Rest-frame Optical Emission Lines in Far-Infrared Selected Galaxies at z<1.7 from the FMOS-COSMOS Survey

TL;DR

This study uses near-infrared spectroscopy of far-infrared selected galaxies at z<1.7 to analyze emission lines, revealing the evolution of AGN diagnostics and identifying new obscured AGN, with implications for future high-redshift surveys.

Contribution

It provides the largest near-infrared spectroscopic sample of infrared galaxies at these redshifts and develops a redshift-dependent AGN classification line.

Findings

A large fraction of (U)LIRGs are BPT-selected AGN.

The new classification line accurately identifies >70% of X-ray AGN.

Identified 35 new obscured AGN not detected in X-ray.

Abstract

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.