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arxiv: 2604.19896 · v1 · submitted 2026-04-21 · 🌌 astro-ph.GA

Recognition: unknown

HI 21-cm absorption in low- and high-excitation radio-loud AGNs at z<0.5 from MALS

F. Combes, H.-R. Kl\"ockner, H.-W. Chen, J-.K. Krogager, N. Gupta, P. Noterdaeme, P. P. Deka, S. A. Balashev

Pith reviewed 2026-05-10 01:39 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords HI absorptionradio-loud AGNLERGHERGredshift evolutionneutral hydrogenMeerKAT survey
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The pith

HI 21-cm absorption occurs at similar rates in radio-loud AGNs from low redshifts up to z ~ 0.5.

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

The paper examines cold neutral gas in radio-loud active galactic nuclei at redshifts less than 0.5 by measuring HI 21-cm absorption with the MeerKAT telescope. After classifying hundreds of sources into low-excitation and high-excitation radio galaxies, it focuses on a bright subsample to search for absorption. Five new cases are detected, yielding an overall rate of 3 percent. This matches rates from earlier surveys at redshifts below 0.2 after accounting for sensitivity. The result implies that the presence of cold gas and its interaction with radio jets does not change much over this redshift interval.

Core claim

The authors report five new HI 21-cm absorption detections in a sample of 99 radio-loud AGNs at 0.29 < z < 0.47. The detection rate of 3+3-2 percent at 3-sigma is consistent with sensitivity-matched samples at z<0.2, with no evidence for redshift evolution or radio luminosity dependence. Absorption profiles indicate disturbed gas kinematics in LERGs with velocity offsets over 350 km/s.

What carries the argument

Classification of AGNs into LERGs and HERGs using BPT diagnostics and radio luminosity, followed by targeted HI 21-cm absorption search in the radio-bright subsample from MALS.

If this is right

  • The cold gas in these systems shows disturbed kinematics likely due to jet or lobe activity.
  • Detection rates do not depend on radio luminosity over six orders of magnitude.
  • Redshifted and blueshifted absorption profiles suggest both inflow and outflow motions.

Where Pith is reading between the lines

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

  • If confirmed, this would mean that models of AGN feedback can use similar gas fractions across this redshift range.
  • The color difference between LERGs and HERGs may reflect different stages of galaxy quenching.
  • Future higher-sensitivity observations could reveal weaker absorption missed in the current threshold.

Load-bearing premise

The selected radio-bright subsample accurately represents the broader population of radio-loud AGNs without selection biases, and prior surveys are correctly matched in sensitivity and resolution.

What would settle it

Observing a detection rate much higher or lower than 3% in an expanded sample at similar redshifts or luminosities would falsify the claim of no evolution.

Figures

Figures reproduced from arXiv: 2604.19896 by F. Combes, H.-R. Kl\"ockner, H.-W. Chen, J-.K. Krogager, N. Gupta, P. Noterdaeme, P. P. Deka, S. A. Balashev.

