The Multiphase CGM in the Epoch of Reionization: CII and CIV absorbers around [OIII] Emitters

A. Tortosa; A. Travascio; C. Mazzucchelli; C. Piscitelli; C. Vignali; F. Ricci; F. Salvestrini; G. Cupani; K. Knudsen; L. Paquereau

REVIEW

The Multiphase CGM in the Epoch of Reionization: CII and CIV absorbers around [OIII] Emitters

C. Piscitelli, V. D'Odorico, M. Galbiati, M. Bischetti, G. Cupani, R. Dutta, S. Cantalupo, S. Di Stefano

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S.T. Guida, K. Knudsen, C. Mazzucchelli, L. Paquereau, L. Pentericci, R. Rana, F. Ricci, F. Salvestrini, A. Tortosa, A. Travascio, C. Vignali, L. Zappacosta

Astrophysics of Galaxies · Submitted 14 Sep 2026

Pith's verdict: A useful first statistical look at CII/CIV around z~6 galaxies, but the headline claim that CIV is more extended than CII is largely a group-multiplicity effect that the paper's own Figure A.1 undercuts.

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arXiv ID2609.15547 (version 2)

Pith IDRJQ76W54, Pith's permanent record of this paper and its review

CitationA. Tortosa, A. Travascio, C. Mazzucchelli, C. Piscitelli, C. Vignali, F. Ricci, F. Salvestrini, G. Cupani, K. Knudsen, L. Paquereau, L. Pentericci, L. Zappacosta, M. Bischetti, M. Galbiati, R. Dutta, R. Rana, S. Cantalupo, S. Di Stefano, S.T. Guida, V. D'Odorico. "The Multiphase CGM in the Epoch of Reionization: CII and CIV absorbers around [OIII] Emitters." arXiv:2609.15547 (2026). https://pith.science/paper/RJQ76W54

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This is a genuinely useful paper, and the stress-test note lands on a real soft spot that the authors themselves document in §3.1. The new data are solid: first systematic cross-correlation of CII and CIV absorbers with [OIII] emitters at z~6, using the EIGER galaxy sample and six quasar sightlines, plus a new X-Shooter spectrum of PSO J159-02. The methodology is careful — Wilson-score intervals for covering fractions, field-shuffling for the background, power-law fits with fixed slopes, and a Bayesian fit that treats upper limits properly. The checks are also good: bin-shifting, excluding systems with associated low-ionization lines, and a conservative carbon mass estimate. Credit where due: this is a serious observational study with reproducible methods and clearly stated limitations.

The problem is the central comparative claim in the abstract and conclusions. The paper says CIV covering fraction stays enhanced to ~1 pMpc while CII drops by ~0.5 pMpc, and interprets that as the warm-ionized phase being more extended around galaxies. But the paper's own Figure A.1 shows that when each CIV absorber is matched only to its nearest galaxy, the extended plateau vanishes and the covering fraction drops to background beyond 300–400 pkpc, just like CII. The body text admits this in one sentence, but the abstract and conclusions do not qualify it. That is not a minor omission — the claim as stated is not supported by the data. What is really shown is that CIV absorbers live in group environments with multiple galaxy associations, not that the warm-ionized CGM is more extended around individual star-forming galaxies.

The velocity-window concern in the reader's weakest_assumption is secondary. The multiplicity-driven fragility is more fundamental. On the other hand, the comparison with lower-redshift CIV covering fractions is on firmer ground, since it uses the inner bins and the combined CII+CIV measurement, and the decline with redshift is consistent with known evolution of ionizing conditions. The small sample sizes (16 CII, 14 CIV) are acknowledged and handled reasonably.

Who is this for? Anyone working on the high-redshift CGM and metal enrichment. It deserves a serious referee, but the revision should either reframe the extended-CIV claim as a group-environment effect or remove it from the abstract. With that change, I would be happy to see it in print.

