A New Diagnostic Diagram of Ionization Sources for High-redshift Emission Line Galaxies

Abstract

We propose a new diagram, the kinematics–excitation (KEx) diagram, which uses the [O iii ] λ 5007/H β line ratio and the [O iii ] λ 5007 emission line width ( σ [O iii ] ) to diagnose the ionization source and physical properties of active galactic nuclei (AGNs) and star-forming galaxies (SFGs). The KEx diagram is a suitable tool to classify emission line galaxies at intermediate redshift because it uses only the [O iii ] λ 5007 and H β emission lines. We use the main galaxy sample of SDSS DR7 and the Baldwin−Phillips−Terlevich (BPT) diagnostic to calibrate the diagram at low redshift. The diagram can be divided into three regions: the KEx-AGN region, which consists mainly of pure AGNs, the KEx-composite region, which is dominated by composite galaxies, and the KEx-SFG region, which contains mostly SFGs. LINERs strongly overlap with the composite and AGN regions. AGNs are separated from SFGs in this diagram mainly because they preferentially reside in luminous and massive galaxies and have higher [O iii ]/H β than SFGs. The separation between AGNs and SFGs is even cleaner thanks to the additional 0.15/0.12 dex offset in σ [O iii ] at fixed luminosity/stellar mass. We apply the KEx diagram to 7866 galaxies at 0.3 <  z  < 1 in the DEEP2 Galaxy Redshift Survey, and compare it to an independent X-ray classification scheme using Chandra observations. X-ray AGNs are mostly located in the KEx-AGN region, while X-ray SFGs are mostly located in the KEx-SFG region. Almost all Type 1 AGNs lie in the KEx-AGN region. These tests support the reliability of this classification diagram for emission line galaxies at intermediate redshift. At z  ∼ 2, the demarcation line between SFGs and AGNs is shifted by ∼0.3 dex toward higher values of σ [O iii ] due to evolution effects.