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arxiv: 2512.18281 · v2 · pith:3NJ5SSTMnew · submitted 2025-12-20 · 🌌 astro-ph.GA · astro-ph.CO

Measurements of quasar proximity zones with the Lyα forest of DESI Y1 quasars

Ryuichiro Hada , Paul Martini , David H. Weinberg , Zheng Zheng , J. Aguilar , S. Ahlen , D. Bianchi , D. Brooks
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T. Claybaugh A. Cuceu A. de la Macorra S. Ferraro A. Font-Ribera J. E. Forero-Romero E. Gazta\~naga G. Gutierrez J. Guy H. K. Herrera-Alcantar K. Honscheid M. Ishak R. Joyce D. Kirkby T. Kisner A. Kremin C. Lamman M. Landriau L. Le Guillou A. Meisner R. Miquel A. Mu\~noz-Guti\'errez N. Palanque-Delabrouille W. J. Percival C. Poppett F. Prada I. P\'erez-R\`afols G. Rossi E. Sanchez D. Schlegel M. Schubnell J. Silber D. Sprayberry G. Tarl\'e B. A. Weaver H. Zou
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Pith reviewed 2026-05-21 16:08 UTC · model grok-4.3

classification 🌌 astro-ph.GA astro-ph.CO
keywords quasar proximity zonesLy alpha forestintergalactic mediumDESI surveytransverse proximity effectgas overdensityquasar environments
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The pith

Gas clustering near quasars creates stronger Lyman-alpha absorption on nearby sightlines than their ionizing radiation explains.

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

This paper uses over 10,000 quasar pairs from DESI Year 1 data to measure how foreground quasars at redshifts 2 to 3.5 affect Lyman-alpha absorption in background quasar spectra at projected separations under 2 megaparsecs. Enhanced gas clustering around the quasars produces more absorption than the extra ionizing photons from the quasar itself can remove. From the absorption signal and the assumption of steady isotropic emission, the authors infer that the intergalactic medium reaches overdensities of about 10 at distances of 1 megaparsec from the brightest quasars. The absorption strength shows no clear change with the luminosity of the foreground quasar, which the paper attributes to either a balance between extra gas and extra ionization or to quasars whose output varies strongly in time or direction.

Core claim

Enhanced gas clustering near quasars dominates over their ionizing effect, leading to stronger absorption on neighboring sightlines. Under the assumption that quasar ionizing luminosity is isotropic and steady, the IGM overdensity profile reaches Δ ∼ 10 at comoving distance ∼ 1 h^{-1} Mpc from the most luminous systems, yet the proximity profile shows no significant dependence on foreground quasar luminosity.

What carries the argument

The transverse proximity effect, measured as excess Lyman-alpha absorption in background quasar sightlines at small projected separations from a foreground quasar.

If this is right

  • The intergalactic medium within 1-2 h^{-1} Mpc of luminous quasars is overdense by factors of several to ten relative to the cosmic mean.
  • Gas overdensity effects outweigh the ionizing influence of the quasar in determining the observed absorption on transverse sightlines.
  • The absence of luminosity dependence in the absorption profile implies either a precise cancellation between gas and radiation or that observed luminosity does not trace the time-averaged ionizing output.
  • Models of quasar environments at z ∼ 2-3.5 must incorporate strong small-scale clustering to match the measured absorption signal.

Where Pith is reading between the lines

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

  • If quasar emission is highly anisotropic, the ionizing flux reaching nearby sightlines may be much lower than the observed luminosity suggests.
  • Strong time variability would mean the current observed luminosity is a poor proxy for the radiation that has shaped the surrounding gas over the light-travel time.
  • These measurements could be combined with halo-mass estimates to test whether more luminous quasars occupy denser environments in a way that exactly offsets their higher ionizing output.

Load-bearing premise

Quasar ionizing luminosity is both isotropic and steady on the timescales and directions relevant to nearby sightlines.

What would settle it

A statistically significant increase in absorption strength with increasing foreground quasar luminosity, measured in finer luminosity bins with the same or larger sample, would contradict the reported lack of dependence.

Figures

Figures reproduced from arXiv: 2512.18281 by A. Cuceu, A. de la Macorra, A. Font-Ribera, A. Kremin, A. Meisner, A. Mu\~noz-Guti\'errez, B. A. Weaver, C. Lamman, C. Poppett, David H. Weinberg, D. Bianchi, D. Brooks, D. Kirkby, D. Schlegel, D. Sprayberry, E. Gazta\~naga, E. Sanchez, F. Prada, G. Gutierrez, G. Rossi, G. Tarl\'e, H. K. Herrera-Alcantar, H. Zou, I. P\'erez-R\`afols, J. Aguilar, J. E. Forero-Romero, J. Guy, J. Silber, K. Honscheid, L. Le Guillou, M. Ishak, M. Landriau, M. Schubnell, N. Palanque-Delabrouille, Paul Martini, R. Joyce, R. Miquel, Ryuichiro Hada, S. Ahlen, S. Ferraro, T. Claybaugh, T. Kisner, W. J. Percival, Zheng Zheng.

