Galaxy spectral synthesis. I - Stellar populations in the nuclei of giant ellipticals

We develop an automatic spectral synthesis technique based on linear programming and apply it to narrow-band spectrophotometry covering 3300-10800 Angstroms for the nuclei of M31 and three giant elliptical galaxies in Virgo. The mean energy distribution of the gE nuclei (r < 400 pc) is well defined and, within the uncertainty in E(B-V), does not differ significantly from that for M31 (r < 50 pc). The observations are consistent with only a small range of main-sequence turnoffs, which depends on the relative rates of main-sequence and subgiant evolution. Using Iben's (1967) models of stellar interiors, we find the bulk of the stars in gE nuclei formed 8-11 Gyr ago, with star formation possibly continuing at a much reduced level until ~4 Gyr ago. The maximum mean rate of postburst star formation consistent with the data is ~2.9 x [L_v/ 10^11 L_v_sun] M_sun/yr. There is no evidence of a significant Population II component. Na I and Mg I exhibit distinct enhancements with respect to Ca II and the Fe-peak, but the evidence for Fe-peak enrichment is not strong. M giants dominate M dwarfs throughout the spectrum, and M/L_v > 30 (solar units) is excluded if stars with absolute V magnitude > 18 are not important. The best models yield 2 < M/L_v < 15, in agreement with recent dynamical estimates. Infrared spectral features indicate that short-lived, low-gravity phases near the tip of the asymptotic giant branch make significant contributions. There is marginal evidence that the lower main sequence for absolute V magnitude > 11 is enriched, as suggested for M31 by Spinrad and Taylor (1971). Weak [O II] 3727 Angstrom emission is present in two of the gE's, and we find M(H II) < 4 x 10^6 M_sun for the nucleus of NGC 4552. We describe a new method for estimating the H I content of elliptical nuclei, which here yields M(H I) < 5 x 10^6 M_sun. The observations do not exclude a complete recycling into stars of mass shed during stellar evolution at current rates. Aside from marginal evidence for a weak optical nonthermal component in NGC 4552, there are no marked optical peculiarities associated with the radio sources in the gE's, and their general properties are consistent with a model in which the radio energy is produced by supernovae which form in the nucleus following mass loss during stellar evolution.