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A blind FAST search for OH 18-cm absorption in 19 HI-selected systems re-detects the known absorber toward PKS 1413+135, finds no new ones, and places the tightest limits to date on [OH]/[HI].

Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →

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2026-08-01 22:49 UTC pith:6F7Z5NKP

load-bearing objection Solid survey paper with genuinely new HI absorbers and the tightest [OH]/[HI] limits to date, but the combined-sample limit is a ratio of weighted stacks that may be biased low, so the headline claim is slightly oversold. the 2 major comments →

arxiv 2607.15595 v1 pith:6F7Z5NKP submitted 2026-07-17 astro-ph.GA

The FAST HI 21-cm Absorption Blind Survey. III. OH Absorption Search in the 21-cm Absorber Sample and Continued HI Absorption Search

Wenkai Hu , Yougang Wang , Jeremy Darling , Zheng Zheng , James R. Allison , Elaine M. Sadler , Wenxiu Yang , Yichao Li
show 6 more authors
Yidong Xu Jie Wang Fengquan Wu Di Li Ming Zhu Xuelei Chen
This is my paper
classification astro-ph.GA
keywords OH 18-cm absorptionHI 21-cm absorptionmolecular gas in galaxiesspectral stackingsurvival analysisradio absorption lineshydroxyl radicalblind survey
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.

The paper extends a blind HI 21-cm absorption survey on FAST to also search for OH 18-cm absorption in the 19 systems whose redshifted OH lines fall in the telescope band. It re-detects the known OH absorption toward PKS 1413+135 — the first blind survey to catch OH absorption — but finds no new OH absorbers. Stacking the non-detections yields 3-sigma upper limits on the OH-to-HI abundance ratio, [OH]/[HI] < 1.66e-8, < 1.42e-8, and < 0.90e-8 for associated, intervening, and combined samples, under stated assumptions. A survival analysis of column densities finds no statistically significant correlation between N_OH and N_HI, nor redshift evolution of their ratio. The paper argues this establishes an unbiased benchmark for molecular-gas studies in HI-selected systems.

Core claim

In the survey's own terms, the result is that OH 18-cm absorption is rare in unbiased, HI-selected systems. The only detection is the previously known absorber toward PKS 1413+135, whose 1612 MHz absorption and 1720 MHz emission are re-detected. All other searched spectra are non-detections; spectral stacking turns these into 3-sigma upper limits of [OH]/[HI] < 1.66e-8, < 1.42e-8, and < 0.90e-8 (associated, intervening, combined) under adopted Tex=10 K, Ts=100 K, covering-factor-unity, 30 km/s assumptions. The survival analysis finds no statistically significant N_OH-N_HI correlation or redshift evolution, suggesting earlier reported trends came from targeted, molecule-biased samples.

What carries the argument

The central machinery is the OH 18-cm ground-state transitions (1612, 1665, 1667, 1720 MHz) observed toward a parent sample of HI 21-cm absorbers found in a blind drift-scan survey. Because OH absorption is typically optically thin, non-detections are converted into 3-sigma upper limits on integrated optical depth; these are combined across sources by spectral stacking, which weights each OH spectrum by completeness and noise, and by Bayesian censored regression that treats detections and upper limits together. The load-bearing comparison is the OH-to-HI column-density ratio [OH]/[HI] derived by converting optical-depth limits into column densities under fixed excitation and spin temperature

Load-bearing premise

The headline upper limits assume optically thin OH lines, an OH excitation temperature of 10 K, an HI spin temperature of 100 K, unit covering factors, and a 30 km/s line width; if the real excitation temperature is lower or the covering factor is below unity, the limits weaken proportionally.

What would settle it

Take any one of the 19 HI absorbers, integrate with FAST in ON-OFF mode until the 3-sigma sensitivity on the OH 1667 MHz integrated optical depth is three times deeper than the current stacked limit. A detection at the expected redshifted frequency would falsify the claim that these systems contain little OH. Alternatively, a direct measurement of OH excitation temperature (e.g., from the 1612/1667 ratio in PKS 1413+135) below 10 K would rescale the quoted column-density limits upward.

