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REVIEW 3 major objections 6 minor 97 references

Brightest-first stacking of associated HI 21 cm non-detections at 0.4<z<1 yields tentative absorption that vanishes as weaker sources raise stack optical depth; intervening stacks stay null, and one new direct detection matches the survey’s

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2026-07-31 21:34 UTC pith:367AEFZV

load-bearing objection One solid new associated absorber plus careful FLASH limits and a useful sensitivity meta-check; the ordered-stacking “tentative detections” are the soft part and already undercut by the paper’s own bootstrap. the 3 major comments →

arxiv 2607.27977 v1 pith:367AEFZV submitted 2026-07-30 astro-ph.GA

A search for HI absorption in distant star-forming galaxies with ASKAP-FLASH - II. Direct observations and stacking of 21 cm line

Sophie L. Eden , Elaine M. Sadler , Kevin A. Pimbblet , Stephen J. Curran , Julia Healy , Filippo M. Maccagni , Elizabeth K. Mahony , Hyein Yoon This is my paper
classification astro-ph.GA
keywords HI 21 cm absorptionASKAP-FLASHspectral stackingstar-forming galaxiesradio galaxiesintermediate redshiftcovering factoroptical depth
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved

The pith

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

This paper asks whether cool neutral hydrogen can be recovered at intermediate redshift by pairing UV-selected star-forming galaxies (and a radio-galaxy control sample) with ASKAP-FLASH continuum sources, both by direct 21 cm absorption searches and by stacking non-detections. It reports one new associated absorption line toward NVSS J214954-004657 at z_HI=0.863, finds no significant difference in associated detection rate between star-forming hosts and the general radio-source population, and shows that a meta-analysis of prior surveys predicts roughly one detection given FLASH sensitivity and sample size. For stacking, the authors argue that order matters: when associated spectra are co-added from brightest continuum source downward, FLASHFINDER flags candidate absorption near rest velocity at N=5 that disappears as fainter sources drive the stack optical-depth rms upward; intervening stacks never show a signal. They conclude that usable stacked absorption at these redshifts depends on keeping background optical-depth limits low, and that covering factor effectively weakens intervening backgrounds more than associated ones. A sympathetic reader cares because this frames why absorption stacking has historically failed beyond the local Universe and what survey design must change before statistical HI studies at 0.4

Core claim

For the first time at 0.4<z<1, ordered (brightest-first) stacking of associated non-detections produces FLASHFINDER-flagged candidate HI 21 cm absorption features at N=5 that become undetectable as progressively weaker continuum sources raise the stack optical-depth rms, while intervening stacks remain null; the single direct detection and a meta-analysis of HI 21 cm surveys imply that about one detection is expected for these survey parameters, with no significant star-forming versus radio-galaxy rate difference.

What carries the argument

Brightest-first optical-depth stacking: spectra are converted to observed optical depth, shifted to the optical redshift prior, and median-coadded in descending continuum brightness so that stack optical-depth rms rises as fainter sources enter, testing when any average absorption remains detectable.

Load-bearing premise

That the absorption-like features flagged in the five-brightest associated and spare-fibre stacks are real average HI signals rather than continuum-subtraction residuals or noise that adds constructively.

What would settle it

Rebuild the brightest-first associated stack with an independent sample of similar continuum flux and redshift (or with deeper spectra that hold optical-depth limits fixed while increasing N) and test whether a near-zero-velocity feature is recovered whose minimum amplitude is supported by bootstrap resampling and is free of continuum-subtraction artefacts.

Watch this falsifier — get emailed when new claim-graph text bears on it.

If this is right

  • Associated HI 21 cm stacking at 0.4<z<1 only remains informative while background sources keep the stack optical-depth limit low.
  • Covering factor suppresses intervening stacks more than associated stacks by effectively dimming the background continuum.
  • UV star-forming selection does not raise associated 21 cm detection rates above the general radio-galaxy population at these redshifts.
  • For FLASH-like sensitivity and sample sizes, of order one associated detection is the expected yield.
  • Uniform optical-depth survey design or substantially deeper spectra are required before complete non-detection stacks can trace cool HI statistically at intermediate redshift.

