  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.
