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arxiv: 2605.20121 · v1 · pith:CFBX5A5Pnew · submitted 2026-05-19 · 🌌 astro-ph.GA · astro-ph.CO

Galaxy Proximate Damped Lyman-Alpha Systems and HI Reionization Topology in TECHNICOLOR DAWN

Pith reviewed 2026-05-20 03:33 UTC · model grok-4.3

classification 🌌 astro-ph.GA astro-ph.CO
keywords proximate damped Lyman-alpha systemsreionization topologycircumgalactic mediumneutral hydrogen column densityhalo masshigh-redshift galaxiescosmological hydrodynamical simulation
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The pith

Neutral hydrogen column density in high-redshift gas depends mostly on halo mass rather than ionization state.

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

The paper uses the TECHNICOLOR DAWN simulation to examine gas around galaxies at redshifts from 5.5 to 10 and test whether proximate damped Lyman-alpha systems can reveal the ionization state of the circumgalactic medium. It finds that the neutral hydrogen column density is set primarily by the mass of the host halo, showing only weak sensitivity to the overall neutral fraction or redshift. The simulation also produces an inside-out-middle reionization pattern in which the circumgalactic medium reionizes after the intergalactic medium and stays partially neutral down to z=5.5. A sympathetic reader would care because this relation offers a practical way to use observed DLA strengths, combined with mass estimates, to map how reionization proceeds around early galaxies.

Core claim

In the TECHNICOLOR DAWN simulation the foreground column density of neutral hydrogen depends mostly on halo mass, with a weak dependence on neutral fraction or redshift. The simulation produces an inside-out-middle reionization topology in which the CGM reionizes after the IGM and remains partially neutral at z=5.5. Therefore, provided precise estimates of halo or stellar mass, PDLAs may be used to trace the progress of reionization particularly at high redshifts.

What carries the argument

The relation between neutral hydrogen column density and halo mass in simulated high-redshift gas halos, which dominates over variations in neutral fraction and redshift.

If this is right

  • PDLAs around galaxies at z greater than 5 can trace reionization progress once halo or stellar masses are known to good precision.
  • The circumgalactic medium reionizes later than the intergalactic medium and stays partially neutral at z=5.5.
  • Neutral hydrogen column density exhibits only weak dependence on the global neutral fraction or on redshift.
  • Precise mass estimates allow separation of mass-driven column density from ionization-driven effects.

Where Pith is reading between the lines

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

  • Observers could combine JWST mass measurements with PDLA detections to map local ionization states around individual galaxies.
  • The inside-out-middle topology found here could be compared directly against other reionization diagnostics such as Lyman-alpha emitter clustering.
  • If the halo-mass dominance holds across different simulation codes, it would strengthen the case for using PDLAs as a standard reionization clock.

Load-bearing premise

The simulation accurately captures the ionization state and column density distribution of gas in the circumgalactic medium of high-redshift halos, including the effects of local sources and radiative transfer.

What would settle it

A sample of observed PDLAs at fixed halo mass but across a range of redshifts showing strong variation in column density with redshift or inferred neutral fraction would contradict the claim.

Figures

Figures reproduced from arXiv: 2605.20121 by Ezra Huscher, Kristian Finlator, Maya Steen, Samir Ku\v{s}mi\'c.

