5.3 Elliptical Galaxies and Globular Clusters
The most direct way to constrain the formation and evolution of
galaxies certainly is to trace back their evolution with redshift.
The price to be paid, however, is that high-redshift data naturally
have lower quality and are therefore more difficult to interpret. A
clear complication is the so-called progenitor bias, which implies
that galaxies observed at low and high redshift are not necessarily
drawn from the same sample (van Dokkum et al. 2000). The alternative
approach is the detailed investigation of the stellar populations in
local galaxies, which has been pioneered by analyzing slopes and
scatter of color-magnitude and scaling relations of early-type
galaxies, followed by a number of detailed studies of absorption line
indices. We call this the 'archaeology approach'. The confrontation
with predictions from models of galaxy formation is certainly most
meaningful, when the two approaches, the mining of the high-redshift
universe and the archaeology of local galaxies set consistent
constraints.
The main challenge in the archaeology of stellar population is the
disentanglement of age and metallicity effects. The use of absorption
line indices to lift this degeneracy is powerful (Worthey 1994), but
has been up to now hampered by the fact that different metallic line
indices yield different metallicities and therefore different ages.
We have solved this problem by developing stellar population models
that include element abundance ratio effects and now allow for an
un-ambiguous derivation of age, total metallicity, and element ratios
from (Lick) absorption line indices (Thomas, Maraston, Bender 2003a).
Note that a complication that remains, however, is the degeneracy
between age and horizontal branch morphology, which stems from the
fact that the presence of warm horizontal branch stars (which cannot
be excluded) strengthens the Balmer absorption and can mimic a younger
stellar population age.
The principal aim is to constrain the formation epochs of the stellar
populations in early-type galaxies as a function of their type
(elliptical and lenticular), mass and environmental density. The
α/Fe element ratio plays a key role for the accomplishment of
this goal. While the so-called α-elements O, Ne, Mg, Si, S, Ar,
Ca, Ti (particles that are build up with α-particle nuclei)
plus the elements N and Na are delivered mainly by Type II supernova
explosions of massive progenitor stars, a substantial fraction of the
Fe-peak elements Fe and Cr comes from the delayed exploding Type Ia
supernovae. Hence, the α/Fe ratio quantifies the relative
importance of Type II and Type Ia supernovae, and therefore carries
information about the timescale over which star formation occurs.
Thus, the α/Fe ratio can be considered as an additional measure
of late star formation, and we use it both to constrain formation
timescales and to lift the degeneracy between age and horizontal
branch morphology.
From the analysis of a homogeneous, high-quality data sample of 124
early-type galaxies in various environmental densities, we find clear
evidence for an influence of the environment on the stellar population
properties. Massive early-type galaxies in low-density environments
appear on average ~2 Gyrs younger and slightly
(~0.05-0.1 dex) more metal-rich than their counterparts in high density
environments. No offsets in the α/Fe ratios, instead, are
detected. With the aid of a simple chemical evolution model, we
translate the derived ages and α/Fe ratios into star formation
histories. We show that most star formation activity in early-type
galaxies is expected to have happened between redshifts ~3 and 5
in high density and between redshifts 1 and 2 in low density
environments.
last update:
10/2004, editor of this page: Roberto Philip Saglia
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