LBDS 53W091: An old, red galaxy at z = 1.552

The weak radio source LBDS 53W091 is associated with a very faint (R ≈ 24.5) red (R-K ≈ 5.8) galaxy. Long spectroscopic integrations with the W. M. Keck telescope have provided an absorption-line redshift, z = 1.552 ± 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 ≳ 3.5 Gyr, similar to that inferred for the intermediate-age population in M32. Depending upon the assumed Hubble constant and the value of Ω₀, only certain cosmological expansion times are consistent with the age of LBDS 53W091; in particular, for Ω₀ = 1, only models with H₀ ≲ 45 km s^-1 Mpc^-1 are permitted. For H₀ = 50 km s^-1 Mpc^-1 and Ω₀ = 0.2, we derive a formation redshift, z,z≥ 5.