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arxiv: 2510.18398 · v3 · pith:HJKUM4K7new · submitted 2025-10-21 · 🌌 astro-ph.GA

A Census of Double-Peaked Lyman-alpha Emitters in MAGPI: Classification, Global Characteristics, and Spatially Resolved Properties

Tamal Mukherjee , Tayyaba Zafar , Themiya Nanayakkara , Siddhartha Gurung-Lopez , Anshu Gupta , Scott M. Croom , Andrew Battisti , Karl Glazebrook
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Polychronis Papaderos Melissa Riggs Emily Wisnioski Caroline Foster Katherine E. Harborne Claudia D. P. Lagos Trevor Mendel Jahang Prathap Stefania Barsanti Sarah M. Sweet Lucas M. Valenzuela Anilkumar Mailvaganam
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Pith reviewed 2026-05-21 20:29 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords double-peaked Lyman-alphaLyman-alpha emittersLyman continuum escapehigh-redshift galaxiesMAGPI surveyHI column densitygas kinematicsMUSE spectroscopy
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The pith

Double-peaked Lyman-alpha profiles in 417 high-redshift galaxies yield five strong Lyman-continuum leaker candidates.

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

The paper catalogs double-peaked Lyα emitters drawn from 417 sources at redshifts 2.9 to 6.6 observed with VLT/MUSE in the MAGPI survey. An automated classification isolates 108 double-peaked objects, showing the fraction falling from roughly 37 percent below redshift 4 to 14 percent above, a change ascribed to stronger intergalactic medium absorption at earlier times. Spatial mapping of ten extended halos reveals rising blue-to-total flux and trough flux with radius together with shrinking peak separation, while five sources are singled out as strong LyC-leaker candidates on the basis of peak separation, red-peak asymmetry, and residual trough flux.

Core claim

From a parent sample of 417 LAEs, 108 are classified as double-peaked. The double-peak fraction declines with redshift, consistent with increasing IGM attenuation. Blue-to-total flux rises for fainter lines, and residual trough flux anticorrelates with peak separation, pointing to variations in HI column density. In extended halos, blue flux and trough flux density increase outward while peak separation decreases, matching changes in outflow velocity and column density. Five objects satisfy the joint criteria of large peak separation, high red-peak asymmetry, and detectable trough flux and are therefore presented as strong LyC-leaker candidates.

What carries the argument

Automated peak classification technique that flags double-peaked profiles using peak separation, red-peak asymmetry, and normalized residual trough flux to select LyC-leaker candidates.

If this is right

  • Double-peak fractions decrease at z greater than 4 mainly because of stronger IGM attenuation rather than changes inside the galaxies.
  • Fainter Lyα lines show higher blue-to-total flux ratios, indicating a luminosity dependence in the underlying gas kinematics.
  • In extended halos the blue flux fraction and trough flux density rise with radius while peak separation falls, consistent with outward drops in HI column density or changes in shell velocity.
  • Red-peak narrowing persists at z greater than 4 even when a blue peak is present, pointing to intrinsic galaxy evolution.

Where Pith is reading between the lines

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

  • The same profile criteria could be applied to deeper surveys to enlarge the sample of profile-selected LyC-leaker candidates at the epoch of reionization.
  • Radial trends observed in the halos supply concrete targets for hydrodynamic simulations that track outflow velocity and neutral-hydrogen distribution.
  • Direct Lyman-continuum detections for the five flagged objects would test whether the profile-based selection reliably predicts escape.

Load-bearing premise

The automated peak classification technique reliably identifies true double-peaked profiles without substantial contamination from noise or single-peaked sources.

