Testing Diagnostics of Nuclear Activity and Star Formation in Galaxies at z>1. (arXiv:1212.2218v1 [astro-ph.CO]):
We present some of the first science data with the new Keck/MOSFIRE
instrument to test the effectiveness of different AGN/SF diagnostics at z~1.5.
MOSFIRE spectra were obtained in three H-band multi-slit masks in the GOODS-S
field, resulting in two hour exposures of 36 emission-line galaxies. We compare
X-ray data with the traditional "BPT" line ratio diagnostics and the
alternative mass-excitation and color-excitation diagrams, combining new
MOSFIRE infrared data with previous HST/WFC3 infrared spectra (from the 3D-HST
survey) and multiwavelength photometry. We demonstrate that a high [OIII]/\Hb
ratio is insufficient as an AGN indicator at z>1. For the four X-ray detected
galaxies, the classic BPT diagnostic ([OIII]/Hb vs. [NII]/Ha and [SII]/Ha)
remains consistent with X-ray AGN/SF classification. The X-ray data also
suggest that "composite" galaxies (with intermediate AGN/SF classification)
host bona-fide AGNs. Nearly 2/3 of the z~1.5 emission-line galaxies have
nuclear activity detected by either X-rays or the BPT diagnostic. Compared to
the X-ray and BPT classifications, the mass-excitation method remains effective
at z>1, but we show that the color-excitation method requires a new calibration
to successfully identify AGNs at these redshifts.
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