By Jason Palmer
Science and technology reporter, BBC News
A red spiral galaxy (c), flanked by blue spiral (l) and red elliptical galaxies (r)
Astronomers have identified a type of galaxy that represents a "missing link" in our understanding of the Universe.
Spiral and elliptical galaxies used to be known exclusively as "blue" and "red", respectively.
But two studies, published in a Royal Astronomical Society journal, show that one in five galaxies is a red spiral.
It is now thought the red spirals occur when spiral galaxies grow old without any violent collisions, such as with other galaxies.
Astronomers have seen that, in dense regions of space where there is a high concentration of galaxies, there are many old, "red and dead" galaxies which are elliptical or spherical in shape.
These galaxies are presumed to have formed rapidly, early in the history of the Universe. The red light we now see from them is that of their twilight years, showing that new star formation has long since stopped.
Elsewhere, comparatively young spiral galaxies still promote star formation, whose emitted light tends towards the blue.
But the new findings demonstrate the existence of red spirals, the first evidence that there are galaxies whose star formation ceased in the last few billion years - relatively recently when one considers the entire history of the Universe.
"There have been hints that red spirals that would exist in clusters and groups of galaxies before and people had seen handfuls of objects, but never enough to say that they are a major player in the galaxy population," said co-author Steven Bamford of the University of Nottingham.
Some galaxies are easier to classify than others
Dr Bamford used data from the Galaxy Zoo project, which has seen some 170,000 members of the public help classify galaxies imaged with the Sloan Digital Sky Survey.
The survey imaged galaxies in colour in a vast swathe of sky, with Galaxy Zoo participants then classifying more than a million of them as spiral or elliptical.
What the study found was that some 20% of the galaxies classified were red spirals. They were primarily the largest ones, and tended to be towards the outskirts of their clusters.
"A lot of studies recently have really concentrated on colour because it's an easy thing to measure, so conclusions have been made about how galaxies form and evolve based on splitting them up by colour," Dr Bamford told BBC News.
"But now, by actually looking at pictures of galaxies, we see there are red ellipticals, red spirals, blue spirals and even a small blue elliptical population."
Another, more directed search was led by Christian Wolf of the University of Oxford and Meghan Gray, also at the University of Nottingham, in a project dubbed Stages.
The team made use of the Hubble Space Telescope, pointing it at a dense region of space known as the A901/902 supercluster. Studying a region of just four clusters, the team found an equally high proportion of red spirals.
"Our two projects have approached the problem from very different directions, and it is gratifying to see that we each provide independent pieces of the puzzle pointing to the same conclusion," Dr Gray said.
Red hot questions
The two groups will publish their results in a forthcoming issue of the Monthly Notices of the Royal Astronomical Society.
The research raises further questions however: what has stopped these galaxies building new stars to keep the blue light shining? One way for that to happen is a grand collision between galaxies, which would heat up matter too much for the stable formation of large bodies.
However, such a violent event would also destroy the delicate spiral arms of a young galaxy. That supports the observation that the red spirals were found in the less dense regions of their clusters.
The Sloan Survey, unlike Hubble, can capture large regions of the sky
Whatever the process that creates these galaxies from their blue cousins, it is a gentle one in cosmic terms.
The results will also help to resolve a long-standing debate about how to classify a third, lens-shaped type of galaxy, called lenticular. Though many astronomers place them in a category with elliptical galaxies, recent studies have shown signs that lenticular galaxies are younger.
The existence of red spirals seems to suggest that young, blue spirals can age through the time of star formation, slowly becoming lenticular as the arms of the spiral die away.
The two groups will continue their work by studying the red spirals in detail, finding the region of the galaxy that the red light is coming from. That in turn will tell them which parts ceased forming stars first.
Dr Chris Lintott, Galaxy Zoo team leader at the University of Oxford, said: "These results are possible thanks to a major scientific contribution from our many volunteer armchair astronomers. No group of professionals could have classified this many galaxies alone."