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

摘要

We report the first detections of the [NⅡ] 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 EarlyUniverse 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-tocontinuum deficits seen in nearby sources. The line strengths also indicate large ionized gas fractions, ～8%–17% of the molecular gas mass. The [OⅢ]/[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 [OⅢ]/[N Ⅱ] line ratio for SMMJ02399-0136 indicates that the dominant source of the line emission is either stellar Hii 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 [NⅡ] line. The remainder may come from the NLR. This work demonstrates the utility of the [NⅡ] and [O Ⅲ] lines in constraining properties of the ionized medium.