18 july 2004

Massive galaxies are surprisingly old

A team of researchers, including Oxford astrophysicist Isobel Hook, has found that certain massive galaxies are older than anyone expected.

Astrophysicists had assumed that massive galaxies were formed from an assembly of smaller units, which would imply that the biggest galaxies were formed the most recently. Yet when the scientists looked back in space and time, they found that some of the largest galaxies formed when the universe was very young. The findings will mean that astrophysicists will have to look again at their models for galaxy formation.

The team, who published their results in Nature, used the Gemini telescope in Hawaii to look far out into space - and, because of the time it takes for the light from there to reach Earth, far back in time too - as part of the Gemini Deep Deep Survey (GDDS), which is probing areas of the universe which scientists have previously been unable to see.

Because of redshift, the process whereby the wavelengths of objects travelling away from the observer (such as galaxies) shift towards the red end of the light spectrum, becoming infrared rather than visible light, certain points in the universe have been difficult to observe. The GDDS team developed a special observing technique at the Gemini telescope to allow observation of those areas.

The team studied the masses and ages of about 100 massive galaxies. Existing models predicted that all but a few of the most massive galaxies would have formed after the universe reached half its present age, because of hierarchical formation, the process by which smaller units merge over time to form bigger units.

Dr Hook said: 'Detailed explanations of how matter condensed over time into ever larger structures and ultimately formed galaxies are still being refined, and the data we have obtained feeds into those models. The existing models predicted that most of the huge galaxies would be relatively young. These new observations show that to be wrong. Either the massive galaxies grew much earlier than the hierarchical model would have predicted, or the stars in these massive galaxies formed in a much more efficient, faster way than we thought.'

The UK national office for Gemini is based in the Astrophysics department at Oxford. The UK is the second largest partner in the telescope, and the national office here provides user support and takes part in decisions about how the observatory is run.