Large survey of distant galaxy clusters

The types of galaxies that populate our universe have changed over time. The population evolution can be at least partly unraveled by studying the cores of rich clusters of galaxies, then comparing them across cosmic time—at different cosmological redshifts z. But few such clusters are known with z > 1. At those higher redshifts, the most prominent spectral feature of the elliptical galaxies at rich clusters’ cores—the 4000-Å break in the continuum that occurs when there is a dearth of hot, blue stars—is shifted into the IR. A proven detection method is to scan the sky in two wavelength bands, one on either side of the 4000-Å break, but only recently have near-IR detectors become available that allow ground-based telescopes to see the “blueward” side of the shifted spectral break in distant clusters. A new study, named the Spitzer Adaptation of the Red-sequence Cluster Survey, found hundreds of new galaxy cluster candidates using relatively modest observational resources. The astronomers surveyed high-z galaxies by combining wide-field observations from telescopes in Hawaii and Chile with archival IR data from the Spitzer Space Telescope. Follow-up observations of the first three candidates confirmed them to be massive clusters at z ~ 1.2-1.3, firmly in the realm where evolutionary effects can begin to be studied. The authors expect to confirm many additional distant clusters from the SpARCS survey. (A. Muzzin et al., Astrophys. J. 698 , 1934, 2009 http://dx.doi.org/10.1088/0004-637X/698/2/1934 ; G. Wilson et al., Astrophys. J. 698 , 1943, 2009 http://dx.doi.org/10.1088/0004-637X/698/2/1943 .)