Figure 1
Figure 1. Figure 1: Distribution of z (first panel), S1.4 GHz (second), L1.4GHz (third) and S/N of [O iii]λ5007 (fourth) for the initial sample of 1908 radio sources (yellow histograms). Of these, 681 are classified as AGNs (blue; see Sections 2.3 and 2.4 for details). Finally, 99 LERGs/HERGs from this subsample were searched for associated H i 21-cm absorption (green hatched; details in Section 3). The low-z deficit in the g… view at source ↗
Figure 2
Figure 2. Figure 2: S/N distribution of various lines used for AGN / SF classification, and the location of sources in the L1.4 GHz–z plane based on the classifica￾tion method (details in Section 2.4). Sources with line S/N< 1 are not shown. The dashed vertical and horizontal lines denote an S/N = 3, except in the bottom right panel where the dashed horizontal line (L1.4 GHz = 1024 W Hz−1 ) marks the boundary segregating SF a… view at source ↗
Figure 3
Figure 3. Figure 3: BPT1 classified AGN, SF, and Composite (COMP) galaxies color-coded according to their 1.4 GHz radio luminosity (L1.4 GHz) [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figure 5
Figure 5. Figure 5: Distribution of α 1.4 1.0 (first), L1.4GHz (second), S1.0GHz (third), z (fourth), and k-corrected g − r (fifth) for the LERG and HERG samples. The red and black dashed lines mark the median values for LERGs and HERGs, respectively. 0.00 0.10 0.20 0.30 0.40 0.50 0 1 2 3 4 5 Probability density 23.6 24.0 24.4 24.8 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0.40 0.60 0.80 1.00 1.20 0 2 4 6 8 10 MALS LERG BH12 LERG 0… view at source ↗
Figure 6
Figure 6. Figure 6: Histograms comparing the distributions of z, L1.4 GHz, and k-corrected (g − r) for LERGs (top panel) and HERGs (bottom panel), using the MALS and Best & Heckman (2012) samples matched in L1.4 GHz. HERGs. Of these, 22 were classified as AGN using BPT1, 36 using the LHα - L1.4GHz criteria and 41 based on L1.4GHz > 1024 W Hz−1 . The sample spans nearly six decades in radio lumi￾nosity and reaching flux densit… view at source ↗
Figure 7
Figure 7. Figure 7: Comparison of LERG fractions between the BH12 (Best & Heckman 2012) and MALS samples, evaluated within two redshift bins. Horizontal error bars denote the width of each redshift bin, while verti￾cal error bars represent the binomial uncertainties on the derived frac￾tions. kinematically disturbed absorbing gas, possibly driven by jet￾ISM interaction. This is consistent with studies of more power￾ful (L1.4 … view at source ↗
Figure 8
Figure 8. Figure 8: The radio continuum images (left) and normalized absorption profiles (right) for the five sources listed in [PITH_FULL_IMAGE:figures/full_fig_p009_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: Distributions of absorption line properties for the five detections. FWBN denotes the full width between nulls (FWBN) in km s−1 , Ap measures the asymmetry of the line profile and |Vpeak| represents the shift of the absorption peak from zem. The median uncertainty in log L1.4GHz is 0.003 dex, and is not visible at the scale of the figure [PITH_FULL_IMAGE:figures/full_fig_p010_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: Top row: MALS sources; Bottom row: 185 (170 LERGs, 15 HERGs) sources from Maccagni et al. (2017). Legends are same across all panels. Vertical and horizontal dashed lines are median values. For better visibility 22 MALS sources (19 LERGs, 4 HERGs) are excluded from the panels with R τ3σ dv > 7 km s−1 . 0.1 0.2 0.3 0.4 0.5 z 20 22 24 26 28 lo g 1 0 L 1.4 G H z (W H z 1 ) L-band coverage Other Literature Gu… view at source ↗
Figure 11
Figure 11. Figure 11: L1.4 GHz vs z at z < 0.5 for MALS HERG/LERGs, Gupta et al. (2006), WSRT (Maccagni et al. 2014) and JVLA (Murthy et al. 2021) samples. Filled and open symbols represent H i detections and non￾detections, respectively. Other literature sources shown for context in￾clude Glowacki et al. (2017); Chandola & Saikia (2017); Allison et al. (2012); Aditya & Kanekar (2018a,b); Chandola et al. (2020); Murthy et al. … view at source ↗
read the original abstract

