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 M giants dominate M dwarfs throughout the spectrum, and M/Lᵥ > 30 (solar units) is excluded if stars with absolute V magnitude > 18 are not important. The best models yield 2 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 10⁶ Mₛun 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 10⁶ Mₛun. 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.