Copyright © 2006 by Annual Reviews. All rights reserved
|
| Annu. Rev. Astron. Astrophys. 2006. 44:
141-192 Copyright © 2006 by Annual Reviews. All rights reserved |
Reprinted with kind permission from Annual Reviews, 4139 El Camino Way, Palo Alto, California, USA
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Abstract. 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-
,
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.
Keywords galaxy formation, galaxy evolution, galaxy surveys, stellar populations
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