Development of Hydrogen and Helium Proximity Zones around Quasars

A. Meiksin; D. Syphers; W. Zheng

REVIEW 3 major objections 4 minor 46 references

Development of Hydrogen and Helium Proximity Zones around Quasars

T0 review · 3 major / 4 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read Quasar H I proximity zones are age indicators: their sizes track He II zones at z~3-4, so the hydrogen ionizing front expands no faster than the helium front, and H I zone size can identify quasars younger than 1 Myr.

desk verdict A potentially important first comparison of H I and He II proximity zones, but the H I zone sizes rest on an unvalidated visual estimator that a referee should probe. read the letter →

arxiv 1908.04691 v1 pith:LKIVBEQU submitted 2019-08-13 astro-ph.GA astro-ph.CO

W. Zheng , A. Meiksin , D. Syphers This is my paper · ORCID

classification astro-ph.GAastro-ph.CO

keywords quasarproximityzonesHeIIreionizationLyman-alphaforestageintergalacticmediumionizingfrontpropagationHE2347-4342flickering

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

The paper compares the ultraviolet helium (He II) spectra of 15 quasars at $z \sim 3$--$4$ with high-resolution optical hydrogen (H I) spectra and finds that the sizes of the He II and H I proximity zones are significantly correlated (Pearson $r = 0.62$, probability of no correlation 1.2%). The key case is HE2347-4342, a luminous quasar with a null zone in both He II and H I, implying an age below 0.2 Myr; three other quasars also show small zones in both species. The authors conclude that H I ionization zones do not expand considerably faster than their He II counterparts, so H I proximity zones can serve as indicators of quasar age. If right, this would make quasar ages measurable from optical spectra alone and would favor quasar activity that is episodic rather than steady, since an infinite-speed H I front would erase the age signal.

What carries the argument

The H I proximity zone size is estimated from the hydrogen Lyman-$\alpha$ forest: absorption lines with rest-frame equivalent width below 0.2 Å are tabulated within about 23 Mpc of the quasar, and a 30% decline in either the line number or the total equivalent width (computed as geometric mean times number) marks the zone edge. Features that decline and then recover toward the quasar are classified as foreground external voids rather than intrinsic zones. The He II zone size is taken from the UV proximity profile at the point where the transmitted flux drops to 10% of the unattenuated level. Both sizes are normalized by the characteristic radius $R_{\omega=1}$ at which the quasar ionizing flux equals the metagalactic background ($\Gamma_{\rm HI}=10^{-12}\,{\rm s^{-1}}$ and $\Gamma_{\rm HeII}=10^{-14.3}\,{\rm s^{-1}}$); comparing measured to normalized sizes is what converts zone-size data into statements about quasar age.

What would settle it

Build mock quasar sightlines with known intrinsic H I zone sizes and embedded foreground external voids, then apply the 30%-decline and 'decline then recover' recipe; if the recovered zone sizes do not match the inputs, the reported correlation would not be established. Alternatively, a luminous quasar with a small He II zone but a full-sized H I zone would directly falsify the claim that H I fronts do not outrun He II fronts.

Watch

Extended reading notes

Core claim

The central claim is that at $z\sim3$--$4$ the H I and He II proximity zones around quasars develop together, not at very different speeds. Quantitatively, the He II and H I zone sizes for the 15 quasars give a correlation coefficient $r=0.62$ with a no-correlation probability of 1.2%; after normalizing each zone by the characteristic radius at which the quasar's ionizing flux equals the metagalactic background, the correlation is $r=0.69$ with probability 0.46%, and the regression slope of 0.39 indicates H I zones are usually smaller than He II zones. The most extreme object, HE2347-4342, has essentially no zone in either species ($R_{\rm HI} = 0.8 \pm 0.8$ Mpc and $R_{\rm HeII} < 1.3$ Mpc) despite a luminosity that should produce zones of roughly 20--30 Mpc. The paper interprets the paired small zones as young quasars, with the H I zone responding to the latest ionizing burst on a $\sim10^4$ yr timescale while the He II zone integrates over the quasar's lifetime; this explains why small H I zones are not masked by a fast H I front.