Referee report

Significance

This work provides one of the first statistical characterizations of the multiphase CGM during the Epoch of Reionization using two ionization states of the same element (carbon). The combination of JWST galaxy surveys and quasar absorption-line spectroscopy at z~6 is novel and exploits unique datasets. If the results hold, they offer direct observational constraints on the extent of early metal enrichment and the ionization state of the CGM, with implications for feedback models and the role of faint dwarf galaxies. The inclusion of a new X-Shooter spectrum for one quasar field adds value. The analysis uses appropriate statistical tools (Wilson-score intervals, field-shuffling, Bayesian fitting with upper limits) and is transparent about limitations, including the group-multiplicity test in Appendix A.

Major objections (2)

[Abstract and §5 (Conclusions)] The abstract states that the CIV covering fraction remains enhanced to ~1 pMpc, 'demonstrating that the warm-ionized phase is more spatially extended than cooler gas.' The conclusions repeat this wording. However, as shown in §3.1 and Fig. A.1, the extended CIV signal vanishes when each absorber is matched only with its nearest galaxy, and the authors themselves write that 'the extended highly ionized gas signal at large distances is primarily driven by group environments.' The current phrasing in the abstract and conclusions is ambiguous and could be misinterpreted as a property of individual galaxy halos. Please add an explicit caveat in both places stating that the extended CIV covering fraction is largely due to multiple galaxy associations in group environments.
[§3 and §3.1] The choice of |Δv| ≤ 500 km/s to define associated galaxy-absorber pairs is adopted from lower-redshift studies, but no test of sensitivity to this threshold is presented. At z~6, peculiar velocities and redshift uncertainties may be larger. The covering fractions and cross-correlation function values depend on this choice. The authors should test the robustness of the main results to different velocity thresholds (e.g., 300 km/s, 700 km/s) or at least discuss the potential impact of this assumption on the conclusions.

Minor objections (5)

[Table 1] The column headings are abbreviated (cols 6 and 7). Please expand or add a footnote to clarify that columns (6) and (7) are the redshift windows for CII and CIV detection, respectively.
[Fig. 3] The shaded regions for the random background (1σ and 3σ) are difficult to distinguish in grayscale. Consider using different hatching patterns or labelling the contours directly.
[§3.4] The description of the likelihood split into two factors (α1, α2) for upper limits within and beyond 100 pkpc is clear, but the rationale for the specific 100 pkpc boundary could be stated more explicitly. Does it correspond to a typical virial radius at z~6?
[Appendix B.1.1] The acronym LBLRTM in the telluric correction paragraph is not defined. Please define it.
[Throughout] The term 'pMpc' is used for proper Mpc. This is fine, but consider adding a short note at first use for readers less familiar with the prefix convention.

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The paper

The paper claims

At z~6, warm ionized carbon gas extends to ~1 pMpc around star-forming galaxies, while cool gas is confined to half that radius.

The reading

This paper combines JWST EIGER [OIII] emitters with six background quasar spectra to map the circumgalactic medium (CGM) at z~6. It finds that CIV absorbers (tracing warm ionized gas) have a covering fraction that remains significantly enhanced out to ~1 proper Mpc, whereas CII absorbers (cooler gas) drop to background beyond ~0.5 pMpc. The galaxy-absorber cross-correlation function is significantly weaker than the galaxy-galaxy autocorrelation, indicating that a substantial fraction of early carbon enrichment is not confined to the virial radii of massive star-forming galaxies but resides in the diffuse IGM or in faint dwarf galaxies. Comparing to lower-redshift samples, the CIV covering fraction declines more rapidly at z~6, but the combined CII+CIV covering fraction matches the lower-z CIV signal, suggesting that a larger fraction of circumgalactic carbon is in lower ionization states during reionization. The paper reports a conservative lower limit of M_CII+CIV >= 2.8e6 M_sun within 300 pkpc.

What carries the argument

CII λ1334 and CIV λ1548,1550 absorption lines as tracers of cool and warm ionized gas phases, respectively; covering fraction computed in concentric annuli with a field-shuffling technique to define random background; 3D galaxy-absorber cross-correlation function (Peebles-Hauser estimator) measured in comoving distance bins; and Bayesian log-linear fits to the rest-frame equivalent width radial profiles including upper limits.

