LBDS 53W091 - AN OLD, RED GALAXY AT Z=1.552

The weak radio source LBDS 53W091 is associated with a very faint (R a pproximate to 24.5) red (R-K approximate to 5.8) galaxy. Long spectros copic integrations with the W. M. Keek telescope have provided an abso rption-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 sing le-burst old stellar population that matches the IR colors, the optica l energy distribution and the spectral discontinuities has a minimum a ge of 3.5 Gyr. We present detailed population synthesis analyses of th e observed spectrum in order to estimate the time since the last major epoch of star formation. We discuss the discrepancies in these estima tes resulting from using different models, subjecting the UV spectrum of M32 to the same tests as a measure of robustness of these technique s. The models most consistent with the data tend to yield ages at z = 1.55 of greater than or similar to 3.5 Gyr, similar to that inferred f or the intermediate-age population in M32. Depending upon the assumed Hubble constant and the value of Omega(0), only certain cosmological e xpansion times are consistent with the age of LBDS 53W091; in particul ar, for Omega(0) = 1, only models with H-0 less than or similar to 45 km s(-1) Mpc(-1) are permitted. For H-0 = 50 km s(-1) Mpc(-1) and Omeg a(0) = 0.2, we derive a formation redshift, z(f) greater than or equal to 5.