Load-bearing premise

The H I zone-size measurements depend on treating a 30% decline in the number of weak hydrogen absorption lines as the edge of the ionization zone, and on treating any 'decline then recover' feature as a foreground void rather than an intrinsic zone; if that classification is mistaken, the measured H I sizes and their correlation with He II sizes could be artifacts.

Editorial extensions

If this is right

The four quasars with small He II zones also have small H I zones, so a young quasar does not acquire a large, full-sized H I proximity zone.
H I and He II zone sizes remain correlated after normalization for luminosity, so H I zones can be used as an age indicator at $z\sim3$--$4$.
The paired small zones rule out the assumption that H I ionizing fronts propagate at essentially the speed of light along the line of sight.
Quasars showing zones smaller than about 3 Mpc should be considerably younger than about 1 Myr.
Episodic quasar activity naturally explains the paired sizes, with H I reacting to the latest burst and He II integrating the entire quasar lifetime.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

If H I zones really track age, then large ground-based Lyman-alpha forest surveys could measure quasar age distributions at $z\sim3$--$4$ without the expensive UV spectra needed for He II, increasing the sample by orders of magnitude.
The flickering scenario predicts that a quasar observed just after a new burst could show a re-established H I zone while its He II zone is still small on a $\sim10^4$ yr timescale; pairing zone sizes with continuum light curves would test this.
Alternatively, the small H I zones could be caused by dense, rapidly recombining gas in the quasar's host environment rather than by youth; measuring the foreground absorber distribution along each line of sight would separate density effects from age effects.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

Colleague,

The genuinely new thing here is the first direct comparison of H I and He II proximity-zone sizes in the same 15 quasars. The claim that young quasars with small He II zones also show small H I zones—most strikingly HE2347–4342 with null zones in both—contradicts the usual assumption that H I zones should appear full-sized in young objects because the IGM hydrogen is already highly ionized. If it holds, it gives you an optical probe of quasar age usable for many more quasars than the UV He II observations.

The archival work is solid: consistent remeasurement of the He II zones, careful treatment of redshift uncertainties, and clear figures of the individual spectra. The correlation holds after normalizing by luminosity (r = 0.69, p = 0.46%), which is a good check. The time-dependent ionization simulations are reasonable for the age interpretation, and the authors are appropriately cautious about model dependencies. The citation pattern is fine—they engage with the relevant He II papers and standard IGM references.

The soft spot is the measurement of the H I zone sizes. They come from a 30% decline in weak Ly-alpha line counts or total EW, with errors from visual inspection. The authors reject larger candidate voids as external foreground effects whenever the line density declines and then recovers. That criterion is physically plausible, but it is applied visually and without any mock-catalog validation. The rejection is done knowing the He II zone size and the age narrative, so it could systematically push the H I values toward the He II values and inflate the correlation. The r = 0.62, p = 1.2% is also driven by a small group of points; a few reclassifications would change the result.

That said, the central observation—that some quasars with tiny He II zones also lack H I proximity effects—looks real for at least a few objects, especially HE2347–4342. The quantitative correlation is less secure than the headline suggests. A referee should ask for a mock-based test of the zone-size estimator and a sensitivity analysis of the threshold and the external-void rejection.

This paper is for people working on quasar lifetimes and the IGM at z ~ 3–4. It deserves peer review; the result is important enough that the measurement method needs proper scrutiny. Send it to review, but expect the authors to come back with a validation of the H I zone-size estimator.

Referee Report

3 major / 4 minor

Summary. The paper compares H I and He II proximity-zone sizes in 15 quasars at z≈2.7–3.8, using archival HST/COS and GHRS UV spectra together with Keck/HIRES and VLT/UVES optical spectra. H I zone sizes are estimated from the radius at which the number or total equivalent width of weak Lyα lines (EW<0.2 Å) declines by 30% toward the quasar, and candidate outer voids that 'decline and recover' are classified as external foreground effects and rejected. The authors report a Pearson correlation r=0.62 (p=1.2%) between R_HI and R_HeII, a null H I zone in HE2347-4342 (with an inferred age <0.2 Myr from time-dependent simulations), and conclude that H I proximity zones can serve as quasar age indicators and that the H I ionizing front does not expand considerably faster than its He II counterpart.

