Exploring the Connection Between Star Formation and AGN Activity in the Local Universe

摘要

We study a combined sample of 264 star-forming, 51 composite, and 73 activegalaxies using optical spectra from SDSS and mid-infrared (mid-IR) spectra fromthe Spitzer Infrared Spectrograph. We examine optical and mid-IR spectroscopicdiagnostics that probe the amount of star formation and relative energeticcontributions from star formation and an active galactic nucleus (AGN). Overallwe find good agreement between optical and mid-IR diagnostics.Misclassifications of galaxies based on the SDSS spectra are rare despite thepresence of dust obscuration. The luminosity of the [NeII] 12.8 \mu memission-line is well correlated with the star formation rate (SFR) measuredfrom the SDSS spectra, and this holds for the star forming, composite, andAGN-dominated systems. AGN show a clear excess of [NeIII] 15.6 \mu m emissionrelative to star forming and composite systems. We find good qualitativeagreement between various parameters that probe the relative contributions ofthe AGN and star formation, including: the mid-IR spectral slope, the ratio ofthe [NeV] 14.3 \mu m to [NeII] \mu m 12.8 fluxes, the equivalent widths of the7.7, 11.3, and 17 $\mu m$ PAH features, and the optical "D" parameter whichmeasures the distance a source lies from the locus of star forming galaxies inthe optical BPT emission-line diagnostic diagram. We also consider the behaviorof the three individual PAH features by examining how their flux ratios dependupon the degree of AGN-dominance. We find that the PAH 11.3 \mu m feature issignificantly suppressed in the most AGN-dominated systems.