The MOSDEF Survey: Optical AGN Diagnostics at z~2.3
  We present results from the MOSFIRE Deep Evolution Field (MOSDEF) survey on
rest-frame optical AGN identification and completeness at z~2.3. With our
sample of 50 galaxies and 10 X-ray and IR-selected AGN with measured H-beta,
[OIII], H-alpha, and [NII] emission lines, we investigate the location of AGN
in the BPT, MEx (mass-excitation), and CEx (color-excitation) diagrams. We find
that the BPT diagram works well to identify AGN at z~2.3 and that the z~0
AGN/star-forming galaxy classifications do not need to shift substantially at
z~2.3 to robustly separate these populations. However, the MEx diagram fails to
identify all of the AGN identified in the BPT diagram, and the CEx diagram is
substantially contaminated at high redshift. We further show that AGN samples
selected using the BPT diagram have selection biases in terms of both host
stellar mass and stellar population, in that AGN in low mass and/or high
specific star formation rate galaxies are difficult to identify using the BPT
diagram. These selection biases become increasingly severe at high redshift,
such that optically-selected AGN samples at high redshift will necessarily be
incomplete. We also find that the gas in the narrow-line region appears to be
more enriched than gas in the host galaxy for at least some MOSDEF AGN.
However, AGN at z~2 are generally less enriched than local AGN with the same
host stellar mass.
