What drives the disappearance of the highly ionized circumgalactic medium at $z\gtrsim 5.7$: separating ionization evolution from early chemical impoverishment in reionization-era metal absorbers

At redshifts approaching the epoch of hydrogen reionization ($z\gtrsim 5.7$), absorption from the highly ionized circumgalactic phase traced by C IV largely disappears from quasar spectra, while low-ionization absorption (C II, O I, Mg II) persists and the surviving absorbers are narrow, cold, and metal-poor. Cooper et al. (2019) established this transition but could not separate its two candidate drivers: softening or fluctuations of the metagalactic ultraviolet background during the end of reionization, versus a genuine decline in the heavy-element abundance of circumgalactic gas. The problem is to determine the physical mechanism(s) behind the suppression of high-ionization absorption, to constrain the total carbon density of the early circumgalactic medium and its partition among C II, C III, and C IV (with C III currently unobservable at $z>5$), and to characterize the low-ionization absorber population and its host galaxies sufficiently to test whether the circumgalactic medium indeed transitions from a multiphase, highly ionized state to a predominantly neutral, metal-poor state at early epochs. No published analysis has yet succeeded in separating the two contributions, so the question remains open.

Background

When light from a distant quasar traverses intervening gas, chemical elements heavier than helium imprint narrow absorption lines whose strengths (column densities) reveal the amount, ionization state, and enrichment of that gas. The circumgalactic medium (CGM) — the diffuse gas extending hundreds of kiloparsecs around galaxies — is studied this way, and at redshifts $z>5$ absorption lines are the primary probe, because the galaxies themselves are too faint to study in detail. Carbon is the most widely used tracer: triply ionized carbon (C IV; three electrons removed, strongest lines at rest wavelengths 1548 and 1550 Angstroms) traces highly ionized, hot-phase gas, while singly ionized carbon (C II; 1334 Angstroms) traces cooler, mostly neutral gas. Because C II and C IV are ions of the same element, their ratio encodes the intensity and spectral shape of the metagalactic ultraviolet background (UVB) — the summed, nearly isotropic radiation from galaxies and active galactic nuclei — independently of abundance ratios between different elements. Carbon's C IV stage requires photons above about 3.5 Rydbergs (1 Rydberg = 13.6 eV, the ionization energy of hydrogen), so C IV is especially sensitive to the hard tail of the UVB. The same Roman-numeral ion convention applies to the other tracers used below: Si IV is triply ionized silicon (another high-ionization tracer), Mg II is singly ionized magnesium, and O I is neutral oxygen, which survives mainly in gas that is itself predominantly neutral. The epoch of hydrogen reionization (roughly the first billion years of cosmic time, ending near $z\sim 6$) is expected to soften and fluctuate this radiation field as the last neutral hydrogen absorbs and filters ionizing photons.

At $z>6$ the ultraviolet sky is dominated by the fully saturated Lyman-alpha forest — the blend of countless hydrogen absorption lines — which blocks measurement of the neutral-hydrogen column density $N_{\mathrm{HI}}$ of individual absorbers and hides the C III line at 977 Angstroms, the ionization stage between C II and C IV. Absorbers are conventionally classified by $N_{\mathrm{HI}}$: damped Lyman-alpha systems (DLAs, the densest neutral gas), sub-DLAs (systems with somewhat lower neutral-hydrogen columns), and Lyman limit systems (LLSs, more diffuse and highly ionized). Photoionization modeling (computing the ionization equilibrium of a gas slab illuminated by a specified radiation field, as with the Cloudy code) is the standard tool for converting observed ion ratios into density, metallicity, and ionization-parameter inferences, but a given C IV column can usually be matched by several combinations of lower metallicity, higher density, or softer radiation — the ionization-abundance degeneracy.

Cooper et al. (2019) analyzed 69 intervening absorption systems at $5.0<z<6.8$: 63 found in near-infrared spectra of 47 quasars, with the remainder from one high signal-to-noise spectrum from HIRES, the high-resolution echelle spectrograph on the Keck telescope, of the $z=6.3$ quasar J0100+2802. They found that half of all absorbers at $z\gtrsim 5.7$ show no C IV or Si IV at all, that most of the remainder have more C II than C IV, and that the C IV-free systems — which they called low-ionization absorbers (LIAs) — exhibit narrow kinematics (within roughly 100 km/s) and temperatures near $10^4$ K or below. By incidence-rate arguments the LIAs are consistent with a population of very metal-poor DLAs and sub-DLAs. Their photoionization models indicated that circumgalactic gas at $z\sim 6$ has systematically lower chemical abundances and experiences a softer ionizing background than at $z\sim 3$, but the two effects could not be separated. This matters because the cosmic mass density of C IV, $\Omega_{\mathrm{C\,IV}}$ (the comoving mass fraction of the Universe in that ion), declines by nearly an order of magnitude from $z=1$ to $z=5$ and drops rapidly at $z\gtrsim 5$: whether that downturn is an ionization signature of reionization's end or a fossil record of early enrichment changes what metal absorbers can tell us about the first galaxies.

