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

We report the discovery of strongly absorbed [O I] 63 mu m 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 mu m 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 mu m 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 mu m and [N II] 205 mu m. We identify several compact (0.05-0.2 '') regions with [O I] 63 mu m 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 mu m 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 mu m line, particularly when concentrating on the regions with the densest foreground material along the line of sight. The [O I] 63 mu m 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 mu m -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 mu m line emission. We argue that the high [O I] 63 mu m 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.