ALMA Observations of [O i ] 145 μ m and [N ii ] 205 μ m Emission Lines from Star-forming Galaxies at z  ∼ 7

Abstract

We present results of new observations of [O i ] 145 μ m and [N ii ] 205 μ m emission lines from four star-forming galaxies at redshifts between z = 6.58 and 7.68 that have previous detections of [C ii ] 158 μ m and dust continua. Using the Atacama Large Millimeter/submillimeter Array, we successfully detect [O i ] 145 μ m emission from all targets at >4 σ significance. However, [N ii ] 205 μ m emission is undetected in all galaxies (signal-to-noise ratio < 3.5 σ ) except for a tentative detection from A1689-zD1. From the observed high [C ii ]/[N ii ] emission line ratios (≳20−80), we find that most of the [C ii ] 158 μ m emission arises from neutral gas regions (3 σ lower limits of ≳74%−96%). From [O i ] 145 μ m, [C ii ] 158 μ m lines, and IR luminosities, we estimate the neutral gas densities of n H  = 10 3.5 –10 6 cm −3 and the far-UV (FUV) radiation strengths of G 0  ∼ 10 2.5 –10 3 . While the neutral gas densities are similar to those of high-redshift starburst galaxies, the FUV strengths are lower compared to both local and high-redshift starbursts. Finally, we estimate atomic hydrogen masses using [O i ] 145 μ m emission lines and the oxygen abundances measured from recent JWST observations. We find gas mass ratios of f gas  ∼ 0.3–0.8, which are similar to earlier studies using [C ii ] 158 μ m. Starting from this pilot observation, future large [O i ] 145 μ m emission line surveys will provide us with currently little-known neutral gas properties of star-forming galaxies in the early Universe.