The Star Formation Reference Survey. II. Activity demographics and host-galaxy properties for Infrared-selected galaxies

We present activity demographics and host-galaxy properties of infrared-selected galaxies in the local Universe, using the representative Star Formation Reference Survey (SFRS). Our classification scheme is based on a combination of optical emission-line diagrams (BPT) and IR-color diagnostics. Using the weights assigned to the SFRS galaxies based on its parent sample, a far-infrared-selected sample comprises 71% H,$\text{textsc}ii$ galaxies, 13% Seyferts, 3% Transition Objects (TOs), and 13% Low-Ionization Nuclear Emission-Line Regions (LINERs). For the SFRS H,$\text{textsc}ii$ galaxies we derive nuclear star-formation rates and gas-phase metallicities. We measure host-galaxy metallicities for all galaxies with available long-slit spectroscopy and abundance gradients for a subset of 12 face-on galaxies. The majority of H,$\text{textsc}ii$ galaxies show a narrow range of metallicities, close to solar, and flat metallicity profiles. Based on their host-galaxy and nuclear properties, the dominant ionizing source in the far-infrared selected TOs is star-forming activity. LINERs are found mostly in massive hosts (median of ${10}^{10.5}$ M${}_{\odot }$), median $L(60\mu m)={10}^9$ L${}_{\odot }$, median dust temperatures of $F60\mathrm{/}F100=0.36$, and median $L_{\text{H}\alpha }$ surface density of ${10}^{40.2}$ erg s${}^{-1}$kpc${}^{-2}$, indicating older stellar populations as their main ionizing source rather than AGN activity.