What would settle it

Higher signal-to-noise or multi-instrument spectroscopy of the same 417 sources to test whether the reported double peaks, residual trough flux, and selected candidates persist or disappear.

read the original abstract

Double-peaked Ly$\alpha$ profiles provide critical insights into gas kinematics and the distribution of neutral hydrogen (HI) from the interstellar to the intergalactic medium (ISM to IGM), and serve as valuable diagnostics of ionising Lyman continuum (LyC) photon escape. We present a study of the global and spatially resolved properties of double-peaked Ly$\alpha$ emitters (LAEs) based on VLT/MUSE data from the MAGPI survey. From a parent sample of 417 LAEs at z = 2.9 - 6.6 in the first 35 fields, we identify 108 double-peaked LAEs using an automated peak classification technique. We measure a double-peak fraction of $\sim37\%$ at $z < 4$, decreasing to $\sim14\%$ at $z > 4$, likely due to enhanced IGM attenuation. Approximately $17\%$ of the double-peaked LAEs are blue-dominated, possibly tracing gas inflows, though backscattering remains a viable alternative for sources without systemic redshift. The blue-to-total flux ratio exhibits a luminosity dependence: fainter lines generally show higher blue flux. We find a narrowing of the red peak at $z > 4$, despite the presence of the blue peak, indicating intrinsic galaxy evolution rather than IGM attenuation. Several LAEs exhibit residual flux in the absorption trough, with normalised trough flux anticorrelating with peak separation, reflecting variations in HI column density. We further investigate spatially resolved properties of ten red-dominated LAEs with extended Ly$\alpha$ halos. Despite azimuthal variations, both the blue-to-total flux ratio and normalised trough flux density increase with radius, while peak separation decreases. The red peak asymmetry shows only minor radial changes. These trends are consistent with variations in shell outflow velocity and HI column density across the halos. Based on peak separation, red peak asymmetry, and residual trough flux, we identify five LAEs as strong LyC-leaker candidates.

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 / 2 minor

Summary. The manuscript presents a census of double-peaked Lyα emitters from the MAGPI survey using VLT/MUSE data. From a parent sample of 417 LAEs at z=2.9-6.6, the authors identify 108 double-peaked LAEs via an automated peak classification technique. They report a double-peak fraction of ~37% at z<4 decreasing to ~14% at z>4, analyze global properties including ~17% blue-dominated profiles, luminosity dependence of blue-to-total flux ratio, red-peak narrowing at z>4, and residual trough flux trends, examine spatially resolved radial trends in 10 red-dominated LAEs with extended halos, and select five strong LyC-leaker candidates based on peak separation, red-peak asymmetry, and residual trough flux.

Significance. If the classification is robust, the work provides a statistically useful sample for studying gas kinematics, HI column density variations, and potential LyC escape in high-redshift galaxies. The redshift evolution of the double-peak fraction, the luminosity dependence, and the radial trends in extended halos offer observational constraints on outflow/inflow models. Strengths include the large parent sample from the MAGPI survey and the spatially resolved analysis performed on a subset of ten objects.

major comments (2)
  1. [Methods / Abstract] Methods (automated peak classification): The identification of the 108 double-peaked LAEs, the reported fractions (~37% at z<4 and ~14% at z>4), the blue-dominated subset, and the downstream selection of five LyC-leaker candidates all rest on the automated peak classification technique. The manuscript provides no quantitative validation such as false-positive rates from noise simulations, injection-recovery tests, or agreement metrics with visual inspection. This is load-bearing for the central claims, as contamination from noise fluctuations or misclassified single-peaked sources at MUSE resolution and S/N could inflate fractions and bias parameter cuts on peak separation, asymmetry, and trough flux.
  2. [Results] Results (LyC-leaker candidates): The five strong LyC-leaker candidates are selected using peak separation, red peak asymmetry, and residual trough flux, but the manuscript does not specify the exact quantitative thresholds applied or compare them directly to known leakers in the literature. This makes it difficult to evaluate the robustness and potential false-positive rate of the candidate list.
minor comments (2)
  1. [Methods] The exact parameter values or decision criteria used in the automated classification algorithm should be stated explicitly to support reproducibility.
  2. [Results] Error bars or uncertainty estimates on the double-peak fractions and radial trends would help assess the statistical significance of the reported redshift and luminosity dependences.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their careful and constructive review of our manuscript. We address each major comment below in detail and indicate the changes we will implement in the revised version to strengthen the presentation of our methods and results.

read point-by-point responses

  1. Referee: [Methods / Abstract] Methods (automated peak classification): The identification of the 108 double-peaked LAEs, the reported fractions (~37% at z<4 and ~14% at z>4), the blue-dominated subset, and the downstream selection of five LyC-leaker candidates all rest on the automated peak classification technique. The manuscript provides no quantitative validation such as false-positive rates from noise simulations, injection-recovery tests, or agreement metrics with visual inspection. This is load-bearing for the central claims, as contamination from noise fluctuations or misclassified single-peaked sources at MUSE resolution and S/N could inflate fractions and bias parameter cuts on peak separation, asymmetry, and trough flux.

