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-σ, 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 (a) 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; (b) 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.