Integrated Nebular Abundances of Disk Galaxies

Abstract

Chemical abundance measurements of distant galaxies are needed to constrain the chemical evolutionary histories predicted by theoretical models of galaxy formation. However, abundance measurements based on integrated spectroscopy may suffer a number of systematic effects due to abundance gradients, diffuse-ionized gas (DIG) emission, and dust reddening. Using spatially integrated spectrophotometry of 12 nearby spiral galaxies with well-defined H II-region abundance gradients, we investigate the reliability of integrated spectroscopy for deriving the nebular abundances of disk galaxies. Our principal findings are: (1) Integrated abundances based on the reddening-corrected R23 parameter reflect the gas-phase metallicity at an average galactocentric radius of (0.43+/-0.12) R25 with a precision of +/-20%. The observed scatter correlates with the slope of the abundance gradient, but not with the nebular reddening. (2) Integrated R23 abundances based on emission-line equivalent widths offer comparable precision to the reddening-corrected R23 parameter, but may be susceptible to additional systematic effects of order 0.1 dex. (3) We find a modest contribution from DIG emission to the integrated [N II] and [S II] fluxes of our galaxy sample, which may bias abundance diagnostics based on these emission lines. However, abundances based on R23 are relatively insensitive to DIG emission because R23 depends relatively weakly on variations in ionization.