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

If this is right

  • Every known extragalactic OH absorber also shows HI 21-cm absorption, so HI-selected samples provide an efficient route to finding OH absorbers; this survey demonstrates that route blind.
  • Because the stacked limits sit at or below the diffuse-Galactic value of [OH]/[HI] ~ few x 1e-8, the typical HI absorber in this sample contains very little OH, meaning molecular gas is either confined to small clumps or absent.
  • The absence of a significant N_OH-N_HI correlation or redshift evolution in the unbiased sample indicates previously reported correlations were likely driven by targeted, dust- and molecule-biased samples.
  • Wide instantaneous bandwidth instruments can search HI and OH absorption simultaneously, so future large drift-scan surveys can expand the census of molecular absorbers across redshift.

Where Pith is reading between the lines

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

  • Editorial inference: if deeper integrations still show no OH in these systems, the simplest reading is that the bulk of HI-selected absorbers are not molecule-rich; this would lower estimates of the 'dark molecular gas' fraction in such sightlines.
  • Editorial inference: the PKS 1413+135 re-detection, now reclassified as intervening, suggests that red, dusty sightlines (V-K > 6) are the high-yield subset of HI absorbers; a testable extension is to compare stacked [OH]/[HI] limits for red versus blue HI absorbers within the same survey.
  • Editorial inference: the quoted limits scale linearly with the assumed OH excitation temperature and inversely with covering factor. If OH excitation in these diffuse gas clouds is actually colder than 10 K, the true OH content could be higher; measuring Tex or cf along one absorber would calibrate the whole stacking result.

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

2 major / 5 minor

Summary. The paper reports the third installment of the FAST HI 21-cm absorption blind survey. It adds 394.4 hr and 1622.1 deg^2 of 2024 CRAFTS/FASHI data plus FATHOMER, detecting three known and four new HI absorbers, bringing the catalog to 41 systems. For 18 of these systems (19 sightlines including the HVC toward NVSS J090150+030422) with OH transitions in the FAST band, it conducts an OH 18-cm search, re-detecting OH 1612 MHz absorption and 1720 MHz emission toward PKS 1413+135 and finding no new OH absorbers. Survival analysis with Bayesian censored regression finds no statistically significant N_OH-N_HI correlation or redshift evolution. Stacking yields 3-sigma upper limits N_OH < 4.93, 1.64, 1.72 x 10^12 T_ex/c_f cm^-2 K^-1 and [OH]/[HI] < 1.66e-8, 1.42e-8, 0.90e-8 for associated, intervening, and combined samples, claimed as the strongest constraints to date.

Significance. The paper's strengths are the first OH search in a blind HI-selected sample, the re-detection of PKS 1413+135, the use of survival statistics appropriate for ~85% censoring, and stacking that improves on prior limits. If the estimator issues are resolved, the sample and upper limits provide a valuable benchmark for future OH surveys. The paper is honest about the assumptions (T_ex = 10 K, T_s = 100 K, c_f = 1, Delta-v = 30 km/s) and about the large uncertainties in the correlation analysis.

major comments (2)
  1. [Section 6.2, 'OH abundance relative to H i'] The combined [OH]/[H i] limit is calculated as the ratio of two separately stacked weighted means, with OH weights 1/(C rms_OH^2) and H i weights 1/(C rms_HI^2). Because the weighting is not identical, this quantity is not the mean of per-source [OH]/[H i] ratios; the resulting combined limit (0.90e-8) is lower than both the associated (1.66e-8) and intervening (1.42e-8) limits, which the text does not explain. This suggests that the OH stack is dominated by low-noise intervening sightlines while the H i stack includes high-N_HI associated systems. Please define the estimator explicitly and either justify it as an upper limit on a physically meaningful average abundance, or stack per-source ratio upper limits and report those; if the combined number is not a valid mean ratio, the abstract's headline constraint should be qualified.
  2. [Section 6.1 and Figure 13] The censored-regression fits are performed only for associated and intervening absorbers; the 11 systems with undetermined classifications are plotted but omitted from the fits. This exclusion is not stated in the text, and no robustness test is given. Because these systems could differ systematically (e.g., in redshift or host properties), the conclusion of no statistically significant correlation/evolution formally applies only to the classified subset. Please state the exclusion explicitly and assess its impact on the conclusions.
minor comments (5)
  1. [Abstract and Sections 1, 5] The abstract says 19 HI absorption systems were searched for OH, but Section 5 says 'the remaining 18 absorbers' and Table 5 lists 19 rows including the HVC toward NVSS J090150+030422. Please clarify systems vs. sightlines and use consistent counts.
  2. [Section 5.1] In the PKS 1413+135 paragraph, 'N_OH ~ 0.15 T_s 10^15 cm^-2 K^-1' should use T_ex rather than T_s, since this is an OH column density; this appears to be a typo.
  3. [Abstract and Section 1] The term 'blind' for the OH search should be qualified. The OH search is not a blind sky survey but a targeted search within a blind HI-selected sample; as written, 'first blind survey to detect OH absorption' could be overstated.
  4. [Section 6.2] Please state explicitly whether the 'combined' sample includes the 11 systems with undetermined classifications and how they are weighted; currently 'combined' is ambiguous.
  5. [Section 3.2, Eq. (1)] Equation (1) assumes Gaussian uncorrelated noise. Please specify whether the 1 km/s rebinned channels used for the stacked RMS remain uncorrelated and whether baseline systematics are included in the jackknife error estimates.

