First detections of the [n ii] 122 μm line at high redshift: demonstrating the utility of the line for studying galaxies in the early universe

We report the first detections of the [N II] 122 μm line from a high-redshift galaxy. The line was strongly (>6σ) detected from SMMJ02399–0136, and H1413+117 (the Cloverleaf QSO) using the Redshift (z) and Early Universe Spectrometer on the Caltech Submillimeter Observatory. The lines from both sources are quite bright with line to far-infrared (FIR) continuum luminosity ratios that are ~7.0 × 10–4 (Cloverleaf) and 2.1 × 10–3 (SMMJ02399). With ratios 2-10 times larger than the average value for nearby galaxies, neither source exhibits the line-to-continuum deficits seen in nearby sources. The line strengths also indicate large ionized gas fractions, ~8%-17% of the molecular gas mass. The [O III]/[N II] line ratio is very sensitive to the effective temperature of ionizing stars and the ionization parameter for emission arising in the narrow-line region (NLR) of an active galactic nucleus (AGN). Using our previous detection of the [O III] 88 μm line, the [O III]/[N II] line ratio for SMMJ02399–0136 indicates that the dominant source of the line emission is either stellar H II regions ionized by O9.5 stars, or the NLR of the AGN with ionization parameter log(U) = –3.3 to –4.0. A composite system, where 30%-50% of the FIR lines arise in the NLR also matches the data. The Cloverleaf is best modeled by a superposition of ~200 M82-like starbursts accounting for all of the FIR emission and 43% of the [N II] line. The remainder may come from the NLR. This work demonstrates the utility of the [N II] and [O III] lines in constraining properties of the ionized medium