Star-forming Rings in Lenticular Galaxies: Origin of the Gas ∗

Abstract

Rings in S0s are enigmatic features that can, however, betray the evolutionary paths of particular galaxies. We have undertaken long-slit spectroscopy of five lenticular galaxies with UV-bright outer rings. The observations have been made with the Southern African Large Telescope to reveal the kinematics, chemistry, and ages of the stellar populations and the gas characteristics in the rings and surrounding disks. Four of the five rings are also bright in the H α emission line, and the spectra of the gaseous rings extracted around the maxima of the H α equivalent width reveal excitation by young stars betraying current star formation in the rings. The integrated level of this star formation is 0.1–0.2 yr −1 , with the outstanding value of 1 yr −1 in NGC 7808. The difference of chemical composition between the ionized gas of the rings, which demonstrate nearly solar metallicity, and the underlying stellar disks, which are metal-poor, implies recent accretion of the gas and star formation ignition; the star formation history estimated by using different star formation indicators implies that the star formation rate decreases with an e -folding time of less than 1 Gyr. In NGC 809, where the UV ring is well visible but the H α emission line excited by massive stars is absent, the star formation has already ceased.