Uncovering the absorbed atomic Universe with the [OI]63 μm line

¹ European Southern Observatory, Karl Schwarzschild Straße 2, 85748 Garching, Germany
² Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago de Chile, Chile
³ National Astronomical Observatory of Japan, Los Abedules 3085 Oficina 701 Vitacura 763 0414, Santiago, Chile
⁴ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago de Chile, Chile
⁵ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y 455 Ciencias, Universidad Diego Portales, Av. Ejército Libertador 441 Santiago, Chile
⁶ Sterrenkundig Observatorium, Ghent University, Krijgslaan 281–S9, B-9000 Gent, Belgium
⁷ Max Planck Institute für Astrophysik, Karl-Schwarzschild-Strasse 1, 85748 Garching, Germany
⁸ Cornell Center for Astrophysics and Planetary Sciences, Cornell University, Ithaca, NY 14853, USA
⁹ Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 19 December 2025
Accepted: 24 February 2026

Abstract

We report the discovery of strongly absorbed [O I] 63 μm in a sample of 12 dusty star-forming galaxies (DSFGs) at 4.2 < z < 5.8. This is the first systematic survey of the [O I] 63 μm fine-structure line at z > 4, targeting a sub-sample of gravitationally lensed DSFGs selected from the South Pole Telescope survey. Using ALMA Bands 9 and 10, we obtained spatially and spectrally resolved observations that probe the interstellar medium on sub-kiloparsec scales. Despite reaching sensitivities one to two orders of magnitude deeper than most previous studies, we detect [O I] 63 μm in emission in only two sources at low significance, with the remaining galaxies yielding stringent non-detections over the full velocity range covered by robust detections of other far-infrared lines, including [C II] 158 μm and [N II] 205 μm. We identify several compact (0.05–0.2″) regions with [O I] 63 μm absorption against the far-infrared dust continuum, some of which are possibly reaching below rest-frame cosmic-microwave-background (CMB) radiation level. This suggests the presence of low-excitation-temperature (Tex ≤ T_CMB(z)), low-density gas along those lines of sight. We also detect narrow, spatially localised [O I] 63 μm emission ’escape channels’ preferentially detected in regions with weak or absent dust continuum emission. We also predict that similar absorption effects may appear in the [C II] 158 μm line, particularly when concentrating on the regions with the densest foreground material along the line of sight. The [O I] 63 μm line appears to be strongly affected by the influence of extended star forming regions, with a mix of compact, high-optical-depth [O I] 63 μm -emitting regions and sub-thermally excited, oxygen-rich molecular clouds dispersed throughout high-redshift starbursts that are capable of absorbing the ground-state line emission. Combined with a comparison to cosmological radiation hydrodynamical simulations, this supports the interpretation that regions with higher gas and dust column densities may lead to weakening an intrinsically strong [O I] 63 μm line emission. We argue that the high [O I] 63 μm optical depth is the dominant effect causing the strong absorption, limiting the diagnostic power of this line to trace regions of massive star formation in high-redshift DSFGs.