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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 +/- 0.002.The galaxy has a rest frame ultraviolet spectrum very similar to that of an F6Vstar, and a single-burst old stellar population that matches the IR colors, theoptical energy distribution and the spectral discontinuities has a minimum ageof 3.5 Gyr. We present detailed population synthesis analyses of the observedspectrum in order to estimate the time since the last major epoch of starformation. We discuss the discrepancies in these estimates resulting from usingdifferent models, subjecting the UV spectrum of M32 to the same tests as ameasure of robustness of these techniques. The models most consistent with thedata tend to yield ages at z=1.55 of >3.5Gyr, similar to that inferred for theintermediate-age population in M32. Depending upon the assumed Hubble constantand the value of Omega_0, only certain cosmological expansion times areconsistent with the age of LBDS 53W091; in particular, for Omega_0=1, onlymodels with H_0 < 45 km/s/Mpc are permitted. For H_0=50 km/s/Mpc andOmega_0=0.2, we derive a formation redshift, z_f >= 5.Comment: 34 pages (Latex text + tables), 18 pages of postscript figures;  Accepted for publication in The Astrophysical Journa

LBDS 53W091: An Old, Red Galaxy atz= 1.552