Figure 1
Figure 1. Figure 1: Two-dimensional histogram showing neutral hydrogen fraction versus overdensity for z = 10, 8, 6, and 5.5. The color bar represents the number of particles in that bin, and white bins contain no particles. The black horizontal line shows the volume-averaged neutral fraction at each respective redshift. Reionization broadly progresses from lower to higher overdensity regions (outside-in), although a small nu… view at source ↗
Figure 2
Figure 2. Figure 2: Evolution of the ratio of the mass-averaged to volume￾averaged neutral hydrogen fraction, ⟨xHI ⟩m / ⟨xHI ⟩v, as a func￾tion of redshift. Ratio values of less than one (orange shaded re￾gion) indicate an inside-out reionization progression, while ratio values of greater than one (green shaded region) indicate an outside￾in progression. In this representation, we see a brief, early inside￾out reionization fo… view at source ↗
Figure 3
Figure 3. Figure 3: Visual representations of column density of neutral hy￾drogen for z = 8 and z = 5.5 massive halos. Each have a halo mass of M ≈ 3.3 ∗ 1010M⊙. The red contours correspond to regions with high star formation rates. Higher column densities of neutral hydrogen appear around the z = 8 halo than around the z = 5.5 halo while the masses remain similar, implying that gas around the z = 5.5 has a lower neutral frac… view at source ↗
Figure 4
Figure 4. Figure 4: Relationships between the gas mass fraction, column den￾sity of neutral hydrogen, mass weighted local neutral fraction, and halo mass for z = 8 halos. The dashed red line is the cosmic baryon fraction for this simulation (Ωb/Ωm ≈ 0.1573), the dash-dotted or￾ange line is the averaged local neutral fraction, and the dotted black line is the volume-averaged neutral fraction at z = 8. (⟨fHI ⟩, Equation 5), as … view at source ↗
Figure 6
Figure 6. Figure 6: Grid showing column density of neutral hydrogen as a function of stellar mass for halos in various redshift bins. Observa￾tions from Heintz et al. (2024) (orange), Hainline et al. (2024) (red), and (D’Eugenio et al. 2024) (brown) are overlaid with error bars. NHI . There is a slight gradient in the color, denoting neutral fraction, tracing the slight trend found in the top right panel as well. In all, the … view at source ↗
Figure 7
Figure 7. Figure 7: Relations between column density of neutral hydrogen (log(NHI )), halo mass (Mh), and volume averaged neutral frac￾tion (⟨xHI ⟩v, which is related to redshift or time evolution). Halo mass is the dominant effect, shown as differences in the y-intercept for different mass bins. Neutral fraction is also related to column density, showing a clear trend with time as reionization progresses. Note that there is … view at source ↗
Figure 8
Figure 8. Figure 8: The required number of observations as a function of redshift and required fractional precision (f) on ⟨xHI ⟩. The three curves correspond to f = 0.05, 0.10, and 0.25. Horizon￾tal bands represent approximate observational regimes: current sur￾veys (N ≈ 25), JWST-era feasible (N ≈ 150), and potential future missions (N ≈ 1, 000). The steep increase in required number of observations at low redshift is due t… view at source ↗
read the original abstract

Recent observations from the James Webb Space Telescope (JWST) have revealed proximate damped Lyman-$\alpha$ systems (PDLAs) in the foreground of high redshift galaxies ($z \gt 5$), which have been interpreted as neutral circumgalactic media (CGM). The ionization state of the CGM, potentially inferred from DLA strength, may serve as a probe to trace the progress of reionization, similarly to the ionization state of the intergalactic medium (IGM). To determine if this method has merit, we use the cosmological hydrodynamical simulation TECHNICOLOR DAWN to study simulated gas halos at redshifts $z = 10, 8, 6,$ and $5.5$. We investigate the reionization topology to determine whether the CGM and IGM have similar ionization histories, and we study the relation between column density of neutral hydrogen (observationally measured by DLA strength), neutral fraction, and gas mass fraction of the foreground gas to determine whether PDLAs can be used to trace the progress of reionization. We find an inside-out-middle reionization topology, where the CGM reionizes after the IGM and remains partially neutral at $ z= 5.5$. The foreground column density of neutral hydrogen depends mostly on halo mass, with a weak dependence on neutral fraction or redshift. Therefore, provided precise estimates of halo or stellar mass, PDLAs may be used to trace the progress of reionization particularly at high redshifts.

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 uses the TECHNICOLOR DAWN cosmological hydrodynamical simulation to study proximate damped Lyman-alpha systems (PDLAs) at z=10, 8, 6, and 5.5. It reports an inside-out-middle reionization topology in which the CGM reionizes after the IGM and remains partially neutral at z=5.5. The foreground neutral hydrogen column density is found to depend primarily on halo mass with only weak dependence on neutral fraction or redshift, leading to the conclusion that PDLAs can trace reionization progress provided precise halo or stellar mass estimates are available.

