The white arrows point to a few of the old, massive galaxies at a distance of 10 billion light years, discovered in the UKIDSS Ultra-Deep survey. This cut-out image represents just 1/150th of the full survey. Image: UKIDSS UDS survey team.

Galaxies which look old early in the history of the Universe reside in enormous clouds of invisible dark matter, and will eventually evolve into the most massive galaxies that exist in the present day, say astronomers from the University of Nottingham presenting their research at the National Astronomy Meeting in Belfast this week.

Data was collected using the United Kingdom Infra-red telescope (UKIRT), the world's largest telescope dedicated solely to infrared astronomy, and used to estimate the mass of dark matter surrounding the old galaxies by measuring how strongly the galaxies cluster together. The galaxies are considered elderly because they are rich in old, red stars, and because the light from these systems has taken up to ten billion years to reach Earth, they are seen as they appeared just four billion years after the Big Bang. The presence of such fully evolved galaxies so early in the life of the Universe is hard to explain and has been a major puzzle to astronomers studying how galaxies form and evolve.

"We can see that the old, red galaxies clump together far more strongly than the young, blue galaxies," says Will Hartley, who led the study. "And even if we don't know what dark matter is, we can understand how gravity will affect it and make it clump together."

All galaxies are thought to form within massive halos of dark matter which collapse under their own gravity after the Big Bang. Although the halos are invisible to normal telescopes, their mass can be estimated through analysis of galaxy clustering. "Because the red galaxies clump together so strongly, we know that their invisible dark matter halos must be more massive than the young, blue galaxies," says Hartley. The halos contain material which is one hundred thousand billion times the mass of our Sun, which, in the nearby Universe, halos of this size are known to contain giant elliptical galaxies, the largest galaxies known.

"This provides a direct link to the present day Universe," says Hartley, "and tell us that these distant old galaxies must evolve into the most massive but more familiar elliptical-shaped galaxies we see around us today. Understanding how these enormous elliptical galaxies formed is one of the biggest open questions in modern astronomy and this is an important step in comprehending their history."