Chemical Abundances At Large Galactocentric Radii: A Progress Report

Abstract

We present a progress report on our ongoing project to study gas-phase chemical abundances at large galactocentric radii in a sample of nearby spirals. Initial results presented by Ferguson et al (1998a) are augmented with abundance determinations for several new outer disk HII regions in the galaxies NGC 628, NGC 1058 and NGC 6946. The mean oxygen abundance in the extreme outer disks (≳1.5–2R₂₅) is observed to be 10–20% solar, which although low, does indicate significant prior enrichment in these parts. Low dust extinction is also observed. Abundances generally decline smoothly with increasing radius, however detailed study of the shape of the gradient in the outer disk is hampered by the uncertainties present in chemical abundance determinations for low metallicity, low excitation HII regions. We discuss the preliminary implications of our results and briefly mention our plans to improve upon these measurements in the future.