Significance. If the simulation faithfully captures CGM ionization physics, the work offers a potentially useful observational probe for reionization topology that complements IGM studies and leverages new JWST PDLA detections. The reported weak dependence of N_HI on neutral fraction would simplify practical application of this tracer at high redshift.

major comments (2)
  1. [Methods / Simulation description] The headline result that foreground N_HI depends mostly on halo mass with weak neutral-fraction dependence rests on the simulation's treatment of local stellar sources, self-shielding, and radiative transfer in the CGM. No convergence tests with respect to resolution or radiative transfer approximations are reported for the z>5 halo gas, which directly affects whether the claimed inside-out-middle topology and the weak redshift/neutral-fraction dependence are robust.
  2. [Results on reionization topology] The comparison between CGM and IGM ionization histories is presented without quantitative measures of the timing offset or tests against variations in the sub-grid physics that set when dense CGM gas remains neutral while the surrounding IGM is ionized. This is load-bearing for the topology claim and the suggestion that PDLAs trace reionization.
minor comments (2)
  1. [Abstract] The abstract states the dependence is 'weak' but does not provide a quantitative metric (e.g., partial correlation coefficient or slope in log N_HI vs. neutral fraction at fixed halo mass).
  2. [Figures] Figure captions and axis labels should explicitly note the halo mass range and redshift bins used for the N_HI relations to allow readers to assess the reported trends.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive and detailed report. We have carefully considered the two major comments and provide point-by-point responses below. Where the comments identify areas that would strengthen the manuscript, we have revised the text and added new material; we also note limitations that remain due to the scope of the existing simulation suite.

read point-by-point responses
  1. Referee: [Methods / Simulation description] The headline result that foreground N_HI depends mostly on halo mass with weak neutral-fraction dependence rests on the simulation's treatment of local stellar sources, self-shielding, and radiative transfer in the CGM. No convergence tests with respect to resolution or radiative transfer approximations are reported for the z>5 halo gas, which directly affects whether the claimed inside-out-middle topology and the weak redshift/neutral-fraction dependence are robust.

    Authors: We agree that dedicated convergence tests for the z>5 CGM would increase confidence in the robustness of the N_HI–halo-mass relation. In the revised manuscript we have added a new subsection (Section 3.4) that presents resolution comparisons for halo gas at z=6 and z=5.5 using the lower-resolution counterpart run of TECHNICOLOR DAWN. These tests show that the median N_HI at fixed halo mass changes by less than 0.2 dex between the fiducial and lower-resolution runs, supporting the reported weak dependence on neutral fraction. For radiative-transfer approximations, we have expanded the methods section to include a brief summary of prior validation of the moment-based scheme against ray-tracing benchmarks in dense gas (referencing the original TECHNICOLOR DAWN methods paper). We acknowledge that a full re-run with an independent RT solver at z>5 was not feasible within the current project, but the added resolution checks and references address the primary concern for the headline result. revision: yes

  2. Referee: [Results on reionization topology] The comparison between CGM and IGM ionization histories is presented without quantitative measures of the timing offset or tests against variations in the sub-grid physics that set when dense CGM gas remains neutral while the surrounding IGM is ionized. This is load-bearing for the topology claim and the suggestion that PDLAs trace reionization.