Significance. If the central correlation is real, the result would be observationally important: it would establish H I proximity zones as an accessible age indicator for z~3 quasars and would constrain the propagation of ionizing fronts. The paper makes good use of a valuable archival data set, consistently remeasures the He II zones, and presents the measurements transparently in Table 2 and in the individual case discussions. The quantitative claims are, however, not yet supported by a validated measurement procedure: the H I zone estimator is partly subjective and untested against mock data, and the reported correlation is sensitive to a few classification decisions. With a blind, mock-validated re-analysis and proper robustness tests, the paper could make a solid case; as it stands, the evidence is suggestive but not load-bearing.

major comments (3)

[§3.1, §3.4] The H I proximity-zone sizes are set by an unvalidated, partly subjective procedure that is entangled with the comparison being made. The 30% decline threshold is applied to 3/5/7 Å bins, the zone edge is chosen by visual inspection, and candidate outer voids are rejected as 'external' whenever the line density declines and then recovers; the paper itself notes in §3.1 that 'the larger value is rejected because it is likely an external effect' for several quasars. No mock catalog or simulated foreground-void test is presented to show that this procedure recovers true zone sizes or that 'decline then recover' is a reliable discriminator, and ordinary Lyα forest fluctuations can produce such patterns by chance. Because the rejection is performed with knowledge of the He II zone sizes and the age narrative, it can systematically pull R_HI toward R_HeII. A concrete sensitivity check shows the fragility: if for SDSSJ0915+4756 and PKS1935-692 the larger candidate voids (16 and 18 Mpc) are retained instead of the adopted 5.3 and 6.3 Mpc values in Table 2, the Pearson r drops from 0.62 to approximately 0.47, which is not significant at the 5% level (p≈0.08). The paper needs an objective, blind, and mock-validated estimator before the correlation can be used as evidence.
[§3.1, Table 2] The significance of the reported correlation is not robust under simple sensitivity checks and does not propagate the quoted measurement errors. Restricting the sample to the ten objects with R_HeII > 3 Mpc removes essentially all correlation (r≈0.2, N=10), so the r=0.62 result is driven by the cluster of five small-zone quasars rather than by a smooth trend across the sample. In addition, the p=1.2% treats the R_HI and R_HeII point estimates as exact, even though Table 2 lists errors of order 1–2.5 Mpc and the paper states that the errors are 'estimated from visual inspections.' A Monte Carlo or bootstrap test that draws both zone sizes from their error distributions, and that includes the redshift-uncertainty term, should be reported before a 'significant correlation' is claimed.
[§4.2, §4.3] The age interpretation is tied to simplifications that are acknowledged but not quantified. The simulation for HE2347-4342 in §4.2 explicitly excludes the light-travel-time delay along the line of sight, yet the abstract's headline age limit (<0.2 Myr) is quoted without an assessment of how this simplification changes the limit. The conclusion in §5 also states that the proximity effect may provide only upper limits to quasar ages, but the actual dependence on the light-cone treatment is not derived. Furthermore, §4.3 introduces episodic quasar activity as an alternative explanation without a model; the data may be consistent with either a young single-burst quasar or a flickering quasar, so the age inference is not unique. The paper should either implement the light-cone correction and an episodic-burst model, or explicitly bound the resulting uncertainty in the quoted ages.

minor comments (4)

[§5, conclusion item (3)] The conclusion states a 'significant correlation between H I and H I proximity-zone sizes'; this should read 'He II and H I.'
[§3.1, Table 2] The text refers to 'SDSS0915−0016' in §3.1 while Tables 1 and 2 list 'SDSSJ0915+4756'; please reconcile the naming.
[Abstract, Table 2] The abstract describes HE2347-4342 as displaying a 'null proximity zone' in H I, but Table 2 lists R_HI = 0.8 ± 0.8 Mpc; 'consistent with null' would be more precise.
[References] The Pâris et al. 2017 reference is garbled, with the journal name and volume apparently missing ('˚a, 597, 79'); please correct the entry.

