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 \gtrsim 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 , & K : 2000 – 9000 Å 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 two .
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 can not be easily reconciled with that of Spinrad et al .
( 1997 ) .

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 & Charlot 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 .