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arxiv: 2606.31546 · v1 · pith:SHUIRRSDnew · submitted 2026-06-30 · 🌌 astro-ph.GA

Tracing Cold Gas in Absorption Across Cosmic Time with the SKA

Elizabeth K. Mahony , Neeraj Gupta , Sergei A. Balashev , Yogesh Chandola , Francoise Combes , Rebecca Davies , Jens-Kristian Krogager , Wenkai Hu
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Filippo M. Maccagni Pasquier Noterdaeme Mamta Pandey-Pommier Elaine M. Sadler Rasha M. Samir Nick Seymour Hyein Yoon
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Pith reviewed 2026-07-01 04:46 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords HI absorptioncold gasSKAOcosmic evolutionOH absorptiongalaxy feedback21-cm lineneutral hydrogen
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The pith

The SKAO will detect HI absorption from redshift zero to beyond six, characterising cold gas properties in galaxies at all epochs.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

Observing the 21-cm HI line in absorption traces cold neutral gas in galaxies across time. The SKAO frequency coverage and sensitivity will enable such detections from the present day out to redshifts greater than six. The work also identifies new opportunities to search simultaneously for OH 18-cm absorption as a tracer of diffuse molecular gas. These capabilities, combined with sub-arcsecond imaging and multi-wavelength data, will address questions on AGN fuelling, feedback, and the evolution of cold neutral gas.

Core claim

The frequency coverage and sensitivity of SKAO Array Assembly 4 will allow detection of HI in absorption from z=0 to beyond z=6, plus simultaneous OH absorption, enabling characterisation of cold atomic and molecular gas in and around galaxies at all epochs, supported by high-resolution spectroscopic imaging and large multi-wavelength surveys.

What carries the argument

The 21-cm HI absorption line and 18-cm OH absorption line, observed across the wide frequency range provided by SKAO receivers and sensitivity.

If this is right

  • Properties of cold gas in and around galaxies can be measured at every cosmic epoch.
  • The cycles of fuelling and feedback in active galactic nuclei can be studied through their effects on cold gas.
  • The overall evolution of cold neutral gas content across cosmic time can be tracked directly.
  • Simultaneous atomic and molecular gas tracers become available in the same observations.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Coordinated use with large optical and infrared surveys would provide environmental context for the detected gas systems.
  • High-redshift absorption detections could test models of how early galaxies assembled their gas reservoirs.
  • Sub-arcsecond resolution might resolve internal gas structures within individual distant galaxies.

Load-bearing premise

The SKA design baseline Array Assembly 4 will achieve the frequency coverage and sensitivity levels stated in the design specifications.

What would settle it

A measurement showing that the deployed SKAO sensitivity or frequency coverage falls short of the levels needed to detect the projected HI absorption lines at redshifts above six.

Figures

Figures reproduced from arXiv: 2606.31546 by Elaine M. Sadler, Elizabeth K. Mahony, Filippo M. Maccagni, Francoise Combes, Hyein Yoon, Jens-Kristian Krogager, Mamta Pandey-Pommier, Neeraj Gupta, Nick Seymour, Pasquier Noterdaeme, Rasha M. Samir, Rebecca Davies, Sergei A. Balashev, Wenkai Hu, Yogesh Chandola.