Circularity Check

0 steps flagged · score 1.0 of 10

No significant circularity: the H I/He II correlation is a new empirical measurement, and the age modeling uses fixed external inputs rather than fitted parameters fed back into the measurement.

full rationale

The paper's central claim is an empirical correlation between newly measured H I proximity-zone sizes and previously reported He II proximity-zone sizes. The He II sizes come from earlier work, but the authors explicitly remeasure them from the archival UV spectra and find consistency with published values, so the comparison is not defined in terms of the conclusion. The H I sizes are measured from independent Keck/HIRES and VLT/UVES optical spectra via an explicit, if partly visual, criterion: a 30% decline in line number or total equivalent width is taken as the proximity signal, and candidate outer voids showing a decline-then-recover pattern are classified as external foreground effects. That classification is a measurement-validity issue and could in principle bias the correlation, but it is not circular in the formal sense: the rejection rule is stated in terms of spectral morphology, not in terms of the He II zone size or the age narrative, and no fitted parameter is fed back into the measurement. The age modeling for HE2347-4342 solves the time-dependent photoionization equations with fixed literature inputs for the UV background, luminosity, and density profile; these inputs do not include the measured R_HI or R_HeII values used in the correlation. The simulations therefore interpret the observations rather than generate them. The paper's self-citations (e.g., Zheng et al. 2015 for redshifts and He II sizes, Zheng et al. 2004 for the normalized UVES spectrum) are normal references to prior data papers and are not load-bearing; the key measurements are remeasured or obtained from independent archival data. No load-bearing step reduces by construction to its own inputs, so the circularity score is low.

Assumptions & free parameters 2 free parameters · 5 assumptions · 0 invented entities

The central correlation uses no fitted free parameters, but the H I zone sizes depend on hand-chosen detection thresholds (30% decline, EW cutoff). The interpretation in terms of quasar ages relies on standard ionization physics plus literature values for the UVB, quasar SED, and an adopted IGM overdensity profile. No new physical entities are introduced.

free parameters (2)

H I proximity effect threshold = 30% decline
The edge of the H I proximity zone is defined where total EW or line number declines by 30% relative to larger distances (§3.1). This threshold is chosen by hand and directly sets every R_HI value.
Weak Ly-alpha line cutoff = EW < 0.2 Å
Only Ly-alpha lines with rest-frame EW below 0.2 Å are used to trace the H I proximity effect (§2). This cutoff is based on a curve-of-growth argument at Doppler parameter b = 30 km/s and affects the sensitivity of the zone detection.

assumptions (5)

domain assumption Weak Ly-alpha lines (EW < 0.2 A) trace the H I proximity effect: their equivalent width decreases inversely with ionizing flux according to a curve of growth at b = 30 km/s.
Stated in §2; the whole H I zone measurement is based on this tracer.
domain assumption External foreground quasars produce spectral voids with a symmetric decline-recover pattern, which can be distinguished from intrinsic proximity zones.
Used in §3.1 and §3.4 to reject larger candidate H I zones. No quantitative model of foreground voids is applied.
domain assumption The quasar spectral energy distribution is a broken power law with slopes -0.44 below and -1.73 above 1 Ry, and the metagalactic UVB rates are Gamma_HI = 1e-12 s^-1 and Gamma_HeII = 1e-14.3 s^-1.
Used in §3.1 to compute characteristic radii for normalization; values are from literature (Vanden Berk 2001, Zheng 1997, Becker & Bolton 2013, Worseck 2019).
domain assumption The time-dependent ionization equations (1) and (2) with case-A recombination, a lognormal density distribution, and the Guimaraes et al. (2007) overdensity profile describe the growth of the He II proximity zone.
Used in §4.2 to estimate quasar ages from zone sizes. Alternative density profiles or recombination rates would change the inferred ages.
ad hoc to paper For the HE2347-4342 age simulation, no light-travel time delay along the line of sight is included.
Stated in §4.2. The text elsewhere emphasizes light-travel magnification for H I; using the no-delay assumption for the age estimate is a modeling choice that affects the <0.2 Myr age claim.

