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Constraining Galaxy Formation Epoch
- Age Estimation of High Redshift Galaxies -
S. Yi et al.
Caltech, USA
The galaxy formation epoch (z f ) is an important and unique prod-
uct of cosmological models, and thus much eort has been made to
constrain it. The most direct way of constraining z f is to estimate
the ages of bright elliptical galaxies at high redshifts arrucately,
but it has not been an easy task. We have nally reached the stage
where the stellar ages can be determined within 15% uncertainty.
Using this advantage, the recent spectral analysis of LBDS
53W091 by Spinrad and his collaborators (1997) has suggested
that this red galaxy at z = 1:552 is at least 3.5 Gyr old. This pro-
vides a signicant constraint to cosmology, i.e., z f > 6:5, as LBDS
53W091 is the most distant passiviely evolving galaxy whose UV
spectrum is available.
The results, based on our population synthesis models, however,
suggest a much smaller age for this galaxy, i.e., 1.5 - 2 Gyr. The
large discrepancy between their age estimate and ours originates
from the dierence in the model integrated spectra: their model
spectra are signicantly bluer than ours. Such a large dierence
in the models for an apparently simple population is unreason-
able. A simple test seems to favor our models. If we adopt our
age estimate, the continuum of this galaxy suggests z f  2 { 3.
This anaysis, when performed to a larger sample, would eфciently
constrain the global galaxy formation epoch.
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