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Session 47 - Old Stellar Populations Beyond The Milky Way - II.
Oral session, Wednesday, June 11
North Main Hall A,

[47.06] Stellar Populations at High Redshift: How Old is Old?

W. C. Keel (U. Alabama)

Recent observations of high-redshift systems in the near-IR (emitted optical and near-IR) bands allows a first estimate of the ages of their stellar populations, which probe the epoch of first significant star formation because of the leverage that their lookback time provides. As an example, the compact star-forming objects in the 53W002 cluster at z=2.39 have observed V-K=4.2-5.9, which samples the emitted 2400-6600 Å\ color. The near-constant flux from 1800-6600 Å\ suggests either highly mixed dust and stars (unlikely given the rather high observed Lyman \alpha/H\alpha emission-line ratio) or an important component older than a single burst. The closest-fit rapid-burst spectral model gives an age for the dominant population 3-5 \times 10^8 years; an exponential decay in star-formation rate will better match the UV continuum shape, while making the populations somewhat older. At z=2.4, an extra 10^9 years equates to a ``formation redshift" z_f = 8 (H_0=80 km/s Mpc, q_0=1/2), so limits to population ages in this regime are very sensitive indicators of when galaxy formation started.

It is not yet clear how representative this population, very similar to the blue compact objects in the HDF, should be of the overall population. More luminous objects such as 53W091 and perhaps some of the bright Lyman-break objects in the HDF are redder, naively giving ages pushing the Hubble time. We would like to know which, if any, of these kinds of high-redshift systems are the precursors of today's old populations. Prospects for progress are strong, with scheduled HST and ISO observations capable of measuring such spectral features as the 4000-Å\ break and the emitted K-band light.

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