Circularity Check

0 steps flagged

No significant circularity: OH upper limits are direct noise-based 3-sigma conversions with explicit physical assumptions; self-citations supply input catalogs/methods, not the derived result.

full rationale

The derivation chain is not circular. The OH search is an observational search over fixed redshifted 1612/1665/1667/1720 MHz windows in a sample defined by previously detected HI absorption; the only input from the authors' earlier papers is the parent HI catalog (Hu et al. 2023, 2025), which is an externally checkable observational input, not an assumed conclusion. Non-detections are converted to limits by the stated error propagation `tau_3sigma ~ 3sigma/S` and `integral tau_3sigma dv = tau_3sigma * delta_v * sqrt(Delta_v/delta_v)` with Delta_v = 30 km/s; column densities then use standard spectroscopic constants (X = 2.38e14 for OH 1667, 1.82e18 for HI) and explicitly stated Tex = 10 K, Ts = 100 K, cf = 1. These formulas are inputs, not outputs of the analysis. The survival analysis treats the resulting limits as censored data; no fitted parameter is later reported as an independent prediction. The only self-citation beyond the parent catalog is the stacking method (Hu et al. 2019), which is a procedural reference rather than a load-bearing result; the known OH absorber is re-detected and compared with independent spectra (Darling 2004; Combes et al. 2023). The combined-sample [OH]/[HI] limit being lower than both subgroup limits is an arithmetic consequence of taking a ratio of two separately sensitivity-weighted stacks; this is a presentation/interpretation choice that could be debated, but it is not a circular reduction because the value is still directly computed from the stacked optical-depth spectra and the stated assumptions. The paper itself flags the main caveat in Section 6.2: 'the assumed excitation temperature or OH covering-factor parameters may systematically bias the inferred abundances,' and Section 8 notes 'The uncertainties of our analysis remain large.' The claim is therefore self-contained conditional on those physical assumptions.

Axiom & Free-Parameter Ledger

4 free parameters · 6 axioms · 0 invented entities

The central upper limits and abundance ratios are not fitted outputs but are set by standard spectroscopic formulas plus four adopted numerical values (Tex, Ts, cf, Δv). No new physical entities are introduced. The survival-analysis slopes are fitted outputs, not inputs, and the paper does not use them as predictions.

