The central finding is that [OI]63um shows strong absorption against the continuum in most of the 12 SPT-selected DSFGs at 4.2
Referee Report
2 major / 2 minor
Summary. The paper reports the first systematic ALMA survey of the [OI]63μm line in 12 SPT-selected DSFGs at 4.2<z<5.8 using Bands 9 and 10. Despite sensitivities 10-100× deeper than prior work, emission is detected in only two sources at low significance; the remainder show non-detections across the velocity range where [CII] and [NII]205μm are robustly seen. Compact (0.05–0.2″) absorption features against the dust continuum are identified in several sources, some possibly below the rest-frame CMB level, together with narrow, spatially localized emission “escape channels” in regions of weak continuum. The authors interpret the pattern as arising from a combination of high-optical-depth compact [OI]-emitting regions and sub-thermally excited foreground molecular gas, supported by comparison to cosmological radiation-hydrodynamical simulations, and conclude that high [OI] optical depth dominates, thereby limiting the line’s diagnostic utility for massive star formation in high-z DSFGs.
Significance. If the absorption features are confirmed to be astrophysical, the work supplies the first statistical view of [OI]63μm at z>4 and demonstrates that optical-depth suppression can be severe in dense, high-column-density environments. This has direct implications for the use of far-IR fine-structure lines as star-formation tracers at high redshift and motivates caution when interpreting [OI] (and potentially [CII]) in DSFGs. The simulation comparison provides an independent physical anchor that strengthens the optical-depth interpretation. The study is observationally grounded and highlights a previously under-appreciated limitation of an important ISM diagnostic.
major comments (2)
- [Observations and Data Reduction] Observations and Data Reduction section: The central claim—that the reported non-detections and compact absorption features are produced by high foreground [OI] optical depth rather than instrumental effects—requires explicit validation against known ALMA Bands 9/10 systematics (atmospheric transmission, receiver noise, continuum-subtraction residuals). The manuscript should present jackknife maps, alternative self-calibration runs, or independent pipeline re-reductions that demonstrate the negative features persist and are not localized to the highest-frequency data. Without these tests the optical-depth interpretation remains vulnerable to the calibration concern raised in the stress-test note.
- [Results and Discussion] Results and Discussion sections: The paper states that absorption occurs in regions with higher gas and dust column densities and that escape channels appear where continuum is weak. It would strengthen the argument to show a quantitative correlation (e.g., absorption depth versus local continuum surface brightness or [CII] velocity dispersion) rather than qualitative statements, and to report the precise velocity ranges and spatial masks used for the non-detection limits.
minor comments (2)
- [Figures] Figure captions and legends should explicitly state the 1σ noise level, the velocity range over which non-detections are claimed, and whether the spectra are continuum-subtracted or not for each source.
- [Abstract and Introduction] The abstract and introduction would benefit from a brief statement of the typical [OI]63μm luminosities or line-to-FIR ratios expected from lower-redshift samples to quantify how extreme the non-detections are.
Simulated Author's Rebuttal
2 responses · 0
unresolved
We thank the referee for their constructive review and recommendation for minor revision. Their comments have prompted us to strengthen the validation of our data reduction and to add quantitative support for the absorption interpretation. We address each major comment below and have revised the manuscript accordingly.
read point-by-point responses
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Authors: We appreciate the referee's emphasis on ruling out instrumental artifacts. In the revised manuscript we have added a new subsection (2.3) that presents jackknife maps constructed by splitting each dataset into independent time chunks, results from two alternative self-calibration strategies (phase-only and amplitude+phase), and an independent re-reduction performed with the ALMA pipeline v2023.1. These tests show that the compact negative features remain at the same locations and depths, are not confined to the highest-frequency spectral windows, and exhibit no correlation with atmospheric transmission windows or receiver noise spikes. We have included the corresponding figures in the revised Appendix B and updated the main text to reference them explicitly.
revision: yes
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Authors: We agree that quantitative correlations and precise methodological details will improve clarity. The revised Results section now includes a new figure (Figure 6) that plots [OI] absorption depth against local 870 μm continuum surface brightness and against [CII] velocity dispersion measured in the same 0.2-arcsec apertures; a statistically significant positive correlation is found in both cases. We have also added explicit statements of the velocity ranges adopted for the non-detection limits (the full velocity interval over which [CII] is detected at >5σ, typically −250 to +250 km s⁻¹ relative to the [CII] systemic velocity) and the spatial masks employed (circular apertures of radius 0.15–0.25 arcsec centered on the peak continuum emission). These additions are referenced in both the Results and Discussion sections.
revision: yes
Circularity Check
0 steps flagged
No circularity: purely observational results grounded in external data and simulations
full rationale
This is an observational paper reporting ALMA Band 9/10 detections, non-detections, and absorption features in [OI]63um for 12 high-z DSFGs, with comparisons to independent cosmological radiation hydrodynamical simulations. No equations, fitted parameters, or derivations are present that reduce to the paper's own inputs by construction. The central interpretation (high optical depth causing absorption) is supported by direct observations and external benchmarks rather than self-referential steps. No self-citations, ansatzes, or uniqueness claims are load-bearing in the provided text.
Axiom & Free-Parameter Ledger
0 free parameters ·
1 axioms ·
0 invented entities
The interpretation rests on standard astrophysical assumptions about line excitation, optical depth, and dust continuum emission in star-forming galaxies; no new free parameters or invented entities are introduced.
axioms (1)
- domain assumption Standard interstellar medium physics for [OI] fine-structure line formation and optical depth effects in dense gas
Invoked to interpret absorption against continuum and non-detections relative to [CII] and [NII]
pith-pipeline@v0.9.0 ·
5748 in / 1320 out tokens ·
64157 ms ·
2026-05-15T19:23:54.554202+00:00
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