Where Pith is reading between the lines

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

  • Absorption stacking pipelines should weight or cut on continuum brightness (or per-spectrum optical-depth limit) instead of co-adding all non-detections equally.
  • Null intervening stacks at 30–120 kpc impact parameters imply that intermediate-z pair searches need much tighter alignments, or secure background redshifts, before stacking helps.
  • If the narrow, redshifted spare-fibre stack feature is physical, associated absorption profiles may differ between star-forming hosts and ordinary radio galaxies—a split larger samples can test.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit.

Referee Report

3 major / 6 minor

Summary. This paper searches for associated and intervening H I 21 cm absorption at 0.4 < z < 1.0 toward UV-selected star-forming (WiggleZ-type) galaxies and a control sample of radio galaxies (SPARE_RADIO), using ASKAP-FLASH spectra. The main solid result is a new associated absorber in NVSS J214954-004657 at z_HI = 0.863 (ln B = 8.0 with optical redshift prior; re-confirmed in prior SBIDs). No other direct detections are found among bright associated/offset targets. The authors stack non-detections ordered by descending continuum brightness and report FLASHFINDER-flagged candidate features in the N = 5 associated and spare-fibre stacks that disappear as weaker sources raise stack optical-depth rms; intervening stacks remain null. A literature meta-analysis of detection rate versus mean line strength implies ~1 detection is expected given FLASH sensitivity, and no significant SF-versus-radio-galaxy rate difference is claimed. UV ionising-photon rates are shown to lie below the critical luminosity of Curran & Whiting (2012).

Significance. A carefully reduced intermediate-redshift FLASH search with spectroscopic priors, multi-SBID confirmation of one absorber, explicit upper limits (Tables 3–4), ionising-photon-rate checks, and a quantitative sensitivity meta-comparison is useful for the field as SKA-pathfinder absorption surveys mature. The null intervening stacks and the demonstration that stack τ_rms rises when faint sources are co-added are methodologically informative even if the N = 5 features are not physical. The paper does not claim a large new absorber sample; its value is in documenting FLASH limits for SF-selected targets and in framing why absorption stacking at these redshifts is hard. Strengths include re-observation consistency (Table 5), dual stacking codes (LINESTACKER/HISS), and transparent bootstrap caveats.

major comments (3)
  1. [Abstract; §5.2.1–5.2.4; §7] Abstract and §5.2 / §7(i): The headline claim of “tentative detections of stacked H I 21 cm absorption… when stacked in order of descending radio source brightness” is not supported by the paper’s own statistics. §5.2.1 notes the associated N = 5 feature (ln B = 36.9, FWHM ~227 km s⁻¹) may arise from imperfect continuum subtraction; §5.2.3 notes the spare-fibre N = 5 feature is redshifted by +175 km s⁻¹ with a narrow 19.7 km s⁻¹ width atypical of associated absorption; and §5.2.4 bootstrap resampling explicitly finds that N = 5 stack minima are not statistically supported and do not overlap the resampled distributions. High ln B on stacked optical-depth spectra therefore cannot be treated as evidence of average cool H I. The abstract, conclusions, and any “first time at these redshifts” phrasing should be rewritten so that the primary stacking result is the null/buried outcome and the op
  2. [§6.5.2; Fig. 12] §6.5.2 and Fig. 12: The expected-detection-rate argument rests on an orthogonal-distance-regression power-law fit to only six literature survey points (plus the Su et al. 2022 endpoint that strongly leverages the slope). The quoted r_det ≈ 0.9 ± 4% for this work’s mean line strength is consistent with one detection, but the uncertainty and leverage of a single faint-source survey should be stress-tested (e.g., jackknife omitting Su+2022; alternative functional forms; explicit Poisson expectation). Without that, the claim that “1 detection is expected given the survey parameters” is suggestive rather than robust and should be softened or accompanied by a sensitivity table.
  3. [§5.2.2; §6.3.3–6.3.4] §5.2.2 and §6.3.3–6.3.4: For intervening (offset) stacks and non-detections, the interpretation leans on unknown background fractions and covering factors (Roster et al. 2026 photometric mix is cited globally, not per sightline). The paper correctly notes that incomplete covering effectively weakens S_c, but the quantitative discussion still treats co-added flux as if geometry were secondary. Either restrict intervening conclusions to “no stacked signal under FLASH limits and unknown f_c” or add a simple Monte Carlo (draw background probability ~0.5 and f_c < 1) showing that a null stack is expected even if associated gas were present. As written, the geometric explanation is plausible but not demonstrated for this sample.
minor comments (6)
  1. [Table 3; §4.2] Table 3 note and offset row 5 (J144546-013041): A non-null ΔS_peak / τ_obs is listed with a dagger in the spare-fibre table convention, but the associated/offset bright search text (§4.2) states no significant detections. Clarify whether this is a non-significant prior-based feature, a table error, or an upper-limit formatting inconsistency.
  2. [Fig. 2] Fig. 2 caption vs text: Caption says 15 bright associated / 21 bright offset before primary-component cuts; body uses 10 / 15 after cuts. State the post-cut numbers in the figure caption to avoid confusion.
  3. [§3.2; Tables 3–4] Eqs. (1)–(3): The optically thin approximation and the N_HI f_c / T_s reporting are standard; briefly state the assumed FWHM = 30 km s⁻¹ template for 3σ limits in the main text near the tables, not only in table notes.
  4. [§5.1; Fig. 4] §5.1: Noise scaling is shown for one “GOOD” cube (Fig. 4). A short note on how many stacked spectra fall in ducting-flagged bands after masking would help readers judge residual non-Gaussianity at N ≳ 300.
  5. [Throughout] Typos / notation: “Y oon” spacing in author list and citations; “T wenty-cm”; “boostrapping” (§5.2.4); inconsistent z_HI vs z_opt precision. Standardise FLASHFINDER / LINESTACKER capitalisation.
  6. [§6.1; Abstract] §6.1 sample-size comparison (10 vs 23): The Gehrels errors are appropriate; consider stating explicitly that the SF versus control rate difference is unconstrained rather than “no significant difference,” which can be read as a null result with power.

