Missing link found between old and young star clusters

One and a half billion years ago the small, inconspicuous galaxy Messier 82 (M82) almost smashed into its large, massive neighbour galaxy Messier 81 (M81), causing a frenzy of star formation.

New research by astronomers from the Universities of Cambridge and Utrecht, Netherlands, has now discovered an elusive phenomenon in this violent “starburst” area. About 100 star clusters have been discovered that are believed to be the ancestors of the so-called “globular clusters” thought to be the oldest building blocks of galaxies.

“Such an intermediate-age population of massive, compact star clusters has been searched for extensively, but unsuccessfully until now,” said Dr Richard de Grijs from the University of Cambridge’s Institute of Astronomy, and lead scientist on the project. “The fact that we have finally found such a population is evidence that formation of long-lived star clusters is indeed happening now, just as it has in the early Universe and ever since.”

Dr de Grijs and his colleagues Henny Lamers and Nate Bastian, using images of M82 from the Hubble Space Telescope, found a population of about 100 star clusters aged 1.5 billion years. This is half-way down its evolution to old age – it is thought the star clusters could survive the next 10 billion years.

M82, which is 11 million light years away, is the closest starburst galaxy to Earth and is close enough to allow Hubble to see many individual star clusters.

During M82’s collision with its neighbouring galaxy, M81, the interstellar gas clouds were strongly compressed because of gravitational effects of the mass of M81, causing intense star formation activity.

Such strong starbursts have not occurred in our Milky Way galaxy for billions of years and none, until this discovery, have yet evolved sufficiently far.

“Clusters are disrupted much faster in M82 than here,” said Dr de Grijs. “This is probably another after-effect of the near collision with M81. M82 has a more disturbed appearance than other nearby, well-behaved galaxies because of the more inhomogeneous distribution of interstellar gas in the galaxy caused by the near collision.”

Stars are most often born in large clusters, originating from gas clouds that collapse due to their self-gravitation.

Astronomers have found thousands of star clusters in and around our Milky Way galaxy, and the populations of star clusters in neighbouring galaxies have been charted extensively as well.

Since all stars in a cluster – ranging from 1000 up to a million or more – are born more or less simultaneously, a cluster’s age can be determined fairly accurately; star clusters can therefore be used as astronomical “clocks”, tracing the events that created them.

The most ancient clusters in our Milky Way galaxy – so-called “globular clusters” – are about 12 billion years old, older than the Milky Way in its present form. These globular clusters have always been considered the oldest building blocks of galaxies.

The majority of galaxies go through epochs in which the rate of star formation rises explosively, a so-called starburst. Such epochs can be traced back through the ages of star clusters: entire populations of star clusters of roughly similar age are found to form during such starburst events.

However, a strange anomaly persisted in the observations: both young, unevolved and old, fully evolved star cluster populations were known – and such populations continue to be detected routinely – but clusters half-way on that career track were missing.

This anomaly gave credibility to the theory that cluster formation in the early Universe was fundamentally different from that at present.

However, since cluster formation is closely associated with the formation and evolution of stars and galaxies in general, fundamental doubts about the evolution of the Universe as a whole were raised.

The research team consists of: Dr Richard de Grijs, Institute of Astronomy, University of Cambridge Professor Henny Lamers and Mr. Nate Bastian, Astronomical Institute, Utrecht University, The Netherlands. This research will be published in The Astrophysical Journal (Letters) of 20 January 2003.