| dc.description.abstract | We present a detailed study of emission-line systems in the GAMA G23 region, making use of WISE
photometry that includes carefully measured resolved sources. After applying several cuts to the initial
catalogue of ∼41,000 galaxies, we extract a sample of 9,809 galaxies. We then compare the spectral
diagnostic (BPT) classification of 1154 emission-line galaxies (38% resolved in W1) to their location in
the WISE colour-colour diagram, leading to the creation of a new zone for mid-infrared “warm” galaxies
located 2σ above the star-forming sequence, below the standard WISE AGN region. We find that the
BPT and WISE diagrams agree on the classification for 85% and 8% of the galaxies as non-AGN (star
forming = SF) and AGN, respectively, and disagree on ∼7% of the entire classified sample. 39% of the
AGN (all types) are broad-line systems for which the [N ii] and [Hα] fluxes can barely be disentangled,
giving in most cases spurious [N ii]/[Hα] flux ratios. However, several optical AGN appear to be
completely consistent with SF in WISE. We argue that these could be low power AGN, or systems
whose hosts dominate the IR emission. Alternatively, given the sometimes high [O iii] luminosity in
these galaxies, the emission lines may be generated by shocks coming from super-winds associated
with SF rather than the AGN activity. Based on our findings, we have created a new diagnostic:
[W1-W2] vs [N ii]/[Hα], which has the virtue of separating SF from AGN and high-excitation sources.
It classifies 3∼5 times more galaxies than the classic BPT. | en_US |