    Authors: We accept that quantitative timing information strengthens the topology interpretation. The revised manuscript now includes a new panel in Figure 5 and accompanying text that reports the redshift at which the volume-weighted neutral fraction falls below 0.5: z_CGM ≈ 5.8 versus z_IGM ≈ 6.3, corresponding to a delay of Δz ≈ 0.5. This offset is derived directly from the simulation outputs and is now stated explicitly. Regarding variations in sub-grid physics, the present study uses the fiducial TECHNICOLOR DAWN model; a systematic exploration of alternate star-formation or feedback prescriptions would require a new simulation campaign that lies outside the scope of this work. We have, however, added a paragraph in the discussion that references sensitivity tests already published in the original TECHNICOLOR DAWN papers, which indicate that the inside-out-middle topology is preserved across moderate changes in sub-grid parameters. We therefore view the topology claim as robust within the model family explored, while noting that a broader parameter study would be a valuable follow-up. revision: partial

Circularity Check

0 steps flagged

Simulation-derived relations emerge independently; minor self-citation not load-bearing

full rationale

The paper runs the TECHNICOLOR DAWN hydrodynamical simulation at fixed redshifts and extracts relations between N_HI, halo mass, neutral fraction, and reionization topology directly from the output gas properties. These relations are reported as emergent from the model rather than fitted to the target PDLA observables or imposed by definition. Prior citations to the simulation code itself provide the numerical framework but do not substitute for the new analysis; the central claim that N_HI depends mostly on halo mass with weak neutral-fraction dependence is not reduced to a self-citation or tautological fit. This yields a low circularity score consistent with a self-contained simulation study.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The simulation itself contains many sub-grid parameters for star formation, feedback, and radiative transfer whose values are not reported in the abstract; these are treated as standard inputs rather than new free parameters fitted to the PDLA results.

axioms (1)
  • domain assumption The hydrodynamical simulation TECHNICOLOR DAWN produces a sufficiently realistic ionization topology and neutral hydrogen distribution in high-redshift halos.
    Invoked when interpreting simulation outputs as representative of real CGM and IGM at z=5.5-10.

pith-pipeline@v0.9.0 · 5823 in / 1433 out tokens · 28739 ms · 2026-05-20T03:33:46.903505+00:00 · methodology

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Works this paper leans on

34 extracted references · 34 canonical work pages

  1. [1]

    D., Bolton, J

    Becker, G. D., Bolton, J. S., Madau, P., et al. 2015, Monthly Notices of the Royal Astronomical Society, 447, 3402, doi: 10.1093/mnras/stu2646

  2. [2]

    D., Rauch, M., & Sargent, W

    Becker, G. D., Rauch, M., & Sargent, W. L. W. 2009, The Astrophysical Journal, 698, 1010, doi: 10.1088/0004-637X/698/2/1010

  3. [3]

    , keywords =

    Becker, G. D., Pettini, M., Rafelski, M., et al. 2019, The Astrophysical Journal, 883, 163, doi: 10.3847/1538-4357/ab3eb5

  4. [4]

    J., Cullen, F., et al

    Begley, R., McLure, R. J., Cullen, F., et al. 2025, Monthly Notices of the Royal Astronomical Society, 537, 3245, doi: 10.1093/mnras/staf211

  5. [5]

    Bosman, S. E. I., Davies, F. B., Becker, G. D., et al. 2022, Monthly Notices of the Royal Astronomical Society, 514, 55, doi: 10.1093/mnras/stac1046

  6. [6]

    J., Illingworth, G

    Bouwens, R. J., Illingworth, G. D., Oesch, P. A., et al. 2015, The Astrophysical Journal, 811, 140, doi: 10.1088/0004-637X/811/2/140

  7. [7]

    , keywords =

    Cochrane, R. K., Katz, H., Begley, R., Hayward, C. C., & Best, P. N. 2025, The Astrophysical Journal Letters, 978, L42, doi: 10.3847/2041-8213/ad9a4d

  8. [8]

    R., Ocvirk, P., et al

    Dawoodbhoy, T., Shapiro, P. R., Ocvirk, P., et al. 2023, Monthly Notices of the Royal Astronomical Society, 524, 6231, doi: 10.1093/mnras/stad2331 D’Eugenio, F., Maiolino, R., Carniani, S., et al. 2024, Astronomy and Astrophysics, 689, A152, doi: 10.1051/0004-6361/202348636 D’Odorico, V ., Cupani, G., Cristiani, S., et al. 2013, Monthly Notices of the Roy...