Problem Statement

Cooper et al. (2019) established that absorption from the highly ionized circumgalactic phase (C IV, Si IV) largely disappears at $z\gtrsim 5.7$ while low-ionization absorption persists, but the physical cause is undetermined. The transition was established over a sample spanning $5.0<z<6.8$, with $5.0<z<5.7$ serving as the comparison baseline; the mechanism question below targets the $z\gtrsim 5.7$ regime. The overarching objective is to establish the physical state of the metal-enriched circumgalactic medium at the end of hydrogen reionization — specifically, what drives its apparent transition from a multiphase, highly ionized state to a predominantly neutral, metal-poor state:

1. What mechanism(s) drive the suppression of high-ionization absorption at $z\gtrsim 5.7$: a softening or increased fluctuation of the metagalactic ultraviolet background during the final stages of hydrogen reionization, a genuine decline in the heavy-element abundance of circumgalactic gas, reduced transport or mixing of metals into outer galactic halos (weaker feedback — a channel coupled to the other two, since reduced mixing would allow for softer UV background spectra and/or higher gas densities), or a combination thereof? The aim is to assess the relative importance of these channels, or to establish that one dominates, consistently with observations of numerous low- and high-ionization species.

2. What is the total carbon density of the $z\gtrsim 5.7$ circumgalactic and intergalactic medium, and how is it partitioned among C II, C III, and C IV — in particular, is a large fraction of carbon hidden in the observationally inaccessible C III phase? The measured decline of $\Omega_{\mathrm{C\,IV}}$ together with a roughly flat $\Omega_{\mathrm{C\,II}}$ implies that the sum $\Omega_{\mathrm{C\,II}}+\Omega_{\mathrm{C\,IV}}$ must decline toward high redshift; what remains open is whether that decline is a signature of early chemical enrichment or is instead dominated by carbon residing in the unobservable C III phase.

3. What is the nature of the neutral low-ionization absorber population and its galactic environment: are the narrow ($\lesssim 100\,\mathrm{km\,s^{-1}}$), cold ($T\sim 10^4$ K or below) kinematics universal for these systems rather than a property of a single high-resolution sightline; what occupies the as-yet unexplored weak-column regime ($\log N_{\mathrm{C\,II}}\lesssim 13.5$); does a distinct population of weaker, more highly ionized absorbers analogous to the lower-redshift sub-DLA/LLS population exist at $z>6$; and which galaxies host these absorption systems? These population characterizations bear directly on whether the circumgalactic medium indeed transitions from multiphase and highly ionized to predominantly neutral and metal-poor at early epochs.

The central question is which of the listed channels — a softening or increased fluctuation of the metagalactic ultraviolet background, a genuine decline in the heavy-element abundance of circumgalactic gas, reduced transport or mixing of metals into outer halos, or a combination thereof — drives the suppression of high-ionization absorption at $z\gtrsim 5.7$. Items 2 and 3 are adjacent questions whose answers constrain any proposed decomposition of this mechanism; the sub-parts of item 3 — universality of the narrow, cold kinematics, the weak-column regime, a possible sub-DLA/LLS-analog population, and host-galaxy identification — are open questions that bear on that decomposition.

