Stellar Evolution in Elliptical Galaxies

Elliptical galaxies are described by models in which the evolution of stars is followed for 12 billion years. Stellar birthrates chosen are mainly power laws in stellar mass, cut off after 10 years; effects of these oversimplifications are discussed. Integrated magnitudes, broad-band colors, narrow-band scanner spectral energy distributions, and strengths of spectral features are derived as a function of time. Comparison of computed with empirical energy distributions sets limits to the galactic age for each initial mass function (IMF). Unless the Universe is extremely young, a fairly shallow IMF is required. There are then few late dwarfs to provide unchanging light, so significant evolution toward fainter luminosities must occur. The magnitude-redshift relation is affected in such a way that q0 appears larger than its true value by at least 0.5. The colors of models with a shallow IMF evolve very slowly, however, because much of their light comes from low-mass giants whose colors depend very weakly on mass; this result agrees with colors of galaxies observed at large redshifts. The possibility of continued star formation in the nuclei of some ellipticals, from gas shed by stars dying throughout the galaxy, can explain the occurrence of Balmer absorption lines and emission lines from H ii regions. Production of metals during the first 10 years is calculated, leading to an explanation of the observed increase in stellar metal abundance and redness toward the center. The computed abundances are lower than observed, however. If a preceding short burst of very massive stars has occurred, these can produce enough metals, leave invisible stellar remnants to provide the observed high mass-to-light ratios, and radiatively ionize the intergalactic medium at the epoch of galaxy formation.