A Comprehensive Analysis of WISE Mid-Infrared Colors for Obscured AGN Selection

Abstract

In this paper, we investigate the robustness of WISE mid-IR color selection (W1-W2) for identifying obscured (Type 2) active galactic nuclei (AGNs) at low redshift (z<0.3), using a sample of ~360,000 SDSS galaxies classified via emission lines into Seyfert 2 (Sy2), LINER, and star-forming (BPT-SF) galaxies. We find that the K-correction is essential to remove non-AGN contamination, and once applied the simple W1-W2>0.5 selection emerges as optimal in terms of purity and completeness of AGN selection. However, we confirm that even this lenient cut selects only ~13% of Sy2 galaxies and that achieving W1-W2>0.5 requires AGN contributing >75% of the total infrared luminosity, which is uncommon. Although mid-IR-selected Sy2s tend to be luminous, the high [OIII] luminosity does not guarantee red W1-W2 (nor does any other tested global or NLR-scale parameter), suggesting the critical role of obscuration on smaller scales. <1% of BPT-SF systems (but making ~20% of all mid-IR selected galaxies) exhibit W1-W2>0.5 colors. Such colors cannot be reproduced by models of star-heated dust alone. Red BPT-SFs tend to have higher W4 luminosities than expected from SF, indicating true AGNs. Intriguingly, mid-IR AGNs in massive bulges ($M_{\mathrm{bulge}} \gtrsim 10^{10} M_{\odot}$) predominantly (84%) manifest themselves as BPT-AGNs, whereas those in low-mass bulges ($\lesssim 10^{10} M_{\odot}$) mostly (60%) manifest as BPT-SF. This BPT-AGN vs.\ BPT-SF dichotomy does not extend to total stellar mass. We conclude that although the mid-IR AGN selection is incomplete, its strength lies in identifying optically inconspicuous AGNs with low-mass bulges, regardless of the total mass.