  The Baldwin, Phillips, and Terlevich emission-line ratio diagnostic
([OIII]/H{\beta} versus [NII]/H{\alpha}, hereafter BPT diagram) efficiently
separates galaxies whose signal is dominated by star formation <BPT-SF> from
those dominated by AGN activity (BPT-AGN). Yet this BPT diagram is limited to z
< 0.5, the redshift at which [NII]{\lambda}6584 leaves the optical spectral
window. Using the Sloan Digital Sky Survey (SDSS), we construct a new
diagnostic, or TBT diagram, that is based on rest-frame g-z color,
[NeIII]{\lambda}3869, and [OII]{\lambda}{\lambda}3726 + 3729 and can be used
for galaxies out to z < 1.4. The TBT diagram identifies 98.7% of the SDSS
BPT-AGN as TBT-AGN and 97% of the SDSS BPT-SF as TBT-SF. Furthermore, it
identifies 97% of the OPTX Chandra X-ray selected AGNs as TBT-AGN. This is in
contrast to the BPT diagram, which misidentifies 20% of X-ray selected AGNs as
BPT-SF. We use the GOODS-N and Lockman Hole galaxy samples, with their
accompanying deep Chandra imaging, to perform X-ray and infrared stacking
analyses to further validate our TBT-AGN and TBT-SF selections; that is, we
verify the dominance of AGN activity in the former and star formation activity
in the latter. Finally, we address the inclusion of the majority of the
BPT-comp (sources lying between the BPT-SF and BPT-AGN regimes) in our TBT-AGN
regime. We find that the stacked BPT-comp source is X-ray hard (<{\Gamma}eff> =
1.0 +/-0.4) and has a high X-ray luminosity to total infrared luminosity ratio.
This suggests that, on average, the X-ray signal in BPT-comp is dominated by
obscured or low accretion rate AGN activity rather than by star formation,
supporting their inclusion in the TBT-AGN regime.

Cosmology and Nongalactic Astrophysics