On the age estimation of LBDS 53W091

The recent spectral analysis of LBDS 53W091 by Spinrad and his collaborators has suggested that this red galaxy at z = 1.552 is at least 3.5 Gyr old. This imposes an important constraint on cosmology, suggesting that this galaxy formed at z greater than or similar to 6.5, assuming recent estimates of cosmological parameters. While their analysis was heavily focused on the use of some UV spectral breaks as age indicators, we have performed chi(2) tests to the continuum of this galaxy using its UV spectrum and photometric data (R, J, H, and K: 2000-9000 Angstrom in rest frame). We have used the updated Yi models that are based on the Yale tracks. We find it extremely difficult to reproduce such large age estimates, under the assumption of the most probable input parameters. Using the same configuration as in Spinrad et al. (conventional solar abundance models), our analysis suggests an age of approximately 1.4-1.8 Gyr. We have improved our models over conventional ones by taking into account convective core overshoot in the stellar model calculations and realistic metallicity distributions in the galaxy population synthesis. Overshoot affects the visible continuum normalized to the UV and raises the photometry-based age estimates by 25%. The use of metallicity mixtures affects the whole spectrum and raises all continuum-based age estimates by up to a factor of 2. If the mean metallicity of the stars in this galaxy is assumed to be twice solar, the models including these two effects match the UV spectrum and photometric data of LBDS 53W091 near the age of 1.5-2.0 Gyr. Our results cannot be easily reconciled with that of Spinrad et al. The discrepancy between Spinrad et al.'s age estimate (based on the Jimenez models) and ours originates from the large difference in the model integrated spectrum: the Jimenez models are much bluer than the Yi models and the Bruzual & Chariot models. We propose to apply some viable tests to them for verification and search for the origin of the difference through a more thorough investigation. Considering the significance of the age estimates of distant galaxies as probes of cosmology, it would be an urgent task.