Red spiral galaxies: a gentle decline

Some galaxies, like some humans, can enter a period of graceful decline rather than suffer violent transformation, according to two new studies.

The findings, from the STAGES project and Galaxy Zoo, suggest that red spiral galaxies make up one fifth of all galaxies and represent a 'missing link' [see BBC Online] between 'young and vigorous' star-forming blue spiral galaxies and mostly 'old, dead and red' elliptical galaxies where star formation has shut down.

The studies were led by researchers from Oxford University and the University of Nottingham and are published online by the Royal Astronomical Society [read the results here and here].

Galaxy Zoo team leader Chris Lintott of Oxford's Department of Physics said: 'Everyone thought they knew that spiral galaxies were blue - but the public's results collected by Galaxy Zoo have shown that what every astronomer knew wasn't quite true;' [more in Universe Today]. 

Whilst Galaxy Zoo enlisted volunteers to help classify and detect the red spirals across a huge chunk of the sky, STAGES used the Hubble Space Telescope to take a detailed look at red galaxies in the A901/902 supercluster.

Oxford's Dr Christian Wolf, who helped lead the STAGES study, said: 'For the STAGES galaxies, the Spitzer Space Telescope provided us with additional images at infrared wavelengths. With them, we were able to go further and peer through the dust to find the missing piece of the puzzle.'

What the team found was that the red spirals in the supercluster were hiding low levels of star formation behind a veil of dust - activity that was only detectable in the infrared spectrum.

It suggests that if a galaxy is large enough and the local environment is relatively benign then a 'lively' blue spiral won't necessarily suffer a violent transformation into a 'dead' red elliptical: instead star formation is more gradually shut down leaving the delicate spiral arms of a galaxy intact - creating a red spiral.

Further work is needed to find out what exactly shuts down star formation in such a gentle fashion.