INTEGRATED ULTRAVIOLET-SPECTRA AND LINE INDEXES OF M31 GLOBULAR-CLUSTERS AND THE CORES OF ELLIPTIC GALAXIES

We present observations of the integrated light of four M31 globular c lusters (MIV, MII, K280, and K58) and of the cores of six elliptical g alaxies (NGC 3605, 3608, 5018, 5831, 6127, and 7619) made with the Fai nt Object Spectrograph on the Hubble Space Telescope. The spectra cove r the range 2200-4800 Angstrom at a resolution of 8 Angstrom with sign al-to-noise ratio of more than 20 and flux accuracy of similar to 5%. To these data we add from the literature IUE observations of the dwarf elliptical galaxy M32, Galactic globular clusters, and Galactic stars . The stellar populations in these systems are analyzed with the aid o f mid-UV and near-UV colors and absorption line strengths. Included in the measured indices is the key NH feature at 3360 8, We compare thes e line index measures with the 2600 - 3000 colors of these stars and s tellar populations. We find that the M31 globular clusters, Galactic g lobular clusters/Galactic stars, and elliptical galaxies represent thr ee distinct stellar populations, based on their behavior in color-line strength correlations involving Mg II, NH, CN, and several UV metalli c blends. In particular, the M31 globular cluster MIV, as metal-poor a s the Galactic globular M92, shows a strong NH 3360 Angstrom feature. Other line indices, including the 3096 Angstrom blend that is dominate d by lines of Mg I and Al I, show intrinsic differences as well. We al so find that the broadband line indices often employed to measure stel lar population differences in faint objects, such as the 4000 Angstrom and the Mg 2800 breaks, are disappointingly insensitive to these stel lar population differences. We find that the hot (T > 20,000 EC) stell ar component responsible for the ''UV upturn'' at shorter wavelengths can have an important influence on the mid-UV spectral range (2400-320 0 Angstrom) as well. The hot component can contribute over 50% of the flux at 2600 Angstrom in some cases and affects both continuum colors and line strengths. Mid-UV spectra of galaxies must be corrected for t his effect before they can be used as age and abundance diagnostics. O f the three stellar populations studied here, M31 globular clusters an d elliptical galaxies are more similar to each other than either is to the Galactic stellar populations defined by globular clusters and nea rby stars. Similarities between the abundance-pattern differences curr ently identified among these stellar populations and those among globu lar cluster stars (N, Al enhancements) present a curious coincidence t hat deserves future investigation.