Galaxy and Mass Assembly (GAMA): A $\textit{WISE}$ study of the activity of emission-line systems in G23

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.