References

1. Cooper, Thomas J.; Simcoe, Robert A.; Cooksey, Kathy L.; Bordoloi, Rongmon; Miller, Daniel R.; Furesz, Gabor; Turner, Monica L.; Bañados, Eduardo (2019). "Heavy Element Absorption Systems at 5.0 < z < 6.8: Metal-poor Neutral Gas and a Diminishing Signature of Highly Ionized Circumgalactic Matter", The Astrophysical Journal 882, 77. DOI: 10.3847/1538-4357/ab3402, https://doi.org/10.3847/1538-4357/ab3402
2. Davies, Rebecca L.; Ryan-Weber, E.; D'Odorico, V.; Bosman, S. E. I.; Meyer, R. A.; Becker, G. D.; Cupani, G.; Keating, L. C.; Bischetti, M.; Davies, F. B.; Eilers, A.-C.; Farina, E. P.; Haehnelt, M. G.; Pallottini, A.; Zhu, Y (2023). "Examining the decline in the C iv content of the Universe over 4.3 ≲ z ≲ 6.3 using the E-XQR-30 sample", Monthly Notices of the Royal Astronomical Society 521, 314–331. DOI: 10.1093/mnras/stad294, https://doi.org/10.1093/mnras/stad294
3. Sebastian, Alma Maria; Ryan-Weber, Emma; Davies, Rebecca L.; Becker, George D.; Keating, Laura C.; D'Odorico, Valentina; Meyer, Romain A.; Bosman, Sarah E. I.; Cupani, Guido; Kulkarni, Girish; Haehnelt, Martin G.; Lai, Samuel; Eilers, Anna-Christina; Bischetti, Manuela; Gallerani, Simona (2024). "E-XQR-30: The evolution of Mg ii, C ii, and O i across 2 < z < 6", Monthly Notices of the Royal Astronomical Society 530, 1829–1848. DOI: 10.1093/mnras/stae789, https://doi.org/10.1093/mnras/stae789
4. Rowlands, Stephanie; Davies, R. L.; Ryan-Weber, E.; Keating, L. C.; Sebastian, A. M.; Becker, G. D.; Bischetti, M.; Bosman, S. E. I.; Chen, H.; Davies, F. B.; D'Odorico, V.; Gaikwad, P.; Gallerani, S.; Haehnelt, M. G.; Kulkarni, G.; Meyer, R. A.; Welsh, L.; Zhu, Y. (2026). "E-XQR-30: evidence for an increase in the ionization state of metal absorbers from z ∼ 6 to z ∼ 2", Monthly Notices of the Royal Astronomical Society 546, article staf2254. DOI: 10.1093/mnras/staf2254, https://doi.org/10.1093/mnras/staf2254
5. Becker, George D.; Pettini, Max; Rafelski, Marc; D'Odorico, Valentina; Boera, Elisa; Christensen, Lise; Cupani, Guido; Ellison, Sara L.; Farina, Emanuele Paolo; Fumagalli, Michele; López, Sebastian; Neeleman, Marcel; Ryan-Weber, Emma V.; Worseck, Gábor (2019). "The Evolution of O i over 3.2 < z < 6.5: Reionization of the Circumgalactic Medium", The Astrophysical Journal 883, 163. DOI: 10.3847/1538-4357/ab3eb5, https://doi.org/10.3847/1538-4357/ab3eb5
6. Becker, George D.; D'Aloisio, Anson; Christenson, Holly M.; Zhu, Yongda; Worseck, Gábor; Bolton, James S. (2021). "The mean free path of ionizing photons at 5 < z < 6: evidence for rapid evolution near reionization", Monthly Notices of the Royal Astronomical Society 508, 1853–1869. DOI: 10.1093/mnras/stab2696, https://doi.org/10.1093/mnras/stab2696
7. Finlator, Kristian; Doughty, Caitlin; Cai, Zheng; Díaz, Gonzalo (2020). "The faint host galaxies of C IV absorbers at z > 5", Monthly Notices of the Royal Astronomical Society 493, 3223–3237. DOI: 10.1093/mnras/staa377, https://doi.org/10.1093/mnras/staa377
8. Doughty, Caitlin; Finlator, Kristian (2021). "The effects of binary stars on galaxies and metal-enriched gas during reionization", Monthly Notices of the Royal Astronomical Society 505, 2207–2223. DOI: 10.1093/mnras/stab1448, https://doi.org/10.1093/mnras/stab1448
9. Suarez, Teresita; Meiksin, Avery (2021). "Modelling intergalactic low ionization metal absorption line systems near the epoch of reionization", Monthly Notices of the Royal Astronomical Society 502, 888–903. DOI: 10.1093/mnras/stab102, https://doi.org/10.1093/mnras/stab102
10. Kashino, Daichi; Lilly, Simon J.; Simcoe, Robert A.; Bordoloi, Rongmon; Mackenzie, Ruari; Matthee, Jorryt; Eilers, Anna-Christina (2023). "Compact [C ii] emitters around a C iv absorption complex at redshift 5.7", Nature 617, 261–264. DOI: 10.1038/s41586-023-05901-3, https://doi.org/10.1038/s41586-023-05901-3