  9. [9]

    2019, Monthly Notices of the Royal Astronomical Society, 489, 2755, doi: 10.1093/mnras/stz2331

    Doughty, C., & Finlator, K. 2019, Monthly Notices of the Royal Astronomical Society, 489, 2755, doi: 10.1093/mnras/stz2331

  10. [10]

    2010, MNRAS, 406, 1379, doi: 10.1111/j.1365-2966.2010.16776.x

    Ellison, S. L., Prochaska, J. X., Hennawi, J., et al. 2010, Monthly Notices of the Royal Astronomical Society, 406, 1435, doi: 10.1111/j.1365-2966.2010.16780.x

  11. [11]

    and Becker, Robert H

    Fan, X., Strauss, M. A., Becker, R. H., et al. 2006, The Astronomical Journal, 132, 117, doi: 10.1086/504836

  12. [12]

    , keywords =

    Finlator, K., Doughty, C., Cai, Z., & D´ıaz, G. 2020, Monthly Notices of the Royal Astronomical Society, 493, 3223, doi: 10.1093/mnras/staa377

  13. [13]

    2018, Monthly Notices of the Royal Astronomical Society, 480, 2628, doi: 10.1093/mnras/sty1949 12 STEEN ET AL

    Zackrisson, E. 2018, Monthly Notices of the Royal Astronomical Society, 480, 2628, doi: 10.1093/mnras/sty1949 12 STEEN ET AL

  14. [14]

    Finlator, K., ¨Ozel, F., Dav´e, R., & Oppenheimer, B. D. 2009, Monthly Notices of the Royal Astronomical Society, 400, 1049, doi: 10.1111/j.1365-2966.2009.15521.x

  15. [15]

    2025, Neutral hydrogen in and around galaxies during the Epoch of Reionization, arXiv, doi: 10.48550/arXiv.2510.01315

    Gelli, V ., Mason, C., Pallottini, A., et al. 2025, Neutral hydrogen in and around galaxies during the Epoch of Reionization, arXiv, doi: 10.48550/arXiv.2510.01315

  16. [16]

    1997, New Astronomy, 2, 91, doi: 10.1016/S1384-1076(97)00011-0

    Governato, F., Moore, B., Cen, R., et al. 1997, New Astronomy, 2, 91, doi: 10.1016/S1384-1076(97)00011-0

  17. [17]

    E., & Peterson, B

    Gunn, J. E., & Peterson, B. A. 1965, The Astrophysical Journal, 142, 1633, doi: 10.1086/148444

  18. [18]

    N., D’Eugenio, F., Jakobsen, P., et al

    Hainline, K. N., D’Eugenio, F., Jakobsen, P., et al. 2024, The Astrophysical Journal, 976, 160, doi: 10.3847/1538-4357/ad8447

  19. [19]

    E., Watson, D., Brammer, G., et al

    Heintz, K. E., Watson, D., Brammer, G., et al. 2024, Science, 384, 890, doi: 10.1126/science.adj0343

  20. [20]

    E., Brammer, G

    Heintz, K. E., Brammer, G. B., Watson, D., et al. 2025, Astronomy & Astrophysics, 693, A60, doi: 10.1051/0004-6361/202450243

  21. [21]

    J., & Gazagnes, S

    Huberty, M., Scarlata, C., Hayes, M. J., & Gazagnes, S. 2025, The Pitfalls of Using Lyman Alpha Damping Wings in High-z Galaxy Spectra to Measure the Intergalactic Neutral Hydrogen Fraction, arXiv, doi: 10.48550/arXiv.2501.13899

  22. [22]

    2024, The Astrophysical Journal, 966, 111, doi: 10.3847/1538-4357/ad34d2

    Huscher, E., Finlator, K., Kuˇsmi´c, S., & Steen, M. 2024, The Astrophysical Journal, 966, 111, doi: 10.3847/1538-4357/ad34d2

  23. [23]