Circularity Check

0 steps flagged

No significant circularity: observational FLASH search with external redshifts, independent spectra, and a literature meta-check; results are not forced by construction.

full rationale

This is a standard observational absorption/stacking paper. The single associated detection (NVSS J214954-004657) is measured from FLASH spectra at an independent optical redshift prior and re-confirmed in prior SBIDs; it is not derived from a fitted model of the same quantity. Ordered stacking and FLASHFINDER ln B flags are empirical co-adds of non-detection spectra; the paper itself reports that bootstrap resampling does not support the N=5 features as robust, so those candidates are not presented as closed-form predictions forced by inputs. The meta-analysis fits an empirical r_det vs mean line-strength trend to external surveys (Vermeulen, Geréb, Maccagni, Murthy, Su, Aditya) and checks consistency of this work’s sensitivity and one detection with that external relation—an ordinary consistency test, not a self-fit renamed as prediction. Dependence on Eden et al. (2025) is sample-construction continuity for the same series, and Curran co-author citations supply context (UV critical luminosity, literature limits), not a uniqueness theorem that forces the detection rate or stacking claim. No step reduces a claimed first-principles or predicted result to its own defining inputs.

Axiom & Free-Parameter Ledger

5 free parameters · 6 axioms · 0 invented entities

Load-bearing content is empirical survey practice plus standard optically-thin HI radiative transfer. Claims rest on FLASH noise/flux cuts, covering-factor and background-redshift assumptions, Gaussian-line upper-limit templates, and a power-law fit to heterogeneous literature detection rates—not on new physical entities.

