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Stellar Population Diagnostics of Elliptical Galaxy Formation

Major progress has been achieved in recent years in mapping the properties of passively-evolving, early-type galaxies (ETG) from the local universe all the way to redshift ~2. Here, age and metallicity estimates for local cluster and field ETGs are reviewed as based on color-magnitude, color-sigma, and fundamental plane relations, as well as on spectral-line indices diagnostics. The results of applying the same tools at high redshifts are then discussed, and their consistency with the low-redshift results is assessed. Most low- as well as high-redshift (z~1) observations consistently indicate 1) a formation redshift z>~3 for the bulk of stars in cluster ETGs, with their counterparts in low-density environments being on average ~1-2 Gyr younger, i.e., formed at z>~1.5-2, 2) the duration of the major star formation phase anticorrelates with galaxy mass, and the oldest stellar populations are found in the most massive galaxies. With increasing redshift there is evidence for a decrease in the number density of ETGs, especially of the less massive ones, whereas existing data appear to suggest that most of the most-massive ETGs were already fully assembled at z~1. Beyond this redshift, the space density of ETGs starts dropping significantly, and as ETGs disappear, a population of massive, strongly clustered, starburst galaxies progressively becomes more and more prominent, which makes them the likely progenitors to ETGs.