KINEMATICS AND STELLAR POPULATIONS IN ISOLATED LENTICULAR GALAXIES

Abstract

By combining new long-slit spectral data obtained with the Southern African Large Telescope for nine galaxies with our previously published observations for 12 additional galaxies, we study the stellar and gaseous kinematics as well as radially resolved stellar population properties and ionized-gas metallicity and excitation for a sample of isolated lenticular galaxies. We have found that there is no particular time frame of formation for the isolated lenticular galaxies: the mean stellar ages of the bulges and disks are distributed between 1 and >13 Gyr, and the bulge and the disk in every galaxy formed synchronously demonstrate similar stellar ages and magnesium-to-iron ratios. Extended ionized-gas disks are found in the majority of the isolated lenticular galaxies, in 72% ±11%. Half of all extended gaseous disks demonstrate a visible counterrotation with respect to their stellar counterparts. We argue that just such a fraction of projected counterrotation is expected if all of the gas in isolated lenticular galaxies is accreted from outside, under the assumption of isotropically distributed external sources. The very narrow range of the gas oxygen abundances we found for the outer ionized-gas disks excited by young stars, [O/H] from 0.0 to +0.2 dex, gives evidence for satellite merging as the most probable source of this accretion. Last, we formulate a hypothesis that the morphological type of a field disk galaxy is completely determined by the outer cold-gas accretion regime.