We have used FMOS on Subaru to obtain near-infrared spectroscopy of 123
far-infrared selected galaxies in COSMOS and obtain the key rest-frame optical
emission lines. This is the largest sample of infrared galaxies with
near-infrared spectroscopy at these redshifts. The far-infrared selection
results in a sample of galaxies that are massive systems that span a range of
metallicities in comparison with previous optically selected surveys, and thus
has a higher AGN fraction and better samples the AGN branch. We establish the
presence of AGN and starbursts in this sample of (U)LIRGs selected as
Herschel-PACS and Spitzer-MIPS detections in two redshift bins (z~0.7 and
z~1.5) and test the redshift dependence of diagnostics used to separate AGN
from star-formation dominated galaxies. In addition, we construct a low
redshift (z~0.1) comparison sample of infrared selected galaxies and find that
the evolution from z~1.5 to today is consistent with an evolving AGN selection
line and a range of ISM conditions and metallicities from the models of Kewley
et al. (2013b). We find that a large fraction of (U)LIRGs are BPT-selected AGN
using their new, redshift-dependent classification line. We compare the
position of known X-ray detected AGN (67 in total) with the BPT selection and
find that the new classification line accurately selects most of these objects
(> 70%). Furthermore, we identify 35 new (likely obscured) AGN not selected as
such by their X-ray emission. Our results have direct implications for AGN
selection at higher redshift with either current (MOSFIRE, KMOS) or future
(PFS, MOONS) spectroscopic efforts with near-infrared spectral coverage.Comment: 7 pages, 3 figures, 2 tables. Accepted for publication in The
Astrophysical Journal Letter