free parameters (5)
  • Bright-source flux cut = 30 mJy
    Direct-search threshold set at peak continuum ≥30 mJy from FLASH 2-hour noise (~5.5 mJy) and 5σ optical-depth practicality; defines which spectra enter direct vs stack-only paths.
  • Associated vs offset angular cuts = 5 and 20 arcsec
    Impact-parameter proxies fixed at <5 arcsec (associated) and 5–20 arcsec (offset) from Paper I geometry; control which gas regime is claimed.
  • Upper-limit Gaussian FWHM template = 30 km/s FWHM, 3σ peak
    Non-detection integrated optical depths assume single Gaussian FWHM=30 km/s and peak=3σ_rms (Tables 3–4).
  • Literature r_det vs mean line-strength power-law fit = log10 r_det ≈ −⟨log N⟩ + 19
    Orthogonal-distance regression on six surveys (≥20 objects, z≤1) yields log10 r_det ≈ −log10⟨N_HI f_c/T_s⟩ + 19 used to predict ~0.9% detection rate for this work.
  • Stack inspection N and ordering = N=5, descending peak flux
    Candidate stacked features highlighted specifically at N=5 brightest-first; ordering and N choice are analysis choices that select where features appear.
axioms (6)
  • domain assumption Optically thin approximation τ≈ΔS/(f_c S_c) and N_HI≈1.823e18 (T_s/f_c)∫τ_obs dv
    Section 3.2; standard 21 cm conversion used for all depths and column-density limits.
  • domain assumption FLASHFINDER ln B ≳8 with spectroscopic redshift prior indicates a credible line; blind threshold much higher (ln B≥30)
    Section 4.1 citing Trotta 2008 and Yoon+2025; gates the single accepted detection.
  • domain assumption Median stacking after rest-frame rebinning preserves any common absorption while rms falls ~N^{-0.5} until non-Gaussian noise dominates
    Section 5.1; justifies stacking methodology and noise tests.
  • domain assumption WiggleZ-type UV selection implies substantial cool HI reservoirs available for absorption
    Introduction and sample motivation from Paper I; frames expectation of higher detection yield that is not observed.
  • standard math Concordance cosmology Ω_Λ=0.7, Ω_m=0.3, H_0=70 km/s/Mpc for physical scales
    Stated at end of Introduction; used for impact parameters and sizes.
  • ad hoc to paper For intervening stacks, many continuum sources can be treated as background with non-negligible covering factor unless proven otherwise
    Sections 6.3.3–6.5.1 acknowledge ~half of FLASH sources may not be background and f_c<1 is common; still used when interpreting null intervening stacks.

pith-pipeline@v1.2.0-daily-grok45 · 38094 in / 3693 out tokens · 69849 ms · 2026-07-31T21:34:22.244374+00:00 · methodology

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read the original abstract

We present results of a search for associated and intervening HI 21 cm absorption in star-forming galaxies and a control sample of radio galaxies at 0.4 < $z$ < 1.0, using ASKAP's FLASH HI 21 cm absorption survey. We report the detection of a new HI 21 cm absorption line in radio galaxy NVSS J214954-004657 at $z_{\mathrm{HI}}$ = 0.863. Additionally, we present results of HI 21 cm stacking for associated and intervening star-forming and radio galaxy catalogues. For the first time at these redshifts, we report tentative detections of stacked HI 21 cm absorption for associated galaxies when stacked in order of descending radio source brightness, and show that these tentative detections become buried as optical depth increases as progressively weaker sources are co-added into the stack. No stacked signal is detected when co-adding intervening galaxy spectra. We speculate that the ability to detect a stacked HI 21 cm detection is dependent on the background source optical depth limits, as well as source covering factors. Covering factor has a greater impact when stacking intervening systems, and can be considered to effectively weaken the observed flux density of a radio source. We find no significant difference in detection rate for associated HI 21 cm absorption in star-forming galaxies versus the general radio-source population at the same redshift. Although detection rates are low, from a meta-analysis of HI 21 cm surveys we find that 1 detection is expected given the survey parameters.

Figures

Figures reproduced from arXiv: 2607.27977 by Elaine M. Sadler, Elizabeth K. Mahony, Filippo M. Maccagni, Hyein Yoon, Julia Healy, Kevin A. Pimbblet, Sophie L. Eden, Stephen J. Curran.

