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

TL;DR

The paper investigates neutral oxygen absorbers as probes of the evolving ionisation structure around faint galaxies during the Epoch of Reionization, using the THESAN-ZOOM radiation-hydrodynamic simulations to link OI absorbers at $z=5-8$ to their galactic environments. It shows that the circumgalactic medium becomes progressively ionised, reducing the neutral OI covering fraction within halos, while the total oxygen content remains roughly constant; importantly, most observable OI absorbers at these redshifts lie outside the CGM, requiring inclusion of distant or undetected low-mass galaxies to reproduce the observed absorber incidence. The work highlights a strong degeneracy between metallicity and ionisation in setting $N_{OI}$ for a given $N_{HI}$, and identifies oxygen clumpiness and recent star formation as key drivers of the scatter in absorber statistics. The findings emphasize OI absorbers as sensitive tracers of the ionisation topology around faint galaxies and provide guidance for interpreting upcoming deep spectroscopic surveys with JWST, ALMA, and ELT-era facilities.

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 that fall below the detection threshold of current instruments. We use the THESAN-ZOOM radiation hydrodynamic simulations to investigate the connection between properties of neutral oxygen (OI) absorbers and galaxies within the redshift range $z = 5 - 8$. We find that the circumgalactic medium of galaxies 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. 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 $R_{\rm{vir}}$, and including these systems is essential to reproduce the observed increase in absorber incidence with redshift. The simulated absorbers preferentially reside in overdensities rich in low-mass galaxies ($M_\star \leq 10^8\,\rm{M}_\odot$), explaining the scarcity of detected counterparts, while not excluding the possibility of nearby star-forming sources ($\geq 5\,\text{M}_\odot\,\text{yr}^{-1}$) similar to those suggested by the latest ALMA observations and, at larger distances, by the JWST. These results establish OI absorbers as sensitive tracers of the evolving ionisation structure around faint galaxies to be probed by forthcoming deep spectroscopic surveys.

Figures (15)

• Figure 1: Column density maps of neutral hydrogen ($N_{\rm HI}$), total oxygen ($N_{\rm O}$), and neutral oxygen ($N_{\ion{O}{i}}$) of four different areas of size $50\times50$ pkpc centred on haloes extracted from the m11.9 and m10.4 zoom-in regions. The first two columns are centred on two different haloes of mass $M_h \approx 10^{10}\,\text{M}_\odot$ at redshift $z=8$ and $z=5$, respectively, while the third and fourth columns represent regions around haloes of mass $M_h \approx 10^{9}\,\text{M}_\odot$ at the same redshifts. In the first row, different colours highlight different types of Hi absorbers, with damped Ly$\alpha$ Absorbers (DLAs) in red and Lyman Limit Systems (LLSs) in green. In the third row, where the neutral oxygen column density is displayed, areas with Oi absorbers with $N_{\rm \ion{O}{i}} > 10^{13}$ cm$^{-2}$ along the line of sight are coloured. In all panels, circles mark the virial radii of haloes more massive than $10^8\,\text{M}_\odot$. Overall, we notice how Oi traces the overlap between O and HI.
• Figure 2: Gas properties of Oi absorbers with $N_{\ion{O}{i}} > 10^{13}$ cm$^{-2}$ inside the virial radii of haloes at $z=8$. The panels show the $n_{\ion{O}{i}}$-weighted hydrogen number density ($n_\text{H})$, temperature ($T$), metallicity ($Z$), and neutral hydrogen fraction ($x_\text{HI}$) as a function of the neutral oxygen column density. The points display values relative to absorbers inside the CGM of high-mass haloes $M_h > 10^{10}\,\text{M}_\odot$, with the gray contours showing the density distribution of these points. The black contours mark the density distribution of absorbers inside the virial radius of lower-mass haloes ($10^9< M_h/\text{M}_\odot < 10^{10}$) which are not explicitly shown. The points are coloured according to the $n_{\ion{O}{I}}$-weighted neutral hydrogen density of absorbers. Overall, $N_{\ion{O}{i}}$ is positively correlated with $n_{\rm H}$, $Z$ and $x_{\ion{H}{i}}$, and anticorrelated with $T$.
• Figure 3: Relation between the neutral hydrogen fraction ($x_{\ion{H}{I}}$) and Oi column density of CGM absorbers in haloes more massive than $10^9\,\text{M}_\odot$. Data are divided in $5$ metallicity bins indicated by different colours. The solid lines mark the median value of the distribution as a function of $N_{\ion{O}{I}}$, while the coloured regions identify the 16$^{\rm th}$ and 84$^{\rm th}$ percentiles. The dashed lines represent the best-fit sigmoid describing the median. The plot highlights the degeneracy between metallicity and ionisation level arising when selecting neutral oxygen absorbers with given $N_{\ion{O}{i}}$ values.
• Figure 4: Median radial profile of hydrogen (left panels) and oxygen (right panels) column density values within the CGM of haloes with masses above $10^{10}\,\text{M}_\odot$ (top panels) and in the range $10^9<M_h/\text{M}_\odot<10^{10}$ (bottom panels). The solid lines trace the total hydrogen or oxygen column density and its evolution from $z=5$ to $z=8$, with darker lines marking the profiles at higher redshift and lighter ones at lower redshifts. The dashed lines delineate the profiles of neutral hydrogen or neutral oxygen column densities, following the same colour code. The dotted black line in the right panels marks the 50% completeness level adapted from Sebastian2024. The neutral oxygen profiles decreases as time progresses, mirroring in both high-mass and low-mass haloes the trend traced by hydrogen.
• Figure 5: Oi and Hi column density relation along equally spaced and parallel lines of sight inside the CGM of individual haloes selected at $z=7$. Each point represents the value along a single line of sight, coloured according to the distance from the centre of the halo. The different shades of gray in the background mark different types of neutral hydrogen absorbers. Both haloes were selected from the same simulated region, and in order to have similar masses. Additionally, the dashed gray lines identify the region where Finlator2013 found 99% of their CGM lines of sight, while the blue circles indicate observational constraints at $z\approx5$Becker2012 and the black crosses at $z\approx6$Sodini2024. A positive correlation between $N_{\ion{O}{i}}$ and $N_{\ion{H}{i}}$ is present, with a large scatter that might increase in some haloes. Observable Oi absorbers are usually associated to LLS or DLAs.