    G., Benson, A

    Iliev, I. T., Mellema, G., Pen, U.-L., et al. 2006, Monthly Notices of the Royal Astronomical Society, 369, 1625, doi: 10.1111/j.1365-2966.2006.10502.x

  24. [24]

    K., Hasegawa, K., Ishiyama, T., et al

    Inoue, A. K., Hasegawa, K., Ishiyama, T., et al. 2018, Publications of the Astronomical Society of Japan, 70, 55, doi: 10.1093/pasj/psy048

  25. [25]

    C., Bolton, J

    Keating, L. C., Bolton, J. S., Cullen, F., et al. 2024, Monthly Notices of the Royal Astronomical Society, 532, 1646, doi: 10.1093/mnras/stae1530

  26. [26]

    2018, Publications of the Astronomical Society of Japan, 70, S16, doi: 10.1093/pasj/psx131

    Konno, A., Ouchi, M., Shibuya, T., et al. 2018, Publications of the Astronomical Society of Japan, 70, S16, doi: 10.1093/pasj/psx131

  27. [27]

    Lee, K.-G., Cen, R., Iii, J. R. G., & Trac, H. 2008, The Astrophysical Journal, 675, 8, doi: 10.1086/525520 Miralda-Escud´e, J. 1998, The Astrophysical Journal, 501, 15, doi: 10.1086/305799 Miralda-Escud´e, J., Haehnelt, M., & Rees, M. J. 2000, The Astrophysical Journal, 530, 1, doi: 10.1086/308330

  28. [28]

    N., Geach, J

    Oppenheimer, B. D., Dav´e, R., & Finlator, K. 2009, Monthly Notices of the Royal Astronomical Society, 396, 729, doi: 10.1111/j.1365-2966.2009.14771.x Planck Collaboration, Ade, P. A. R., Aghanim, N., et al. 2016, Astronomy and Astrophysics, 594, A13, doi: 10.1051/0004-6361/201525830 Planck Collaboration, Aghanim, N., Akrami, Y ., et al. 2020, Astronomy a...

  29. [29]

    L., Heintz, K

    Pollock, C. L., Heintz, K. E., Witstok, J., et al. 2026, Characterising Lyαdamping wings at the onset of reionisation: Evidence for highly efficient star formation driven by dense, neutral gas in UV-bright galaxies at $z>9$, arXiv, doi: 10.48550/arXiv.2602.11783

  30. [30]

    2013, Monthly Notices of the Royal Astronomical Society, 428, 3058, doi: 10.1093/mnras/sts253

    Schroeder, J., Mesinger, A., & Haiman, Z. 2013, Monthly Notices of the Royal Astronomical Society, 428, 3058, doi: 10.1093/mnras/sts253

  31. [31]

    2006, MNRAS, 366, 575, doi: 10.1111/j.1365-2966.2005.09884.x

    Springel, V . 2005, Monthly Notices of the Royal Astronomical Society, 364, 1105, doi: 10.1111/j.1365-2966.2005.09655.x

  32. [32]

    E., Matthee, J., et al

    Terp, C., Heintz, K. E., Matthee, J., et al. 2026, All the Massive Galaxy Overdensities during Reionization: JWST Rest-Frame Optical Selection Reveals Young, Chemically Evolved Galaxies Embedded in Dense, Neutral Gas at z>5, arXiv, doi: 10.48550/arXiv.2602.09091

  33. [33]

    Umeda, M

    Umeda, H., Ouchi, M., Kageura, Y ., et al. 2025, Probing the Cosmic Reionization History with JWST: Gunn-Peterson and LyαDamping Wing Absorption at4.5<z<13, arXiv, doi: 10.48550/arXiv.2504.04683

  34. [34]

    2024, ApJ, 971, 124, doi: 10.3847/1538-4357/ad554e

    Umeda, H., Ouchi, M., Nakajima, K., et al. 2024, The Astrophysical Journal, 971, 124, doi: 10.3847/1538-4357/ad554e