Washington, Nov 26 : Astronomers at the University of Nottingham in the UK have identified a type of galaxy that could be the 'missing link' in our understanding of galaxy evolution.
The discovery is part of the STAGES study, led by the University's Centre for Astronomy and Particle Theory, which is examining galaxy evolution using images from the Hubble Space Telescope.
Astronomers place most normal galaxies into two camps according to their visual appearance: either disk-like systems like our own Milky Way, or round, rugby-ball shaped collections of stars known as ellipticals.
In most cases, a galaxy's shape matches its colour. Spiral galaxies appear blue because they are still vigorously forming hot young stars.
Elliptical galaxies, on the other hand, are mostly old, dead and red, and tend to cluster together in crowded regions of space.
STAGES has identified a population of unusual red spiral galaxies that are setting out on the road to retirement after a lifetime of forming stars.
The team discovered that, despite their colour, the red spirals are actually hiding star formation behind a shroud of dust.
Invisible to Hubble's eye, this star formation is only detectable in the infrared part of the spectrum - radiation emitted from the galaxies at wavelengths longer than visible light. The star formation in blue spiral galaxies is gradually shut off and hidden behind dust, before petering out to form smooth "lenticular" (lens-shaped) red galaxies with no trace of spiral arms.
To go further and transform the galaxy into an elliptical shape would require more violent mechanisms, such as the collision of galaxies.
Dr Christian Wolf, an STFC Advanced Research Fellow at the University of Oxford, trained the Hubble Space Telescope on a region of space crowded with galaxies known as the A901/902 supercluster for the STAGES project.
He uncovered a surprisingly large population of spiral galaxies in the supercluster that are red in colour.
Within the supercluster, Dr Wolf discovered that the red spirals were hiding low levels of hidden star formation, despite their otherwise lifeless appearance in visible light.
The next step for both teams is to find out exactly what shuts off the star formation, by looking inside the galaxies themselves.
They suspect that behind the slow demise of galaxies is a process known as strangulation, in which a galaxy's fuel supply is stripped away as it encounters the crowd.
Starved of the raw material needed to form new stars, it will slowly change colour from blue to red as its existing stars age.