky Survey