The weak radio source LBDS 53W091 is associated with a very faint ( R \approx 24.5 ) red ( R - K \approx 5.8 ) galaxy .
Long spectroscopic integrations with the W. M. Keck telescope have provided an absorption–line redshift , z = 1.552 \pm 0.002. The galaxy has a rest frame ultraviolet spectrum very similar to that of an F6 V star , and a single–burst old stellar population that matches the IR colors , the optical energy distribution and the spectral discontinuities has a minimum age of 3.5 Gyr .
We present detailed population synthesis analyses of the observed spectrum in order to estimate the time since the last major epoch of star formation .
We discuss the discrepancies in these estimates resulting from using different models , subjecting the UV spectrum of M32 to the same tests as a measure of robustness of these techniques .
The models most consistent with the data tend to yield ages at z = 1.55 of \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \mathchar 536 $ } \hss } \raise 2.0 pt% \hbox { $ \mathchar 318 $ } } 3.5 Gyr , similar to that inferred for the intermediate–age population in M32 .
Depending upon the assumed Hubble constant and the value of \Omega _ { 0 } , only certain cosmological expansion times are consistent with the age of LBDS 53W091 ; in particular , for \Omega _ { 0 } = 1 , only models with H _ { 0 } \lesssim 45 km s ^ { -1 } Mpc ^ { -1 } are permitted .
For H _ { 0 } = 50 km s ^ { -1 } Mpc ^ { -1 } and \Omega _ { 0 } = 0.2 , we derive a formation redshift , z _ { f } \geq 5.