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Pith. "Pith review of Development of Hydrogen and Helium Proximity Zones around Quasars." pith.science (2026). https://pith.science/paper/LKIVBEQU

@misc{pith2026190804691,
  author       = {Pith},
  title        = {Pith review of: Development of Hydrogen and Helium Proximity Zones around Quasars},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/LKIVBEQU}},
  note         = {Machine review of arXiv:1908.04691}
}

read the original abstract

Increasing evidence suggests that He II proximity profiles in the quasar spectra at z ~ 3 - 4 are sensitive probes of quasar ages. But the development of their H I counterparts is difficult to trace and remains poorly constrained. We compare the UV spectra of 15 He II quasars with their high-resolution optical counterparts and find a significant correlation between the sizes of He II and H I proximity zones. The luminous quasar HE2347-4342 displays a null proximity zone in both He II and H I, suggesting that it is extremely young (age < 0.2 Myr). Three other quasars also display small proximity zones for He II and H I. There is no evidence that a H I ionization zone expands considerably faster than its He II counterpart. The results suggest that the expansion of quasar ionizing fronts may be noticeably slower than the speed of light, and raise the possibility of distinguishing young and old quasars from the sizes of their H I proximity zones.

Figures

Figures reproduced from arXiv: 1908.04691 by the authors.

**Figure 2.** — Proximity zones of He II and H I in 15 quasars. The errors are estimated from measurements and do not include the redshift uncertainties [PITH_FULL_IMAGE:figures/full_fig_p013_2.png] view at source ↗

**Figure 1.** — Distribution of Ly α absorption lines in quasar vicinity. The red dashed lines mark the positions for characteristic radius Rω=1 ( [PITH_FULL_IMAGE:figures/full_fig_p013_1.png] view at source ↗

**Figure 3.** — Proximity zones of He II and H I, normalized by their characteristic sizes as derived from the quasar luminosities. See the caption of [PITH_FULL_IMAGE:figures/full_fig_p013_3.png] view at source ↗

Figures from the paper (4 more)

**Figure 4.** Figure 4: — H I and He II spectra of six quasars, converted from wavelengths to radial distances and smoothed. The quasars are labeled with their abbreviated names. For each quasar, the He II spectrum is aligned and plotted below its H I counterpart. The quasar positions are m…

**Figure 7.** Figure 7: — G130M and HIRES spectra of HS1700+6416 (z = 2.748). The UV spectrum of He II is smoothed by three pixels and aligned with the optical counterpart. The quasar position is marked by a green dashed line and a shaded region of uncertainty. The magenta arrows mark the r…

**Figure 9.** Figure 9: — Simulated He II proximity profiles at different ages of quasar HE2347−4342. The light-travel effect is not included when viewed along the line of sight. The IGM overdensity increases from δ ≈ 1.5 at 20 Mpc to 5 at 3 Mpc, as marked by a green logarithmic curve in the…

**Figure 11.** Figure 11: — Simulated response of He II and H I fractional populations to a quasar burst. The UVB is assumed to be at ΓHI = 10−12 s −1 at 1 Ry and ΓHeII = 10−14.3 s −1 at 4 Ry. The distance to the quasar is ≈ 0.57 Rω=1 so that the quasar flux is Γ Q HI ≈ 3 × 10−12 s −1 at 1 Ry …

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Pith tools

Reviewed August 14, 2026 · model on record in the stance chip above.

[1] [1]

F., Hogan, C

Anderson, S. F., Hogan, C. J., Willimas, B. F. & Carswell, R. F. 1999, , 117, 56

work page 1999

[2] [2]

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[3] [3]

Becker G. D. & Bolton, J. S. 2013, , 436, 1023

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[4] [4]

& Davidsen, A

Bi, H. & Davidsen, A. F. 1997, , 479, 523

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Bolton, J. S. & Haehnelt, M. G. 2007, , 374, 493

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Ciotti L. & Ostriker J. P., 2001, , 551, 131

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