ALMA reveals a chemically evolved submillimeter galaxy at z = 4.76

¹ The Hakubi Center for Advanced Research, Kyoto University, Kyoto 606-8302, Japan
e-mail: tohru@kusastro.kyoto-u.ac.jp
² Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
³ Cavendish Laboratory, Univerisy of Cambridge, 19 J. J. Thomson Ave., Cambridge CB3 0HE, UK
⁴ European Southern Observatory, Karl Schwarzschild Strasse 2, 85748 Garching, Germany
⁵ School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
⁶ East Asian ALMA Regional Center, National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan

Received: 2 May 2012
Accepted: 21 May 2012

Abstract

The chemical properties of high-z galaxies provide important information for constraining galaxy evolutionary scenarios. However, widely used metallicity diagnostics based on rest-frame optical emission lines are unusable for heavily dust-enshrouded galaxies (such as submillimeter galaxies; SMGs), especially at z > 3. Here we focus on the flux ratio of the far-infrared fine-structure emission lines [N ii] 205 μm and [C ii] 158 μm to assess the metallicity of high-z SMGs. Through ALMA cycle 0 observations, we have detected the [N ii] 205 μm emission in a strongly [C ii]-emitting SMG, LESS J033229.4–275619 at z = 4.76. The velocity-integrated [N ii]/[C ii] flux ratio is 0.043  ±  0.008. This is the first measurement of the [N ii]/[C ii] flux ratio in high-z galaxies, and the inferred flux ratio is similar to the ratio observed in the nearby universe (~0.02−0.07). The velocity-integrated flux ratio and photoionization models suggest that the metallicity in this SMG is consistent with solar, implying that the chemical evolution has progressed very rapidly in this system at z = 4.76. We also obtain a tight upper limit on the CO(12 − 11) transition, which translates into CO(12−11)/CO(2−1)  <  3.8 (3σ). This suggests that the molecular gas clouds in LESS J033229.4–275619 are not significantly affected by the radiation field emitted by the AGN in this system.