We present results from a search of cold neutral gas associated with radio-loud active galactic nuclei (AGNs) at $z < 0.5$ using HI 21-cm absorption measurements from the MeerKAT Absorption Line Survey (MALS). Cross-matching the MALS 1006 MHz and SDSS DR18 catalogs yields 1908 radio sources at $z < 0.5$. Of these, 613 are classified as AGNs using BPT diagnostics and radio luminosity criteria. We further classify 426 AGNs into 327 low-excitation radio galaxies (LERGs) and 99 high-excitation radio galaxies (HERGs). We observe a significant ($>3\sigma$) difference in $k$-corrected $g-r$ color, consistent with LERGs residing in older galaxies with quenched star formation. We searched a radio-bright subsample of 79 LERGs and 20 HERGs ($S_{\mathrm{1.4\,GHz}} > 4$ mJy) for associated HI 21-cm absorption. This spans six decades in radio luminosity ($\log L_{\mathrm{1.4\,GHz}}$ (WHz$^{-1}$) $\sim 21.1-27.0$), probing an order of magnitude fainter than previous targeted HI surveys. We report five new detections (4 LERGs, 1 HERG) at $0.29 < z < 0.47$. The overall detection rate of $3^{+3}_{-2}$% (at a $3\sigma$ threshold of 10.0 kms$^{-1}$) is consistent with sensitivity-matched low-$z$ ($<0.2$) samples, suggesting no significant redshift evolution out to $z \sim 0.5$ or dependence on radio luminosity. Evaluating velocity offset, asymmetry, and width reveals three systems with entirely redshifted absorption and two with predominantly blueshifted absorption. HI profiles in LERGs show diverse asymmetries and velocity offsets exceeding 350 kms$^{-1}$, indicating disturbed cold-gas kinematics likely driven by lobe expansion or jet activity.

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

2 major / 3 minor

Summary. The manuscript reports results from a search for associated HI 21-cm absorption toward radio-loud AGNs at z<0.5 drawn from the MeerKAT Absorption Line Survey (MALS). Cross-matching MALS 1006 MHz sources with SDSS DR18 yields 1908 radio sources at z<0.5, of which 613 are classified as AGNs via BPT diagnostics and radio luminosity; these are further divided into 327 LERGs and 99 HERGs. A radio-bright subsample (S_{1.4 GHz}>4 mJy) of 79 LERGs + 20 HERGs is searched, producing five new detections (4 LERGs, 1 HERG) at 0.29<z<0.47. The overall detection rate is reported as 3^{+3}_{-2}% (3σ optical-depth threshold of 10 km s^{-1}), stated to be consistent with sensitivity-matched low-z (<0.2) samples and therefore indicating no significant redshift evolution or radio-luminosity dependence out to z~0.5. Kinematic analysis of the profiles is also presented.

Significance. If the central consistency result holds after verification of the matching procedure, the work supplies a valuable extension of HI absorption statistics to an order of magnitude fainter radio luminosities (log L_{1.4 GHz} ~21.1-27.0) and to z~0.5, reinforcing the picture of little evolution in cold-gas incidence around radio-loud AGNs over the last ~5 Gyr. The LERG/HERG classification, the noted g-r color difference, and the direct count of five detections with Poisson errors constitute concrete observational anchors that future models of AGN feedback and gas accretion can be tested against.

major comments (2)
  1. [Sample selection and results] Subsample selection (radio-bright cut S_{1.4 GHz}>4 mJy): the claim that the 99-source subsample yields a detection rate representative for comparison to prior low-z surveys rests on the assumption that this flux cut does not introduce differential bias relative to the full 426-AGN parent sample; the manuscript does not quantify how gas properties or completeness might differ for the excluded fainter sources.
  2. [Detection statistics and comparison] Sensitivity matching to low-z samples: the consistency statement (detection rate 3^{+3}_{-2}%) requires that noise, resolution, and selection differences between MALS and the reference low-z surveys have been matched at the level of the 10 km s^{-1} 3σ threshold; without an explicit table or equation showing the matched sensitivity distributions, it is not possible to assess whether residual systematics could shift the rate outside the quoted Poisson errors.
minor comments (3)
  1. [Abstract] The abstract states 'six decades in radio luminosity' while the quoted range is log L_{1.4 GHz} ~21.1-27.0 (exactly six dex); this is minor but the precise span should be stated consistently in the text and any summary table.
  2. [Kinematic analysis] The velocity-offset and asymmetry analysis for the five detections is described qualitatively; a supplementary table listing the measured centroid offsets, FWHM, and asymmetry parameters for each profile would improve reproducibility.
  3. [AGN classification] The BPT/radio-luminosity classification criteria that separate the 327 LERGs from the 99 HERGs are referenced but not reproduced; a brief restatement or citation to the exact demarcation lines used would aid readers.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We are grateful to the referee for their careful review and the recommendation for minor revision. We address each of the major comments below and will make the necessary revisions to the manuscript.

