Integrated Ultraviolet Spectra and Line Indices of M31 Globular Clusters and the Cores of Elliptical Galaxies

Abstract

We present observations of the integrated light of four M31 globular clusters (MIV, MII, K280, and K58) and of the cores of six elliptical galaxies (NGC 3605, 3608, 5018, 5831, 6127, and 7619) made with the Faint Object Spectrograph on the Hubble Space Telescope. The spectra cover the range 2200–4800 A at a resolution of 8 A with signal-to-noise ratio of more than 20 and flux accuracy of ~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 of mid-UV and near-UV colors and absorption line strengths. Included in the measured indices is the key NH feature at 3360 A. We compare these line index measures with the 2600 - 3000 colors of these stars and stellar populations. We find that the M31 globular clusters, Galactic globular clusters/Galactic stars, and elliptical galaxies represent three distinct stellar populations, based on their behavior in color–line strength correlations involving Mg II, NH, CN, and several UV metallic blends. In particular, the M31 globular cluster MIV, as metal-poor as the Galactic globular M92, shows a strong NH 3360 A feature. Other line indices, including the 3096 A blend that is dominated by lines of Mg I and Al I, show intrinsic differences as well. We also find that the broadband line indices often employed to measure stellar population differences in faint objects, such as the 4000 A and the Mg 2800 breaks, are disappointingly insensitive to these stellar population differences. We find that the hot (T > 20,000 K) stellar component responsible for the UV upturn at shorter wavelengths can have an important influence on the mid-UV spectral range (2400–3200 A) as well. The hot component can contribute over 50% of the flux at 2600 A in some cases and affects both continuum colors and line strengths. Mid-UV spectra of galaxies must be corrected for this effect before they can be used as age and abundance diagnostics. Of the three stellar populations studied here, M31 globular clusters and elliptical galaxies are more similar to each other than either is to the Galactic stellar populations defined by globular clusters and nearby stars. Similarities between the abundance-pattern differences currently identified among these stellar populations and those among globular cluster stars (N, Al enhancements) present a curious coincidence that deserves future investigation.