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...

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