Old galaxies stick together

April 2, 2008
Using the most sensitive images ever obtained with the United Kingdom Infra-Red Telescope (UKIRT), astronomers have found convincing evidence that 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.

The distant galaxies identified in the UKIRT images are considered elderly because they are rich in old, red stars. However, because the light from these systems has taken up to 10 billion years to reach Earth, they are seen as they appeared in the very early universe, just 4 billion years after the Big Bang. The presence of such fully evolved galaxies so early in the life of the cosmos is hard to explain and has been a major puzzle to astronomers studying how galaxies form and evolve.

University of Nottingham PhD student Will Hartley, who led the study, and collaborators used the deep UKIRT images to estimate the mass of the dark matter surrounding the old galaxies by measuring how strongly the galaxies cluster together. All galaxies are thought to form within massive halos of dark matter which collapse under their own gravity from a smooth distribution of matter after the Big Bang.

These halos are invisible to normal telescopes but their mass can be estimated through analysis of galaxy clustering.

Hartley explains, "Luckily, even if we don't know what dark matter is, we can understand how gravity will affect it and make it clump together. We can see that the old, red galaxies clump together far more strongly than the young, blue galaxies, so we know that their invisible dark matter halos must be more massive."

The halos surrounding the old galaxies in the early universe are found to be extremely massive, containing material up to one hundred thousand billion times the mass of our Sun. 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."

Hartley spoke at the RAS National Astronomy Meeting in Belfast on Tuesday April 1.