11. Zou, Siwei; Cai, Zheng; Wang, Feige; Fan, Xiaohui; Champagne, Jaclyn B.; Hennawi, Joseph F.; Schindler, Jan-Torge; Farina, Emanuele Paolo; Yang, Jinyi; Inayoshi, Kohei; Bañados, Eduardo; Bosman, Sarah E. I.; Li, Zihao; Lin, Xiaojing; Wu, Yunjing; Sun, Fengwu; Guo, Ziyi; Kulkuarni, Girish; Habouzit, Mélanie; Charlot, Stephane; Chevallard, Jacopo; Connor, Thomas; Eilers, Anna-Christina; Jiang, Linhua; Jin, Xiangyu; Kakiichi, Koki; Li, Mingyu; Meyer, Romain A.; Walter, Fabian; Zhang, Huanian (2024). "A SPectroscopic survey of biased halos In the Reionization Era (ASPIRE): Impact of Galaxies on the Circumgalactic Medium Metal Enrichment at z > 6 Using the JWST and VLT", The Astrophysical Journal Letters 963, L28. DOI: 10.3847/2041-8213/ad23e7, https://doi.org/10.3847/2041-8213/ad23e7
12. Zhu, Yongda; Ji, Zhiyuan; Becker, George D.; Ding, Jiani; Egami, Eiichi; Fan, Xiaohui; Jin, Xiangyu; Liu, Weizhe; Lyu, Jianwei; Ma, Zheng; Narisetty, Suprabhas; Rieke, George H.; Wu, Yunjing; Yue, Minghao; Zhang, Junyu; Rieke, Marcia J. (2026). "Early metal-enriched baryon cycling before the midpoint of cosmic reionization", arXiv:2604.21218. https://arxiv.org/abs/2604.21218
13. Wisz, M. E.; Rafelski, Marc; Oyarzún, Grecco A.; Jorgenson, Regina A.; Fumagalli, Michele; Neeleman, M.; Prochaska, J. Xavier; Christensen, Lise; Fobbs, Eldon; Becker, George D.; Hennawi, Joseph; Worseck, G.; López, Sebastián (2026). "The Qz5 Survey. II. Metallicity Evolution of Damped Lyα Systems Out to z ∼ 5", The Astrophysical Journal 1003, 222. DOI: 10.3847/1538-4357/ae67f8, https://doi.org/10.3847/1538-4357/ae67f8
14. Yu, Xiaodi; Qu, Zhijie; Cai, Zheng; Li, Jiang-Tao; Mao, Huiyang; Wang, Xin (2025). "Probing the He ii Reionization ERA via Absorbing C iv Historical Yield (HIERACHY). III. The C iv Absorber Catalog and Initial Results on Cosmic Abundance Evolution at z ≈ 3–5", The Astrophysical Journal 985, 64. DOI: 10.3847/1538-4357/adc7ad, https://doi.org/10.3847/1538-4357/adc7ad
15. Higginson, Jack; Bordoloi, Rongmon; Simcoe, Robert A.; Matthee, Jorryt; Kashino, Daichi; Mackenzie, Ruari; Kramarenko, Ivan; Lilly, Simon J.; Eilers, Anna-Christina; Naidu, Rohan P.; Yue, Minghao (2026). "EIGER. VIII. First Stars Signatures in the Connection between O i Absorption and Galaxies in the Epoch of Reionization", The Astrophysical Journal 999, 49. DOI: 10.3847/1538-4357/ae3e81, https://doi.org/10.3847/1538-4357/ae3e81
16. Huscher, Ezra; Finlator, Kristian; Kušmić, Samir; Steen, Maya (2024). "Impact of Helium ii Resonant Absorption on the Ultraviolet Background Modeled in Three Dimensions", The Astrophysical Journal 966, 111. DOI: 10.3847/1538-4357/ad34d2, https://doi.org/10.3847/1538-4357/ad34d2
17. Kušmić, Samir; Finlator, Kristian; Huscher, Ezra; Steen, Maya (2024). "Galaxy–Absorber Association in the Epoch of Reionization: Galactic Population Luminosity Distribution for Different Absorbers at 10 ≥ z ≥ 5.5", The Astrophysical Journal 974, 224. DOI: 10.3847/1538-4357/ad713c, https://doi.org/10.3847/1538-4357/ad713c
18. Tie, Suk Sien; Hennawi, Joseph F.; Kakiichi, Koki; Bosman, Sarah E. I. (2022). "Constraining IGM enrichment and metallicity with the C iv forest correlation function", Monthly Notices of the Royal Astronomical Society 515, 3656–3673. DOI: 10.1093/mnras/stac2021, https://doi.org/10.1093/mnras/stac2021
19. Doughty, Caitlin C.; Finlator, Kristian M. (2022). "The environments and hosts of metal absorption at z > 5", Monthly Notices of the Royal Astronomical Society 518, 4159–4171. DOI: 10.1093/mnras/stac3342, https://doi.org/10.1093/mnras/stac3342
20. Davies, Rebecca L.; Ryan-Weber, E.; D'Odorico, V.; Bosman, S. E. I.; Meyer, R. A.; Becker, G. D.; Cupani, G.; Bischetti, M.; Sebastian, A. M.; Eilers, A.-C.; Farina, E. P.; Wang, F.; Yang, J.; Zhu, Y (2023). "The XQR-30 metal absorber catalogue: 778 absorption systems spanning 2 ≲ z ≲ 6.5", Monthly Notices of the Royal Astronomical Society 521, 289–313. DOI: 10.1093/mnras/stac3662, https://doi.org/10.1093/mnras/stac3662