free parameters (4)
  • OH excitation temperature Tex = 10 K (assumed)
    Adopted in §6.2 to convert stacked NOH upper limits and [OH]/[HI]; directly scales the quoted limits (NOH ∝ Tex).
  • HI spin temperature Ts = 100 K (assumed)
    Adopted for [OH]/[HI] ratios and for NHI of this work's sample; appears in the NHI formula and in the ratio.
  • Covering factor cf (OH and HI) = 1
    Assumed in §3.2 and §6.2; if cf<1, optical depths and column densities are underestimated.
  • Assumed OH line width Δv = 30 km/s
    Used in Eq. (1) to convert RMS noise into integrated optical depth upper limits; chosen because known OH absorbers have FWHM ≲30 km/s.
axioms (6)
  • domain assumption The optical-depth formula τ(V) = −ln(1 + ΔT/(cf Tc)) applies under Ts ≪ cf Tc, and for non-detections τ3σ ≈ 3σ/S.
    Stated in §3.2 and §5.2; standard for 21-cm/18-cm absorption but assumes background-dominated, optically thin conditions.
  • domain assumption All six known OH absorbers also show HI 21-cm absorption, so HI-selected samples are a valid parent for OH searches.
    Used in §1 and §7.4 to motivate searching only HI absorbers; if some OH absorbers lack detectable HI, the survey would miss them.
  • standard math The 1667-MHz conversion constant X=2.38e14 and 1612-MHz X=2.14e15 from Curran et al. (2008) and Grasha et al. (2019) are correct.
    Used in §3.2 to compute NOH; taken from literature.
  • domain assumption Noise in the upper-limit integration is Gaussian and uncorrelated between channels with uniform RMS over the velocity range (Eq. 1).
    Stated in §5.2; if noise is correlated, the sqrt(Nchannels) scaling understates the uncertainty.
  • domain assumption PKS 1413+135 is classified as an intervening absorber based on VLBI astrometry (Readhead et al. 2021).
    §6.1; reclassification changes the associated/intervening split in the correlation analysis.
  • standard math The FAST L-band frequency coverage (1.0–1.5 GHz) corresponds to OH 1667 MHz redshifts z=0.15–0.59 and HI z=0–0.35.
    Used to select the 19 systems for OH search; if band edges/RFI differ, coverage changes.

pith-pipeline@v1.3.0-alltime-deepseek · 27228 in / 14021 out tokens · 165984 ms · 2026-08-01T22:49:42.583492+00:00 · methodology

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read the original abstract

We present the first blind search for OH 18-cm absorption with the Five-hundred-meter Aperture Spherical Telescope (FAST), conducted alongside the HI 21-cm absorption search. Our previous FAST blind HI absorption search identified 34 systems. In this work, we extend the search using 2024 and part of the 2025 CRAFTS and FASHI data (394.4 hr and 1622.1 deg$^{2}$) together with FATHOMER observations, yielding three known and four new HI absorbers, for a total of 41 HI absorption systems. We search for OH absorption in 19 HI absorption systems whose OH redshifted frequencies fall within the FAST band. The known OH absorber towards PKS 1413+135 was re-detected, making our survey the first blind survey to detect OH absorption. No new OH absorbers were identified. We examine the relationship between $N_{\rm{OH}}$ and $N_{\rm{HI}}$, applying survival analysis to account for upper limits. The analysis does not provide statistically significant evidence for either an $N_{\rm{OH}}$-$N_{\rm{HI}}$ correlation or redshift evolution of $N_{\rm{OH}}$/$N_{\rm{HI}}$. Finally, spectral stacking sets 3$\sigma$ OH column density upper limits of 4.93, 1.64, and 1.72 $T_{\rm{ex}}$/$c_{\rm{f,OH}}\times$10$^{12}$cm$^{-2}$K$^{-1}$ for associated, intervening, and combined samples, corresponding to [\rm{OH}]/[\rm{HI}] ratios of $<$1.66$\times$10$^{-8}$, $<$1.42$\times$10$^{-8}$, and $<$0.90$\times$10$^{-8}$, assuming $T_{\rm{ex}}$=10K for OH and $T_{\rm{s}}$=100K for HI. These results place the strongest constraints to date on the OH content in radio-selected HI absorbers and establish a blind-survey benchmark for future studies of molecular gas in HI-selected systems. They also demonstrate that known HI 21-cm absorbers provide an effective parent sample for systematic OH absorption searches, paving the way for future larger surveys.

Figures

Figures reproduced from arXiv: 2607.15595 by Di Li, Elaine M. Sadler, Fengquan Wu, James R. Allison, Jeremy Darling, Jie Wang, Ming Zhu, Wenkai Hu, Wenxiu Yang, Xuelei Chen, Yichao Li, Yidong Xu, Yougang Wang, Zheng Zheng.