Figure 1
Figure 1. Figure 1: Schematic of intervening H i absorption (left) and associated H i absorption (right). The principal observable in H i absorption studies is the line optical depth, 𝜏(𝑣), which describes the fractional attenuation of background radio continuum as a function of velocity: 𝜏(𝑣) = − ln  1 + Δ𝑆(𝑣) 𝑐f𝑆c (𝑣)  , (1) where Δ𝑆(𝑣) is the continuum-subtracted absorption line depth (in Jy), 𝑆c (𝑣) is the continuum flu… view at source ↗
Figure 2
Figure 2. Figure 2: Left panel: Distribution of radio luminosities of sources searched for absorption as a function of redshift ( [PITH_FULL_IMAGE:figures/full_fig_p007_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Some examples of fast outflows detected via H i absorption studies: TXS 1245-197 (left panel) at 𝑧 = 1.25 is one of the highest redshift detections of a distinctive blue-shifted wing indicative of a fast outflow (Aditya and Kanekar, 2018a); and 4C12.50 (middle and right panels), where VLBI observations pinpoint the location of the H i outflow to the hotspot of the radio lobe (Morganti et al., 2013). High r… view at source ↗
Figure 4
Figure 4. Figure 4: Molecular gas in BCGs. From left to right; IFU observations of 3C 218 (Hydra A) from Hamer et al. (2014) showing the continuum image made by collapsing the H𝛼 cube (panel A), flux map (B), velocity field relative to the galaxy redshift (C), and FWHM which broadens at the centre of the velocity gradient (D). CO(2–1) emission detected with IRAM (Hamer et al., 2014) and VLA H i absorption toward the Hydra A c… view at source ↗
Figure 5
Figure 5. Figure 5: Left: Zoomed-in plot of OH-main and satellite lines detected in the MeerKAT UHF-band spectrum Right: Corresponding molecular absorption lines obtained with ALMA tracing denser gas. The differences in the absorption line profiles of different species are due to gas physics, structure and the frequency-dependent structure of the background radio source over 0.5 - 200 GHz (details and full L- and UHF-band spe… view at source ↗
Figure 6
Figure 6. Figure 6: Left: Number of DLAs (open circles) and 21-cm absorbers (stars) per unit redshift range as a function of redshift. The incidences of 21-cm absorbers are estimated for optical depth cut-off, To = 0.3 km s−1 based on DLAs and Mg ii systems at 2 < 𝑧 < 3.5 and 0.5 < 𝑧 < 1.3, respectively (Gupta et al., 2012) and nearby galaxies at 𝑧 ∼ 0.1 (Dutta et al., 2017b). The curve for non-evolving population of 21-cm ab… view at source ↗
Figure 7
Figure 7. Figure 7: Left: Total intensity H i 21-cm emission map of Klemola31A, member of a galaxy group at 𝑧 = 0.029, in autumn colour gradient (increasing 𝑁(H i) from brown to yellow) overlaid on the RGB cutout of DSS image. Also shown are the observed (white) and TIRIFIC tilted- ring model (green) moment-0 contours, and the location of background quasar PKS 2020-370 (𝑧𝑒𝑚 = 1.048) probing the outer disk. Right: Comparison o… view at source ↗
Figure 8
Figure 8. Figure 8: Peak optical depth limits reached for integration times ranging from 1 - 12hr, against a 10 mJy, 100 mJy and 1 Jy background continuum source. Assumes a 5𝜎 detection threshold and no spectral binning [PITH_FULL_IMAGE:figures/full_fig_p021_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: Simulations of 12 hour observations of associated H i absorption in GLEAM J0856+0224 at 𝑧 = 5.55 for a Gaussian profile line of width 100 km s−1 and maximum depth of 𝜏 = 0.01 (input spectrum shown by the grey line). The left panel shows what would be observed with MWA Phase 3 with an RMS of 13.4 mJy per 200 kHz channel, and the right panel shows SKA-Low AA4 with an RMS of 2.2 mJy per 18 kHz channel. The ba… view at source ↗
read the original abstract

Observing the 21-cm HI line in absorption provides a powerful means of tracing the cold neutral gas in normal and active galaxies across cosmic time. The frequency coverage and sensitivity of SKAO will allow us to detect HI in absorption from z = 0 to beyond z = 6, enabling the characterisation of the properties of cold gas in and around galaxies at all epochs. This chapter summarises recent advances in absorption-line studies, lessons learned from precursor surveys, and updates the science case presented in Kanekar and Briggs (2004) and Morganti et al. (2015), focusing on the capabilities enabled by the SKA design baseline, Array Assembly 4 (AA4). We expand on these earlier works by presenting new opportunities to simultaneously search for OH 18-cm absorption, an efficient tracer of diffuse molecular gas that complements the atomic gas traced by HI absorption, as well as the need for sub-arcsecond scale spectroscopic imaging and multi-wavelength data from large surveys. These advances will allow SKAO absorption surveys to address key questions surrounding the fuelling and feedback cycles of AGN and the evolution of the cold neutral gas across cosmic time.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 2 minor

