2020/09/13 by H. F. M. Yao, T. H. Jarrett, M. E. Cluver +12
Physics and Astronomy · #Active galactic nucleus #Astronomy and Astrophysical Research #Astrophysics and Star Formation Studies #Extragalactic astronomy #Galaxies: Formation, Evolution, Phenomena #Galaxy #Galaxy formation and evolution #Luminosity #Luminous infrared galaxy #Photometry (optics) #Spurious relationship #astro-ph.GA
paper · pdf · doi:10.3847/1538-4357/abba1a
43 pages, 32 figures, 4 tables
arxiv created 2020/09/13 · openalex created_date 2020/09/21 · openalex publication_date 2020/11/01 · arxiv updated 2020/11/18 · openalex updated_date 2026/08/05
Abstract We present a detailed study of emission-line systems in the Galaxy And Mass Assembly (GAMA) G23 region, making use of Wide-field Infrared Survey Explorer (WISE) photometry that includes carefully measured resolved sources. After applying several cuts to the initial catalog of ∼41,000 galaxies, we extract a sample of 9809 galaxies. We then compare the spectral diagnostic Baldwin, Philips & Terlevich (BPT) classification of 1154 emission-line galaxies (38% resolved in W1) to their location in the WISE color–color diagram, leading to the creation of a new zone for mid-infrared “warm” galaxies located 2 σ above the star-forming sequence, below the standard WISE active galactic nucleus (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. Thirty-nine percent of the AGNs (all types) are broad-line systems for which the [N ii ] and [H α ] fluxes can barely be disentangled, giving in most cases spurious [N ii ]/[H α ] flux ratios. However, several optical AGNs appear to be completely consistent with SF in WISE. We argue that these could be low-power AGNs, 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 AGN activity. Based on our findings, we have created a new diagnostic: [W1 – W2] versus [N ii ]/[H α ], which has the virtue of separating SF from AGNs and high-excitation sources. It classifies 3 to ∼5 times more galaxies than the classic BPT.