Globular clusters show: how galaxies come in several waves

The origin of most galaxies is not uniform, but run in several stages. This is a study of over 900 globular clusters by an international research team. The star clusters from the dawn of galaxy formation to occur mostly in two different chemical variants, the astronomers reported in the journal "Monthly Notices of the Royal Astronomical Society".

"This indicates presence of two types of globular clusters suggest that most large galaxies have gone through two phases of star formation through", Christopher Usher of the Swinburne University of Technology in Australia write Hawthorn and his colleagues. 

Globular clusters are dense clusters of several hundred thousand stars. Large galaxies are surrounded by hundreds of globular clusters. The clusters are at an age of about ten billion years from the formation of galaxies, the astronomers thus allow an insight into this important cosmic epoch.

Back in 2001, observations have shown that in many galaxies are two different color types of globular clusters. So far, the cause of the difference, however, was controversial. Usher and his colleagues have now observed with the Subaru and Keck telescopes in Hawaii systematically the spectra of 903 globular clusters in eleven large galaxies. The new measurements show that differences in the chemical composition are responsible for the color differences. Most galaxies are therefore two types of globular clusters, one of which contains ten times more heavy elements such as calcium and iron than the others.

Such "bimodality" the team found in six of the studied galaxies, another star system also shows evidence for two types of globular clusters. And in a galaxy Usher and his colleagues found even on three chemically different types of globular clusters. "So it appears that galaxies formed in at least two different batches," says Usher's colleague Duncan Forbes. "Unfortunately, galaxies lie at no instructions. Reproduction of our observational results using computer simulations of galaxy formation is therefore now a big challenge for the theoretical astrophysicist "source.