Abstract
Abstract
In this paper, we investigate the robustness of Wide-field Infrared Survey Explorer mid-IR color selection (W1 − W2) for identifying obscured (type 2) active galactic nuclei (AGNs) at low redshift (
z
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 an AGN contributing ≥75% of the total infrared luminosity, which is uncommon. Although mid-IR-selected Sy2s tend to be luminous, the high [O
III
] luminosity does not guarantee red W1 − W2 (nor does any other tested global or narrow-line-region-scale parameter), suggesting the critical role of obscuration on smaller scales. Less than 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
bulge
≳ 10
10
M
⊙
) predominantly (84%) manifest themselves as BPT-AGNs, whereas those in low-mass bulges (≲10
10
M
⊙
) mostly (60%) manifest as BPT-SF. This BPT-AGN versus 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.