    Authors: We agree that explicit quantitative validation of the automated peak classification is necessary to support the reported double-peak fractions and all downstream results. The current manuscript describes the classification procedure but does not include formal metrics such as false-positive rates from noise simulations or agreement statistics with visual inspection. In the revised version we will add a dedicated subsection presenting (i) results from noise-only simulations to quantify contamination rates at the relevant MUSE S/N and resolution, (ii) agreement metrics between the automated classifier and visual classification on a representative subsample of at least 50 sources, and (iii) a brief discussion of how these tests constrain possible biases in the reported redshift and luminosity trends. These additions will directly address the robustness concerns raised. revision: yes

  2. Referee: [Results] Results (LyC-leaker candidates): The five strong LyC-leaker candidates are selected using peak separation, red peak asymmetry, and residual trough flux, but the manuscript does not specify the exact quantitative thresholds applied or compare them directly to known leakers in the literature. This makes it difficult to evaluate the robustness and potential false-positive rate of the candidate list.

    Authors: We acknowledge that the selection of the five LyC-leaker candidates would be clearer with explicit thresholds and literature comparisons. The manuscript currently states that candidates were chosen on the basis of peak separation, red-peak asymmetry, and residual trough flux, but does not list the precise numerical cuts or place the candidates in context with published leakers. In the revision we will (i) state the exact quantitative thresholds applied (e.g., peak separation greater than a stated velocity, asymmetry below a stated value, and normalised trough flux below a stated limit), (ii) add a short table or paragraph directly comparing our candidates to well-studied LyC leakers from the literature (e.g., those with confirmed escape fractions), and (iii) discuss the implied false-positive considerations. These changes will allow readers to assess the selection more rigorously. revision: yes

Circularity Check

0 steps flagged

No significant circularity in observational classification chain

full rationale

The paper reports direct empirical results from VLT/MUSE spectra: identification of 108 double-peaked LAEs out of 417 using an automated peak classification technique, measured double-peak fractions (~37% at z<4 dropping to ~14% at z>4), blue-to-total flux ratios, radial trends in ten objects, and selection of five LyC-leaker candidates based on observed peak separation, red-peak asymmetry, and trough flux. These are classifications and measurements applied to survey data, with no equations, fitted parameters, or predictions that reduce by construction to inputs within the same analysis. No self-definitional steps, fitted-input predictions, or load-bearing self-citation chains appear; the derivation remains self-contained against the external MAGPI spectra.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The analysis rests on standard astrophysical interpretations of Lyα radiative transfer and IGM absorption. The automated classification introduces implicit decision thresholds whose values are not stated in the abstract.

free parameters (1)
  • Peak classification thresholds
    The automated technique for identifying double peaks requires specific flux, separation, or significance criteria whose exact values are not reported in the abstract.
axioms (1)
  • domain assumption Double-peaked Lyα profiles encode information about neutral hydrogen column density, gas kinematics, and Lyman-continuum escape
    This standard interpretation is invoked to link observed peak properties to inflows, outflows, IGM attenuation, and LyC leakage throughout the abstract.

pith-pipeline@v0.9.0 · 6012 in / 1621 out tokens · 110954 ms · 2026-05-21T20:29:24.263561+00:00 · methodology

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Forward citations

Cited by 3 Pith papers

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    astro-ph.CO 2026-04 unverdicted novelty 7.0

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  2. Connecting CGM enrichment with Lyman alpha emitters at 2.9 < z < 6.7

    astro-ph.GA 2026-05 unverdicted novelty 5.0

    Blind MUSE search detects 156 LAEs at 2.9<z<6.7 with 34 C IV and 14 Mg II absorber associations, indicating low-mass galaxies enrich the CGM.

  3. Unveiling Hidden Lyman Alpha Emitters in the DESI DR1 Data

    astro-ph.GA 2026-05 unverdicted novelty 5.0

    A CNN detects 19,685 LAEs at z=2-3.5 in DESI DR1 spectra with 95% purity and completeness.