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 2 hr exposures of 36 emission-line galaxies. We compare X-ray data with the traditional emission-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 [O III]/Hβ ratio is insufficient as an active galactic nucleus (AGN) indicator at z > 1. For the four X-ray-detected galaxies, the classic diagnostics ([O III]/Hβ versus [N II]/Hα and [S II]/Hα) remain 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 classic diagnostics. Compared to the X-ray and line ratio 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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Published in The Astrophysical Journal Letters, v. 763, no. 1, L6, p. 1-7.

© 2013. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

The copyright holder has granted the permission for posting the article here.

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J.R.T. and the authors from UCSC acknowledge support from NASA HST grants GO-12060.10-A and AR-12822.03, Chandra grant G08-9129A, and NSF grant AST-0808133. N.P.K. acknowledges support from NSF grant AST-1106171.

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This work uses data from the 3-D HST Treasury Survey (Program ID 12177).