Figure 1
Figure 1. Figure 1: Spectrum of the Lyα forest of a background quasar that has a minimum projected separation of 1.86 h −1Mpc from a foreground quasar. The sky blue and orange circles indicate the spatial positions of the foreground and background quasars, respectively. The dashed vertical lines mark the Lyα wavelengths of these quasars. The pink-shaded region represents the proximity region, defined as a comoving line-of-sig… view at source ↗
Figure 2
Figure 2. Figure 2: Distributions and stacked Lyα transmissions for three subsamples divided by transverse distance (top), redshift (middle), and luminosity (bottom). Left: Histograms of each subsample, with solid vertical lines showing the median values. Right: Stacked Lyα transmissions around foreground quasars (solid) and corresponding control samples (dashed) as a function of line-of-sight separation from the foreground q… view at source ↗
Figure 3
Figure 3. Figure 3 [PITH_FULL_IMAGE:figures/full_fig_p012_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Luminosity dependence of Lyα optical depth variations in quasar proximity regions (solid lines). For comparison, we also show the expected contribution from quasar ionizing radiation alone (dashed lines), corresponding to the case where the overdensity is fixed at ∆ = 1 in Equation 10. Line colors and vertical lines are the same as in [PITH_FULL_IMAGE:figures/full_fig_p013_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Inferred gas overdensity profiles around quasars for the luminosity subsamples, shown as a function of line-of-sight separation r∥ (left panel) and 3D distance r3D (right panel). The overdensities are derived from the τprx/τIGM profiles in [PITH_FULL_IMAGE:figures/full_fig_p014_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Inferred gas overdensity profiles around quasars for the transverse-distance subsamples (left panel), and the ratio of overdensities between the near side (r∥ < 0) and the far side (r∥ > 0) based on those profiles (right panel). The overdensities are derived from the τprx/τIGM profiles for the transverse-distance subsamples using Equation 10, with (solid lines) and without (dashed lines) including the cont… view at source ↗
Figure 7
Figure 7. Figure 7: Comparison of Lyα transmission uncertainties estimated using two methods: the standard deviation across the entire sample (black) and bootstrap resampling with N = 100 (green). Results are shown for both background quasars (solid lines) and the control samples (dashed lines). Carswell, R. F., Whelan, J. A. J., Smith, M. G., Boksenberg, A., & Tytler, D. 1982, MNRAS, 198, 91, doi: 10.1093/mnras/198.1.91 Chau… view at source ↗
read the original abstract

The intergalactic medium (IGM) around quasars is shaped by their dense environments and by their excess ionizing radiation, forming a "quasar proximity zone" whose size and anisotropy depend on the quasar's halo mass, luminosity, age, and radiation geometry. Using over 10,000 quasar pairs from the Dark Energy Spectroscopic Instrument (DESI) Year 1 data, with projected comoving separations $r_{\perp} < 2\,h^{-1}{\rm Mpc}$, we investigate how the proximity zone of foreground quasars at $z\sim2{\rm-}3.5$ affects Lyman-alpha absorption in their background quasars. The large DESI sample enables unprecedented precision in measuring this "transverse proximity" effect, allowing a detailed investigation of the signal's dependence on the projected separation of quasar pairs and the luminosity of the foreground quasar. We find that enhanced gas clustering near quasars dominates over their ionizing effect, leading to stronger absorption on neighboring sightlines. Under the assumption that quasar ionizing luminosity is isotropic and steady, we infer the IGM overdensity profile in the vicinity of quasars, finding overdensities as high as $\Delta \sim 10$ at comoving distance $\sim 1\,h^{-1}{\rm Mpc}$ from the most luminous systems. Surprisingly, however, we find no significant dependence of the proximity profile on the luminosity of the foreground quasar. This lack of luminosity dependence could reflect a cancellation between higher ionizing flux and higher gas overdensity, or it could indicate that quasar emission is highly time variable or anisotropic, so that the observed luminosity does not trace the ionizing flux on nearby sightlines.

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. Using over 10,000 quasar pairs from DESI Y1 with projected separations r_⊥ < 2 h^{-1} Mpc at z ∼ 2–3.5, the paper measures the transverse proximity effect on Lyα forest absorption in background quasars. It reports that enhanced gas clustering dominates over the ionizing effect of foreground quasars, producing stronger absorption on neighboring sightlines. Under the explicit assumption of isotropic and steady quasar ionizing luminosity, the authors infer an IGM overdensity profile reaching Δ ∼ 10 at comoving distances ∼ 1 h^{-1} Mpc around the most luminous systems. The analysis finds no statistically significant dependence of the proximity absorption profile on foreground quasar luminosity.