Figure 1
Figure 1. Figure 1: Up-to-date (2025-10-25) searched CRAFTS, FASHI, and FATHOMER sky coverage in Equatorial coordinates. Search areas in CRAFTS, FASHI, and FATHOMER regions are highlighted by orange, green, and blue labels, respectively. Zenith angles of 40◦ (maximum zenith angle for FAST) and 26.4 ◦ (zenith angle within which FAST has full gain) are shown as black circles and red circles, respectively. using transit informat… view at source ↗
Figure 2
Figure 2. Figure 2: Left: H i absorption feature of NVSS J080601+190611. The blue solid line shows the absorption spectrum, and the red solid line shows the fit with a three-component Gaussian model. The fitting residual is shown as a black-solid line at the bottom. The optical depth value for H i absorption is shown on the right scale. Middle left: radio map from VLASS centered at NVSS J080601+190611. Middle right: W2 band i… view at source ↗
Figure 3
Figure 3. Figure 3: Left and Middle left: same as [PITH_FULL_IMAGE:figures/full_fig_p007_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Left: same as [PITH_FULL_IMAGE:figures/full_fig_p008_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Left: same as [PITH_FULL_IMAGE:figures/full_fig_p008_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Left: same as [PITH_FULL_IMAGE:figures/full_fig_p009_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Left: same as [PITH_FULL_IMAGE:figures/full_fig_p009_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: Left: same as [PITH_FULL_IMAGE:figures/full_fig_p009_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: Redshift versus 1.4-GHz flux density of radio sources for the systems searched for OH absorption. The flux densities were obtained from the NVSS data available through NED. +135, including information about its associated H i ab￾sorption, were summarized in Hu et al. (2025). The OH 1667 MHz absorption line in PKS 1413+135 was first detected by Kanekar & Chengalur (2002) using the Giant Metrewave Radio Tele… view at source ↗
Figure 10
Figure 10. Figure 10: For the OH 1720 MHz emission line, we measured a peak flux of 8.27±1.10 mJy, an FWHM of 11.78±2.17 km s−1 , and a peak optical depth of τpeak ∼ −0.007. The OH 1612 MHz absorption line exhibited a peak flux of -11.90±1.48 mJy, an FWHM of 6.68±1.83 km s−1and a peak optical depth of τpeak ∼ 0.010, with a corresponding OH column density esti￾mate of NOH ∼ 0.15Ts1015cm−2K−1 . These measure￾ments are consistent… view at source ↗
Figure 11
Figure 11. Figure 11: Calibrated flux density spectra centered on the redshifted frequency of the OH 1667 MHz line for radio targets where H i was detected. The red dashed line represents the FWHM of the corresponding H i absorption. The orange densely dashed dotted lines indicate the 3σ RMS. formation, such as increased pressure (Elmegreen 2009) and turbulence within the interstellar medium (F¨orster Schreiber et al. 2009; Ta… view at source ↗
Figure 12
Figure 12. Figure 12 [PITH_FULL_IMAGE:figures/full_fig_p013_12.png] view at source ↗
Figure 13
Figure 13. Figure 13: The correlation analysis between NH i and NOH. Upper panel: NH i versus NOH. Lower panel: NOH/NH i ratio as a function of redshift. Red stars represent the detection and 3σ upper limits from the sample analyzed in this work. Orange squares mark the six currently known OH absorption detections. The red-solid and blue-dashed lines show the fits for the associated and intervening absorbers from our FAST samp… view at source ↗
Figure 14
Figure 14. Figure 14: Stacked optical depth spectra centered on the redshifted OH lines at 1612 MHz, 1665 MHz, 1667 MHz, and 1720 MHz are presented for all systems (top row), associated systems (middle row), and intervening systems (bottom row). The grey shaded regions represent the jackknife errors, while the orange region indicates the 3σ RMS [PITH_FULL_IMAGE:figures/full_fig_p016_14.png] view at source ↗
Figure 15
Figure 15. Figure 15: Stack of H i absorption spectra of the associated (red dashed line), intervening (green dotted line), and com￾bined (blue solid line) H i absorption samples that are used for OH absorption search. The shaded regions represent the jackknife errors. selected for OH search because they exhibit strong H i 21 cm absorption, are gravitational lenses, or are red quasars (or combinations of these properties). Con… view at source ↗

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