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

Giulia Pruto, Laura Keating, Rahul Kannan, Ewald Puchwein, Aaron Smith, Josh Borrow, Enrico Garaldi, Mark Vogelsberger, Oliver Zier, William McClymont, Xuejian Shen, Sandro Tacchella

First listed 2025-10-15 | Last updated 2026-01-12

Abstract

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.

Short digest

Using the THESAN-ZOOM radiation-hydrodynamic suite at z=5–8, this paper links neutral-oxygen absorbers to their galaxy environments. The CGM becomes more ionised with time, driving a ≈0.2 drop in the OI covering fraction while the total oxygen covering fraction stays flat. Most observable systems (N_OI ≳ 10^13 cm^-2) at z ≥ 5 lie beyond R_vir and cluster around faint galaxies (M* ≤ 10^8 M_sun), with occasional nearby SFR ≥ 5 M_sun/yr sources depending on environment. This establishes OI absorbers as clean tracers of the evolving ionisation structure around faint reionization-era galaxies and sets expectations for deep spectroscopic follow-up.

Key figures to inspect

• Covering-fraction evolution: OI vs total oxygen covering fractions across z=5–8, highlighting the ≈0.2 decline in neutral oxygen despite a flat total-oxygen trend—learn how ionisation, not metal budget, drives the change.
• Radial/2D maps around halos: projected N_OI with R_vir overlaid, showing where N_OI ≥ 10^13 cm^-2 systems sit; verify that ≳60% of observable absorbers lie beyond R_vir at z ≥ 5.
• Incidence vs redshift: simulated OI line density compared to blind quasar measurements, demonstrating that including beyond-halo absorbers is required to recover the rising incidence toward high z.
• Galaxy-association statistics: distributions of nearest-neighbour stellar masses and SFRs around absorbers, showing prevalence of M* ≤ 10^8 M_sun companions and occasional nearby ≥5 M_sun/yr galaxies.
• Environment overdensity: counts-in-cells or halo-number excess around absorbers versus random regions, illustrating that OI systems prefer overdense, faint-galaxy-rich environments.