Extended reading notes

Core claim

The central claim is that during the Epoch of Reionization (z~6) the warm ionized phase of the CGM, traced by CIV absorption, extends to projected distances of ~1 pMpc around star-forming [OIII] emitters, while the cooler phase traced by CII drops to the background level beyond ~0.5 pMpc. This is the first statistical comparison of two ionization states of the same element to probe the multiphase CGM at z>5. The weaker spatial clustering of absorbers relative to galaxies themselves implies that metals are not exclusively bound to massive halos, but are distributed throughout large-scale structure or injected by a population of faint, undetected dwarf galaxies. Additionally, the rapid radial decline of CIV covering fraction relative to z<2 and z~3-4 samples points to an evolving ionization structure where early metals reside predominantly in lower ionization states (CII rather than CIV).

Load-bearing premise

The analysis assumes that the line-of-sight velocity window of |Δv| <= 500 km/s correctly identifies physically associated galaxy-absorber pairs at z~6, but does not test the sensitivity of the results to this specific threshold; if the true velocity window is significantly different (e.g., wider due to larger peculiar velocities), the covering fractions and correlation lengths could change.

Editorial extensions

If this is right

Warm ionized CGM at z~6 is already spatially extended (~1 pMpc), implying that galaxy feedback or pre-enrichment from faint dwarfs operates on scales well beyond the virial radius in the early Universe.
The weaker clustering of absorbers compared to galaxies suggests that a significant fraction of metals in the z~6 CGM/IGM is not directly tied to individual massive halos, requiring models of early chemical enrichment to include widespread metal transport.
When CII and CIV are combined, the total carbon covering fraction matches lower-redshift CIV alone, supporting a scenario where the ionisation state of the CGM evolves strongly from z~6 to z~2, with more carbon in low-ionisation phases at early times.
The lower limit of M_CII+CIV >= 2.8e6 M_sun within 300 pkpc constrains the carbon mass in the CGM around typical star-forming galaxies at the end of reionization, providing a benchmark for cosmological simulations.

Reading between the lines

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

The 500 km/s velocity window adopted from lower-redshift studies may not be optimal at z~6 where peculiar velocities and redshift errors could be larger; testing the sensitivity to this threshold (e.g., 300 or 800 km/s) would strengthen the robustness of the covering fraction and correlation results.
The rapid drop of CIV covering fraction at z~6 compared to lower redshifts could be partly due to the stronger metagalactic UV background during reionization that ionises CIV to higher states (e.g., CIII or CV); future detection of CIII absorption (currently obscured by Lyα forest) would test this interpretation.
The paper's conclusion that metals are not confined to massive halos rests on the assumption that faint dwarf galaxies below the EIGER detection limit (40% complete at L5008 ~ 1e42 erg/s) are the sole extra source; deeper galaxy surveys could directly confirm or rule out the dwarf-galaxy population as the dominant metal supplier.
Combining the EIGER CII+CIV covering fraction with the CIV data from MAGG and QSAGE suggests that the total carbon covering fraction evolves little from z~6 to z<2 when low-ionisation states are accounted for, implying that the metal distribution around galaxies may be established early even as the ionisation state changes.

Original abstract

We investigate the multiphase circumgalactic medium (CGM) during the Epoch of Reionization (EoR, $z>6$) by cross-correlating cool- CII and warm-ionized CIV absorption systems with star-forming [OIII] emitters. JWST/NIRCam wide-field slitless spectroscopy from the EIGER survey is combined with medium- and high-resolution optical/NIR spectra of six background quasars, including new VLT/X-Shooter observations of PSO J159-02. We analyze the relation between 16 CII and 14 CIV absorbers ($\log(N) > 13.0$) and 136 galaxies, within an impact parameter of R$_{\perp} \leq$ 1000 pkpc and a line-of-sight separation of $\Delta v \leq 500$ km/s. We detect a statistically significant excess of both ions around galaxies compared to a randomized background. We observe that the CIV covering fraction remains enhanced up to $\sim 1$ pMpc, whereas CII drops to the background level beyond $\sim 0.5$ pMpc, demonstrating that the warm-ionized phase is more spatially extended than cooler gas. Jointly, the 3D galaxy-absorber spatial clustering is significantly weaker than the galaxy-galaxy auto-correlation. This provides direct physical evidence that early carbon enrichment is not confined to the virial radius of massive star-forming systems; rather, a substantial fraction of these metals permeates the diffuse intergalactic medium (IGM) or is injected by a widespread population of faint, undetected dwarf galaxies. Finally, we note a rapid radial decline of the CIV covering fraction compared with lower redshift samples at $z<2$ and $z\sim3-4$ pointing out an evolving CGM ionization structure where early metals reside predominantly in lower ionization states. In conclusion, we determine a conservative lower limit for the observed carbon mass of $\rm M_{CII + CIV} \geq 2.8 \times 10^6 \, M_{\odot}$ within 300 pkpc.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13.0 via TypeSafe API, question bank abstract-p1-20260923, 2026-09-23: 1 request, 7,538 input tokens.

