ULTRAVIOLET SPECTRAL DATING OF STARS AND GALAXIES

An echelle spectrogram (R = 30,000) of the 2300-3100 Angstrom region i n the ultraviolet spectrum of the F8 V star 9 Comae is presented. The observation is used to calibrate features in the mid-ultraviolet spect ra of similar stars according to age and metal content. In particular, the spectral break at 2640 Angstrom is interpreted using the spectral synthesis code SYNSPEC. We use this feature to estimate the time sinc e the last major star formation episode in the early-type galaxy LBDS 53W091 at redshift z = 1.55, whose rest-frame mid-ultraviolet spectrum , observed with the Keck Telescope, is dominated by the flux from simi lar stars that are at or near the main-sequence turnoff in that system (Spinrad et al.). Our result, 1 Gyr if the flux-dominating stellar po pulation has a metallicity twice solar, or 2 Gyr for a more plausible solar metallicity, is significantly lower than the previous estimate a nd thereby relaxes constraints on cosmological parameters that were im plied by the earlier work.