Christopher Mullis of the University of Michigan and co-workers at the European Southern Observatory (ESO) discovered the distant cluster by sifting through old images from the XMM-Newton satellite. They looked for large X-ray sources that had not been studied before and then took a series of follow-up images of 30 candidate galaxies at optical wavelengths with ESO's Very Large Telescope (VLT) in Chile. One of these galaxies - now named XMMU J2235.3-2557 - had a redshift of 1.4, which means that it is nine billion light years away and must have formed when the universe was less than a third of its present age. Moreover, the spherical shape of the cluster implies that it has a well-organised and mature structure (Astrophysical Journal to be published).

"By observing that 'big cities' were already in place in space nine billion years ago, and that they are not very different from present-day clusters, we can glean fresh information on how the universe evolved," Piero Rosati of ESO told PhysicsWeb.

Meanwhile, James Houck of Cornell University and co-workers used a similar technique to find a population of extremely bright "young" galaxies in which stars are still forming. They combined infrared observations of thousands of galaxies by the Spitzer space telescope with optical images obtained by the ground-based National Optical Astronomy Observatory Deep Wide-Field Survey in the same region of sky. In this way, the astronomers identified 31 candidate galaxies that could only be seen by Spitzer.

Follow-up observations with Spitzer detected the presence of silicate dust in 17 of these galaxies, and by measuring the wavelengths of absorption features caused by this dust, the team were able to determine the redshifts of these objects. The galaxies date back to when the universe was just three billion years old, or a quarter of its present age (Astrophysical Journal Letters at press).

"This is the furthest back in time silicate dust has been detected around a galaxy," says Thomas Soifer of the Spitzer Science Center. "Finding silicate dust at this very early epoch is important for understanding when planetary systems arose in the evolution of galaxies."