What the abstract itself states, as probabilities: claims first 0.05 · states limitation 0.77 · human participants 0.01 · controlled comparison 0.92 · randomized assignment 0.06 · states quantitative result 0.99 · compares with prior results 0.94 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v2. Results and the main claim are underlined; show every role.

We investigate the multiphase circumgalactic medium (CGM) during the Epoch of Reionization (EoR, $z>6$) by cross-correlating cool- CII and warm-ionized CIV absorption systems with star-forming [OIII] emitters. JWST/NIRCam wide-field slitless spectroscopy from the EIGER survey is combined with medium- and high-resolution optical/NIR spectra of six background quasars, including new VLT/X-Shooter observations of PSO J159-02. We analyze the relation between 16 CII and 14 CIV absorbers ($\log(N) > 13.0$) and 136 galaxies, within an impact parameter of R$_{\perp} \leq$ 1000 pkpc and a line-of-sight separation of $\Delta v \leq 500$ km/s. We detect a statistically significant excess of both ions around galaxies compared to a randomized background. We observe that the CIV covering fraction remains enhanced up to $\sim 1$ pMpc, whereas CII drops to the background level beyond $\sim 0.5$ pMpc, demonstrating that the warm-ionized phase is more spatially extended than cooler gas. Jointly, the 3D galaxy-absorber spatial clustering is significantly weaker than the galaxy-galaxy auto-correlation. This provides direct physical evidence that early carbon enrichment is not confined to the virial radius of massive star-forming systems; rather, a substantial fraction of these metals permeates the diffuse intergalactic medium (IGM) or is injected by a widespread population of faint, undetected dwarf galaxies. Finally, we note a rapid radial decline of the CIV covering fraction compared with lower redshift samples at $z<2$ and $z\sim3-4$ pointing out an evolving CGM ionization structure where early metals reside predominantly in lower ionization states. In conclusion, we determine a conservative lower limit for the observed carbon mass of $\rm M_{CII + CIV} \geq 2.8 \times 10^6 \, M_{\odot}$ within 300 pkpc.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#	sentence	role	the authors' own work	states the main conclusion	strength
s0	We investigate the multiphase circumgalactic medium (CGM) during the Epoch of Reionization (EoR, $z>6$) by cross-correlating cool- CII and warm-ionized CIV absorption systems with star-forming [OIII] emitters.	contribution 0.97	0.91	0.04	asserted 0.73
s1	JWST/NIRCam wide-field slitless spectroscopy from the EIGER survey is combined with medium- and high-resolution optical/NIR spectra of six background quasars, including new VLT/X-Shooter observations of PSO J159-02.	method 1.00	0.82	0.01	asserted 0.93
s2	We analyze the relation between 16 CII and 14 CIV absorbers ($\log(N) > 13.0$) and 136 galaxies, within an impact parameter of R$_{\perp} \leq$ 1000 pkpc and a line-of-sight separation of $\Delta v \leq 500$ km/s.	method 0.89	0.94	0.02	asserted 0.91
s3	We detect a statistically significant excess of both ions around galaxies compared to a randomized background.	result 1.00	0.96	0.13	asserted 1.00
s4	We observe that the CIV covering fraction remains enhanced up to $\sim 1$ pMpc, whereas CII drops to the background level beyond $\sim 0.5$ pMpc, demonstrating that the warm-ionized phase is more spatially extended than cooler gas.	result 0.97	0.96	0.14	asserted 1.00
s5	Jointly, the 3D galaxy-absorber spatial clustering is significantly weaker than the galaxy-galaxy auto-correlation.	result 0.99	0.94	0.07	asserted 1.00
s6	This provides direct physical evidence that early carbon enrichment is not confined to the virial radius of massive star-forming systems; rather, a substantial fraction of these metals permeates the diffuse intergalactic medium (IGM) or is injected by a widespread population of faint, undetected dwarf galaxies.	interpretation 1.00	0.86	0.68	asserted 0.95
s7	Finally, we note a rapid radial decline of the CIV covering fraction compared with lower redshift samples at $z<2$ and $z\sim3-4$ pointing out an evolving CGM ionization structure where early metals reside predominantly in lower ionization states.	result 0.80	0.87	0.09	asserted 0.60
s8	In conclusion, we determine a conservative lower limit for the observed carbon mass of $\rm M_{CII + CIV} \geq 2.8 \times 10^6 \, M_{\odot}$ within 300 pkpc.	result 1.00	0.97	0.17	asserted 0.99