Significance. If the central measurements and inference hold after addressing the noted assumption, the work provides a high-precision statistical constraint on quasar environments and IGM structure using the largest sample to date. The quantitative overdensity profile and the reported lack of luminosity dependence would offer new observational input for models of gas clustering around quasars and for the time variability or anisotropy of quasar emission, with direct relevance to studies of the ionizing background and quasar feedback.

major comments (2)
  1. [Abstract] Abstract (and §4): The quantitative inference of the IGM overdensity profile Δ(r) with values as high as Δ ∼ 10 at ∼1 h^{-1} Mpc is derived under the assumption that quasar ionizing luminosity is isotropic and steady, so that observed luminosity directly determines the photoionization rate at transverse distance r_⊥. The same dataset shows no significant luminosity dependence in the proximity absorption profile. This absence of the expected trend (higher L should reduce absorption once clustering is accounted for) creates a direct tension with the assumption used to convert absorption to Δ; the paper notes possible cancellation or variability but does not quantify how these alternatives affect the reported Δ values.
  2. [§3] §3 (data selection and error budget): The robustness of the reported lack of luminosity dependence, which is central to the tension above, depends on the precise definition of luminosity bins, the handling of redshift evolution, and the full covariance matrix for the absorption measurements. These details are needed to confirm that the null result is not driven by sample variance or selection effects that could also bias the inferred Δ profile.
minor comments (2)
  1. [Figures] Figure captions should explicitly define the absorption statistic (e.g., mean transmitted flux or equivalent width) and the exact radial binning used for the proximity profiles.
  2. [Abstract] The abstract states the redshift range as z ∼ 2–3.5; the main text should confirm whether this is the foreground or background quasar redshift and whether any evolution within the bin is modeled.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their careful reading and constructive comments on our manuscript. We address each major comment below and describe the revisions we intend to make.

read point-by-point responses

  1. Referee: [Abstract] Abstract (and §4): The quantitative inference of the IGM overdensity profile Δ(r) with values as high as Δ ∼ 10 at ∼1 h^{-1} Mpc is derived under the assumption that quasar ionizing luminosity is isotropic and steady, so that observed luminosity directly determines the photoionization rate at transverse distance r_⊥. The same dataset shows no significant luminosity dependence in the proximity absorption profile. This absence of the expected trend (higher L should reduce absorption once clustering is accounted for) creates a direct tension with the assumption used to convert absorption to Δ; the paper notes possible cancellation or variability but does not quantify how these alternatives affect the reported Δ values.

    Authors: We agree that the lack of luminosity dependence introduces tension with the assumption of isotropic and steady emission used to derive Δ(r). The reported overdensity profile is presented explicitly under that assumption, as noted in the abstract and §4. The null result on luminosity may indicate that observed luminosity does not trace the ionizing flux on transverse sightlines, for instance due to variability or anisotropy. While the manuscript discusses these possibilities qualitatively, we acknowledge that a more quantitative assessment of their impact on the inferred Δ values would be useful. In the revised manuscript we will expand the discussion in §4 with a simple parametric estimate (e.g., incorporating a duty cycle or opening angle) to illustrate how such effects could modify the conversion from observed absorption to overdensity. revision: yes

  2. Referee: [§3] §3 (data selection and error budget): The robustness of the reported lack of luminosity dependence, which is central to the tension above, depends on the precise definition of luminosity bins, the handling of redshift evolution, and the full covariance matrix for the absorption measurements. These details are needed to confirm that the null result is not driven by sample variance or selection effects that could also bias the inferred Δ profile.

    Authors: We thank the referee for highlighting the importance of documenting these analysis choices. We will revise §3 to state the exact luminosity bin boundaries, describe the procedure used to control for redshift evolution (including any matching or slicing), and provide the full covariance matrix (currently estimated via bootstrap) either in the main text or as supplementary material. We will also add a brief statement on additional robustness tests against sample variance. These clarifications should allow readers to evaluate whether the null luminosity dependence is robust. revision: yes

Circularity Check

0 steps flagged

No significant circularity; observational measurement with explicit assumption

full rationale

The paper reports direct measurements of Lyα absorption excess as a function of projected separation and foreground quasar luminosity from DESI Y1 data. The inference of an IGM overdensity profile Δ(r) is presented explicitly under the stated assumption of isotropic and steady ionizing luminosity (abstract), without deriving or fitting that assumption from the data itself. The reported lack of luminosity dependence is treated as an empirical finding and potential caveat rather than input to a self-referential loop. No equations reduce a prediction to a fitted parameter by construction, no self-citations bear the central load, and no ansatz or uniqueness theorem is smuggled in. The derivation chain consists of empirical measurements conditional on an external assumption, making the result self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The abstract supplies only the high-level measurement and one explicit modeling assumption; no free parameters, invented entities, or additional axioms are described.

axioms (1)
  • domain assumption quasar ionizing luminosity is isotropic and steady
    Invoked when converting the observed absorption signal into an IGM overdensity profile.

pith-pipeline@v0.9.0 · 6092 in / 1186 out tokens · 33213 ms · 2026-05-21T16:08:41.897886+00:00 · methodology

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    work page internal anchor Pith review Pith/arXiv arXiv doi:10.5281/zenodo.831784 2021