Galaxy spectral synthesis. II - M32 and the ages of galaxies

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Abstract
We perform a population synthesis on absolute spectrophotometry for the central 31" of M32. The main-sequence turnoff is at (B-V) ~ 0.5 or F8 equivalent spectral type. A variety of models for cooler turnoffs are considered and found to be excluded by the data. The metallicity of M32 is solar within ~ 0.1 dex. Taken together, these results imply that major star formation continued in M32 until ~ 5 Gyr ago, or 10 Gyr after the oldest globular clusters formed. Perhaps 50% of the giant light could arise in a significantly older population, and the peak star formation rate could have occurred much earlier. However, significant changes in the colors of low-mass elliptical galaxies are expected at lookback times of ~ 5 Gyr. The anomalously blue galaxies now being observed in distant clusters may be such objects. A tentative age dating for gE nuclei indicates an upper limit for the turnoff age of 6-8 Gyr. The synthesis models predict that the rate of mass return to the interstellar medium of M32 from evolving stars is ~8 x 10^-4 M_sun/yr, which is in good agreement with counts of planetary nebulae. However, the upper limit for star formation during the past 1 Gyr is ~ 3 x 10^-3 M_sun/yr. Thus, the available optical observations do not exclude complete recycling of gas lost during stellar evolution into new generations of stars. Ultraviolet observations are required to demonstrate the need for galactic winds or other special gas removal mechanisms in elliptical galaxies.

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