read point-by-point responses

  1. Referee: [Sample selection and results] Subsample selection (radio-bright cut S_{1.4 GHz}>4 mJy): the claim that the 99-source subsample yields a detection rate representative for comparison to prior low-z surveys rests on the assumption that this flux cut does not introduce differential bias relative to the full 426-AGN parent sample; the manuscript does not quantify how gas properties or completeness might differ for the excluded fainter sources.

    Authors: The subsample was selected based on the radio flux density to ensure that the MALS observations have adequate sensitivity to detect HI 21-cm absorption at the 3σ level corresponding to an integrated optical depth of 10 km s^{-1}. Sources below this threshold in the parent sample of 426 AGNs would not yield reliable non-detections or detections at this sensitivity, making the detection rate applicable to the sensitivity-matched subsample. We will revise the manuscript to include a more detailed justification of the cut, including a comparison of the radio luminosity and color distributions between the full sample and the subsample, and a discussion of the implications for completeness. This will clarify that the rate is not claimed to be representative of the entire population but of sources with comparable observational sensitivity to previous studies. revision: partial

  2. Referee: [Detection statistics and comparison] Sensitivity matching to low-z samples: the consistency statement (detection rate 3^{+3}_{-2}%) requires that noise, resolution, and selection differences between MALS and the reference low-z surveys have been matched at the level of the 10 km s^{-1} 3σ threshold; without an explicit table or equation showing the matched sensitivity distributions, it is not possible to assess whether residual systematics could shift the rate outside the quoted Poisson errors.

    Authors: We agree that providing explicit details on the sensitivity matching will improve the robustness of our comparison. In the revised version, we will include a new table that lists the key sensitivity parameters (rms noise, channel width, and the resulting 3σ integrated optical depth limit) for our sample and for the low-redshift reference samples from the literature. Additionally, we will describe the matching procedure in the text, including the equation for calculating the sensitivity limit: τ_{3σ} Δv = 3 * (σ_rms / S_cont) * sqrt(Δv * N), where we normalize to a 10 km s^{-1} velocity width. This will demonstrate that the detection rates are compared at equivalent sensitivities, supporting our conclusion of consistency within the quoted uncertainties. revision: yes

Circularity Check

0 steps flagged

No significant circularity in purely observational analysis

full rationale

The paper reports direct catalog cross-matching (MALS 1006 MHz with SDSS DR18), BPT/radio-luminosity classification of 613 AGNs into LERGs/HERGs, a radio-bright subsample selection, and five new HI absorption detections with a measured rate of 3^{+3}_{-2}%. No equations, fitted parameters, predictions, or derivations are present that could reduce to inputs by construction. The consistency statement with low-z samples relies on explicit sensitivity matching and Poisson uncertainties, with no self-citation chains or ansatzes invoked for the central result. This is a standard observational report self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The central claim rests on standard domain assumptions for AGN classification and a chosen detection threshold. No free parameters are fitted to the target result itself; the detection rate is a direct observational count.

free parameters (1)
  • 3-sigma detection threshold of 10.0 km/s
    Arbitrary but conventional threshold used to define detections and compute the rate; affects the quoted 3% value.
axioms (1)
  • domain assumption BPT diagnostics and radio luminosity criteria reliably separate AGNs from other sources and distinguish LERGs from HERGs
    Invoked to classify 613 sources and split into 327 LERGs and 99 HERGs.

pith-pipeline@v0.9.0 · 5744 in / 1467 out tokens · 64479 ms · 2026-05-10T01:39:14.756626+00:00 · methodology

discussion (0)

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Reference graph

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