Galaxy clusters in formation

Galaxies typically don’t shine alone. Instead, like our Milky Way, they often are members of galaxy clusters, the largest bound objects in the universe. By studying the formation and evolution of those clusters, cosmologists hope to test their models and learn about the nature of dark matter and dark energy (see the article by Josh Frieman, Physics Today, April 2014, page 28 , and the news story in this issue, page 18 ). Now, by combining data obtained from the European Space Agency’s Planck and Herschel observatories, cosmologists may have caught a number of galaxy clusters in the act of forming. Planck, celebrated for its exquisite maps of the cosmic microwave background, sees the entire sky, but with relatively poor resolution. Still, the satellite was able to spot about 230 highly redshifted, bright sources that appeared to be from an epoch when the universe was less than 4 billion years old. Scientists from the Planck collaboration followed up on those observations by procuring images from Herschel’s higher- resolution Spectral and Photometric Imaging Receiver; in the representative figure shown here, the different colors correspond to observations at different wavelengths. The Planck researchers caution against definitively concluding that the overdense regions revealed by the Herschel follow-up are protoclusters—galaxy clusters in the process of formation. However, the number of overdense regions they observe is consistent with theoretical predictions for protoclusters. And as determined by their IR luminosities, the individual members of the putative protoclusters are producing stars at the fantastic rate characteristic of young galaxies—roughly 1000 times that of today’s Milky Way. (N. Aghanim et al., Planck collaboration, Astron. Astrophys., in press, doi:10.1051/0004-6361/201424790 .)