A New Diagnostic Diagram of Ionization Source for High Redshift Emission
  Line Galaxies
  We propose a new diagram, the Kinematic-Excitation diagram (KEx diagram),
which uses the [OIII]/H\beta\ line ratio and the [OIII]5007 emission line width
(\sigma_{[OIII]}) to diagnose the ionization source and physical properties of
the Active Galactic Nuclei (AGNs) and the star-forming galaxies (SFGs). The KEx
diagram is a suitable tool to classify emission-line galaxies (ELGs) at
intermediate redshift because it uses only the [OIII]5007 and H\beta\ emission
lines. We use the SDSS DR7 main galaxy sample and the
Baldwin-Phillips-Terlevich (BPT) diagnostic to calibrate the diagram at low
redshift. We find that the diagram can be divided into 3 regions: one occupied
mainly by the pure AGNs (KEx-AGN region), one dominated by composite galaxies
(KEx-composite region), and one contains mostly SFGs (KEx-SFG region). AGNs are
separated from SFGs in this diagram mainly because they preferentially reside
in luminous and massive galaxies and have high [OIII]/H\beta. The separation of
AGN from star-forming galaxies is even cleaner thanks to the additional
0.15/0.12 dex offset in [OIII] line width at fixed luminosity/stellar mass. We
apply the KEx diagram to 7,866 galaxies at 0.3 < z < 1 in the DEEP2 Galaxy
Redshift Survey, and compare it to an independent X-ray classification scheme
using Chandra observation. 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 of Type1
AGNs lie in the KEx-AGN region. These confirm the reliability of this
classification diagram for emission line galaxies at intermediate redshift. At
z~2, the demarcation line between star-forming galaxies and AGNs should shift
0.3 dex higher in \sigma_{[OIII]} to account for evolution.
