Optical Spectroscopy and Nebular Oxygen Abundances of the Spitzer/SINGS Galaxies

TL;DR

This study provides optical spectrophotometry data for 65 SINGS galaxies, classifies their nuclear activity, and estimates their nebular oxygen abundances, enhancing understanding of galaxy composition and activity.

Contribution

It offers a comprehensive dataset of optical spectra and nebular abundances for SINGS galaxies, combining new observations with literature data to analyze galaxy activity and chemical gradients.

Findings

The fraction of star-forming vs. AGN galaxies depends on the spectroscopic aperture size.

Radial abundance gradients vary among galaxies and are linked to galaxy type.

Estimated nebular oxygen abundances have quantified uncertainties.

Abstract

We present intermediate-resolution optical spectrophotometry of 65 galaxies obtained in support of the Spitzer Infrared Nearby Galaxies Survey (SINGS). For each galaxy we obtain a nuclear, circumnuclear, and semi-integrated optical spectrum designed to coincide spatially with mid- and far-infrared spectroscopy from the Spitzer Space Telescope. We make the reduced, spectrophotometrically calibrated one-dimensional spectra, as well as measurements of the fluxes and equivalent widths of the strong nebular emission lines, publically available. We use optical emission-line ratios measured on all three spatial scales to classify the sample into star-forming, active galactic nuclei (AGN), and galaxies with a mixture of star formation and nuclear activity. We find that the relative fraction of the sample classified as star-forming versus AGN is a strong function of the integrated light enclosed by the spectroscopic aperture. We supplement our observations with a large database of nebular emission-line measurements of individual HII regions in the SINGS galaxies culled from the literature. We use these ancillary data to conduct a detailed analysis of the radial abundance gradients and average HII-region abundances of a large fraction of the sample. We combine these results with our new integrated spectra to estimate the central and characteristic (globally-averaged) gas-phase oxygen abundances of all 75 SINGS galaxies. We conclude with an in-depth discussion of the absolute uncertainty in the nebular oxygen abundance scale.