What kind of paper the classifier reports this is: empirical study 1.00 · evidence from observation or experiment 0.77.

What was examined, and what was not

sources read

arXiv listing abstract v2

not examined

full text
figures and tables
references and citing sentences
relations to other papers

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P abstract

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Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures

**Figure 1.** Projected impact parameter versus line-of-sight velocity di

**Figure 2.** 2D distribution of galaxies associated with C

**Figure 3.** Differential covering fraction of C ii (left panel) and Civ (right panel) absorbers around the surveyed galaxies. The points represent the measured signal in concentric annuli. The vertical error bars denote the Wilson-score confidence interval of 1σ, while the horizontal error bars indicate the width of each radial bin. The solid line denotes the expected random background, and the lighter and darker shaded regions indicate the 1σ and 3σ confidence intervals, respectively

**Figure 4.** Cumulative covering fraction of C ii (left panel) and C iv (right panel) absorbers as a function of the impact parameter. The points represent the measured signal within a given radius. The solid line denotes the expected random background, where the lighter and darker shaded regions indicate the its 1σ and 3σ confidence intervals, respectively. 10 0 10 1 3D Comoving Distance ¢r [cMpc=h] 10 1 10 0 10 1 10 2 C o r r elatio n F u n ctio n »(¢ r) ACF: ° = 1:5, r0 = 10:63 § 0:56 h ¡1cMpc ACF: ° = 1:8, r0 = 7:84 § 0:84 h ¡1cMpc C II: ° = 1:5, r0 = 4:52 § 0:87 h ¡1cMpc C II: ° = 1:8, r0 =…

**Figure 5.** 3D Cii (left panel) and Civ (right panel) absorbers-[O iii] emitting galaxy CCF as a function of the comoving distance at z ∼ 6. The black points represent the measured signal, with vertical error bars indicating the 1σ Poisson error. The solid orange and dotted red lines represent the best-fit power laws with index γ = 1.5 and 1.8, respectively. The shaded orange and red regions denote the associated 1σ confidence intervals. The blue squares denote the ACF of the [O iii] emitters. The blue solid and dotted lines represent the best-fit power laws with index γ = 1.5 and 1.8,…

**Figure 6.** EW profile as a function of transverse separation from the associated galaxy,

**Figure 7.** shows the comparison of the C iv covering fraction for the three samples. In the first impact parameter bin (R⊥ < 75 pkpc), the Civ covering fraction at z ∼ 6 is consistent, within the errors, with the values observed at lower redshift. This means that the highly ionized, metal enriched gas traced by C iv, is already present in the vicinity of galaxies at the end of Reionization. However, at larger impact parameters, the z ∼ 6 data show an evident drop compared with the lower redshift samples. This strong difference at intermediate impact parameters (75 ≤ R⊥ ≤ 400 pkpc) might…

Showing 7 of the paper's 8 figures. See all figures in the PDF ↗ Figures reproduced from arXiv:2609.15547 by the authors.

Discussion

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