Summary. The manuscript presents an updated science case for using the SKA Observatory (SKAO) Array Assembly 4 (AA4) to trace cold neutral gas via 21-cm HI absorption (and simultaneously 18-cm OH absorption) from z=0 to beyond z=6. It summarizes recent advances in absorption-line studies, lessons from precursor surveys, and updates the cases in Kanekar & Briggs (2004) and Morganti et al. (2015), while stressing the requirements for sub-arcsecond spectroscopic imaging and multi-wavelength data to address AGN fuelling/feedback and cold gas evolution.

Significance. If the AA4 design specifications are realized, the paper provides a timely, consolidated roadmap that explicitly adds simultaneous OH searches as a complement to HI for diffuse molecular gas and highlights synergies with large multi-wavelength surveys. This strengthens the justification for absorption surveys as a core SKAO capability for galaxy evolution studies across all epochs.

major comments (1)
  1. [Abstract] Abstract: the headline claim that 'the frequency coverage and sensitivity of SKAO will allow us to detect HI in absorption from z = 0 to beyond z = 6' is presented without any quantitative sensitivity calculations, detection-rate estimates, or error budgets in the manuscript itself; because this projection is the central claim, the absence of even a summary table or cited derivation from the AA4 specs weakens the ability to evaluate robustness.
minor comments (2)
  1. [Abstract] Abstract: the phrase 'this chapter' implies the text is excerpted from a larger volume; adding a sentence on standalone context would improve readability for readers encountering the manuscript independently.
  2. The discussion of 'lessons learned from precursor surveys' is referenced but not illustrated with even one concrete example (e.g., a specific detection-rate limitation or covering-factor issue); a short bullet list or table would make the update to prior science cases more tangible.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their positive assessment of the manuscript, the significance rating, and the recommendation for minor revision. We address the single major comment below.

read point-by-point responses

  1. Referee: [Abstract] Abstract: the headline claim that 'the frequency coverage and sensitivity of SKAO will allow us to detect HI in absorption from z = 0 to beyond z = 6' is presented without any quantitative sensitivity calculations, detection-rate estimates, or error budgets in the manuscript itself; because this projection is the central claim, the absence of even a summary table or cited derivation from the AA4 specs weakens the ability to evaluate robustness.

    Authors: We agree that the central claim in the abstract would benefit from explicit quantitative support. The projection is based on the published AA4 sensitivity specifications and the survey strategies outlined in the SKAO science case documents, which were used to update the earlier estimates in Kanekar & Briggs (2004) and Morganti et al. (2015). To address the referee's concern, we will add a concise summary paragraph (with a short table of key parameters such as expected rms noise, frequency coverage, and estimated detection rates for HI and OH absorption) in Section 2 or a new subsection of the revised manuscript, including direct citations to the AA4 technical specifications. This will provide the requested derivation without altering the overall length or scope. revision: yes

Circularity Check

0 steps flagged

No significant circularity; self-contained science case

full rationale

The manuscript is a forward-looking science case and planning summary that states projected detection capabilities for HI and OH absorption directly from the published SKA AA4 design specifications on frequency coverage and sensitivity. No equations, fitted parameters, model derivations, or internal predictions appear in the text. The central claim follows immediately once the external instrument parameters are granted, with no reduction to self-citation chains, ansatzes, or renamed empirical patterns. References to prior works (Kanekar & Briggs 2004; Morganti et al. 2015) are external and non-overlapping with the present author list, providing no load-bearing self-citation. The paper is therefore self-contained against external benchmarks with no circular steps.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

This is a review and science-case document; no free parameters, axioms, or invented entities are introduced in the provided abstract.

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Reference graph

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