Figure 1
Figure 1. Figure 1: Production process of the associated and offset object samples investigated in this work, as well as outline of the two kinds of H I 21 cm absorption search (direct and stacking) which are carried out. The scope of work covered in Eden et al. (2025) and this work are separated by the grey horizontal dashed line. MNRAS 000, 1–20 (2026) [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Peak flux density of each source in mJy against separation in arc￾sec between the optical object and the FLASH radio source for the associ￾ated (green) and offset (pink) objects, for 0.42 < z < 1.0. The horizontal black dashed line at 30 mJy marks the distinction for ‘bright’ objects, where any￾thing above 30 mJy is considered bright. The 5σ limit for FLASH source de￾tections is peak flux density of 0.5 mJ… view at source ↗
Figure 3
Figure 3. Figure 3: Absorption line detected in spare fibre galaxy NVSS J214954- 004657. The feature was detected close to the optical redshift of the galaxy (zopt = 0.864). The ln B value for this detection is 8.0, which is statistically sig￾nificant with the redshift prior. The grey region is one times the rms spectral￾line noise. 4.3.1 Object NVSS J214954-004657 In the FLASH spectrum of radio galaxy NVSS J214954-004657 we … view at source ↗
Figure 4
Figure 4. Figure 4: shows the relationship between number of spectra being stacked, and the rms noise of the stacked spectra (green points) from FLASH SBID 51446. The expected behaviour of noise decreasing as 𝑁 −0.5 is shown with the dotted black line. The departure of the data from decreasing as 𝑁 −0.5 at 𝑁 ∼ 300, is seen by Fabello et al. (2011) with their H I 21 cm emission stacking as a result of non￾Gaussian noise beginn… view at source ↗
Figure 6
Figure 6. Figure 6: Stacked spectra produced by stacking first 5 and then all 195 bright and non-bright associated FLASH spectra in terms of optical depth in order of decreasing brightness. The FLASHFINDER detection for the 𝑁 = 5 stack is indicated in red. Values of rms opd are included for each stack. The opd rms values increase to a maximum value when all spectra have been stacked. Note that the vertical scale changes to be… view at source ↗
Figure 7
Figure 7. Figure 7: Stacked spectra produced by stacking the first 5 and then all 414 bright and non-bright offset FLASH spectra in terms of optical depth in or￾der of decreasing brightness. Values of rms opd are included for each stack. The opd rms values increase to a maximum value when all spectra have been stacked. Note that the vertical scale changes to best display the data [PITH_FULL_IMAGE:figures/full_fig_p012_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: Stacked spectra produced by stacking the first 5 and then all 22 bright spare fibre FLASH spectra in terms of optical depth in order of de￾creasing brightness, once the spectrum from 62514_component_52a contain￾ing the detection is removed. The FLASHFINDER detection for the 𝑁 = 5 stack is indicated in red. Values of rms opd are included for each stack. The opd rms values increase to a maximum value when al… view at source ↗
Figure 9
Figure 9. Figure 9: Bootstrapping opd results for associated, offset, and spare fibre samples, each resampled 1000 times. Bootstrapped minimum amplitude data is indicated by purple bins, while the minimum opd amplitude of the stacked spectra is indicated by a vertical red line. Left: bootstrapping for 𝑁 = 5 and 𝑁 = 195 associated spectra. The minimum opd amplitude of the 𝑁 = 5 stacked spectrum is -0.026, while for the 𝑁 = 195… view at source ↗
Figure 10
Figure 10. Figure 10: Comparison of the distribution of ionising photon rates for asso￾ciated, offset, and spare fibre galaxies. Sample size and mean rate with un￾certainty for each sample is included. The median UV luminosity for each sample is indicated with the vertical grey dashed line, and the critical UV lu￾minosity with the vertical purple dotted line. nals is partially responsible for the lack of detections we have fou… view at source ↗
Figure 11
Figure 11. Figure 11: Distribution of line strength limits and comparison against detec￾tions from the literature (excluding Su et al. 2022). The red bars are 3σ limits of non-detections from this work, and the filled black bars are from detections in the literature. FLASH sources are at 𝑧 > 1, and ∼ 35 per cent at 0.42 <𝑧 < 1.0. In a simulation of the radio sky, Wilman et al. (2008) find similar results, with ∼ 65 per cent of… view at source ↗
Figure 12
Figure 12. Figure 12: Detection rate versus mean H I 21 cm absorption strength for sur￾veys that contain at least 20 sources at z ≤ 1 (black points; Vermeulen et al. 2003; Geréb et al. 2015; Maccagni et al. 2017; Murthy et al. 2021; Su et al. 2022; Aditya et al. 2024). The red point shows this survey, and the vertical grey line shows the mean 3σ sensitivity of this survey. The bars show the standard error, and the dotted blue … view at source ↗

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Reference graph

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