Age and abundance discrimination in old stellar populations using mid-ultraviolet colors

The rest-frame mid-ultraviolet spectral region (2000 - 3200 Angstrom) is important in analyzing the stellar populations of the red envelope'' systems observed at high redshifts. Here we explore the usefulness of the mid-UV for determining ages and abundances of old populations. We work with a theoretical set of low-resolution spectra and broadband colors because tests show that these are at present more realistic than high-resolution models. A mid-UV to optical/IR wavelength baseline provides good separation of population components because the main-sequence turnoff dominates the integrated light between 2500 and 4000 Angstrom. Mid-UV spectral features are not sensitive to the dwarf/giant mixture in the population, unlike those in the optical region. We find a 6 mag difference in the mid-UV continuum level (normalized at V) over the metallicity range -1.5 < log(Z/Z(.)) < +0.5 and a comparable difference (per unit log t) for ages in the range 4 - 16 Gyr. Logarithmic derivatives of mid-UV colors with respect to age or metal abundance are 3 - 10 times larger than for the UBV region. Most of the spectral information on old populations therefore resides below 4000 Angstrom. Measurement of a single mid-UV color is capable of placing a strong lower bound on the mean metallicity of an old population. We investigate the capability of UBV and mid-UV broadband colors to separately determine age and abundance, taking into account precision in the color measurements. We find that the mid-UV improves resolution in log t, log Z space by about a factor of 3 for a given observational precision. Contamination by hot, post - He. ash evolutionary phases can seriously affect the mid-UV spectra of old populations. A simple estimate shows that contamination can reach over 80% in some cases. However, this is straightforward to remove as long as far-UV measurements are available. We find that extinction should have relatively small effects on parameters derived for old populations from the mid-UV. Finally, we show that a 4 Gyr, solar abundance model based on empirical spectra for nearby stars provides an excellent. t to the mid-UV spectrum of the Local Group elliptical galaxy M32. This indicates that the poorer results obtained from theoretical spectra do arise from limitations of the synthesis models for individual stars.