The THESAN-ZOOM project: The Hidden Neighbours of OI Absorbers during Reionization

Metal absorbers represent a powerful probe of galaxy feedback and reionization, as highlighted by both observational and theoretical results showing an increased abundance of low-ionised metal species at higher redshifts. The origin of such absorbers is currently largely unknown because of the low number of galaxy counterparts detected, suggesting that they might be surrounded by low-mass faint sources below the current detection threshold. We use the THESAN-ZOOM radiation hydrodynamic simulations to investigate the connection between properties of neutral oxygen (OI) absorbers and galaxies across redshift $z = 5 - 8$. We find that the virialised gas in haloes becomes progressively ionised with cosmic time, leading to a decrease of $\approx 0.2$ in the covering fraction of neutral oxygen, while the total oxygen covering fraction remains constant. Comparing the OI line density obtained from our covering fractions with the trend suggested by blind quasar observations, we determine that the observable absorbers ($N_{\rm OI} \gtrsim 10^{13},\text{cm}^{-2}$) are not confined to haloes: at $z \geq 5$ the majority ($\gtrsim 60%$) arise beyond $R_{\rm{vir}}$, consistent with recent JWST results. Close to OI absorbers, low-mass galaxies ($M_\star \leq 10^8,\rm{M}\odot$) are more commonly found, while, depending on the simulated environment, we do not exclude the possibility of nearby more massive star-forming sources ($\geq 5,\text{M}\odot,\text{yr}^{-1}$) similar to those suggested by the latest ALMA observations. These results establish OI absorbers as sensitive tracers of the evolving ionisation structure around faint galaxies to be probed by forthcoming deep spectroscopic surveys.

💡 Research Summary

In this paper the authors investigate the physical connection between neutral oxygen (O I) absorbers and galaxies during the Epoch of Reionization (EoR) using the THESAN‑ZOOM suite of radiation‑hydrodynamic zoom‑in simulations. The study focuses on the redshift interval 5 ≤ z ≤ 8, a period when the intergalactic medium (IGM) transitions from neutral to ionised and metal absorption lines become powerful probes of both galaxy feedback and the progress of reionisation.

The THESAN‑ZOOM simulations consist of 14 high‑resolution zoom‑ins centred on haloes selected from the parent THESAN‑dark‑1 volume. The targeted haloes span a wide mass range (10⁸–10¹³ M⊙ at z = 3) and are re‑simulated with gas particle masses of ≈ 9 × 10³ M⊙, a spatial resolution sufficient to resolve the cold (T ≲ 10⁴ K) phase of the circumgalactic medium (CGM). The code (AREPO‑RT) solves the coupled radiation‑hydrodynamics problem with a moment‑based radiative transfer scheme, seven radiation frequency bins (from infrared to He II‑ionising photons), and a reduced speed of light (c̃ = 0.01 c). Star formation follows a stochastic Jeans‑unstable prescription, and stellar feedback (SN II, SN Ia, stellar winds, early stellar feedback) is implemented via the SMUGGLE model. Metal enrichment tracks seven elements (C, N, O, Mg, Ne, Si, Fe) but does not solve non‑equilibrium metal ionisation; instead, the neutral oxygen fraction is inferred from the on‑the‑fly H I ionisation state and the well‑known charge‑exchange reaction O + H⁺ ↔ O⁺ + H⁰.

Key findings are as follows:

1. Evolution of O I covering fraction – Within the virial radius (R_vir) of haloes, the covering fraction of neutral oxygen declines by ≈ 0.2 from z = 8 to z = 5, reflecting the progressive ionisation of halo gas as reionisation proceeds. By contrast, the total oxygen covering fraction (all ionisation states) remains roughly constant, indicating that metal mass is not lost but simply changes ionisation state.

2. Location of observable absorbers – By converting simulated covering fractions into an O I line density (dN/dX) and comparing with blind quasar surveys, the authors show that absorbers with column densities N_OI ≥ 10¹³ cm⁻² are not confined to halo interiors. At z ≥ 5, more than 60 % of such absorbers arise beyond R_vir, i.e., in the CGM or in dense neutral pockets of the IGM. This result aligns with recent JWST detections of O I absorbers at impact parameters of several hundred kiloparsecs from identified galaxies.

3. Galaxy counterparts – The nearest galaxies to O I absorbers are predominantly low‑mass systems (stellar mass M_* ≤ 10⁸ M⊙). The probability of finding such a faint galaxy within ≈ 30 pkpc of an absorber is significantly higher than for more massive counterparts. Nevertheless, in overdense regions the simulations do not rule out the presence of more massive, actively star‑forming galaxies (SFR ≥ 5 M⊙ yr⁻¹) near absorbers, consistent with ALMA detections of