[Paper Review] Possible Contamination of the Intergalactic Medium Damping Wing in ULAS J1342+0928 by Proximate Damped Ly$α$ Absorption

Frederick B. Davies, Eduardo Bañados+2 more|arXiv (Cornell University)|Dec 11, 2023

Galaxies: Formation, Evolution, Phenomena4 citations

TL;DR

This paper investigates whether the damping wing signal in the $z=7.54$ quasar ULAS J1342+0928 could be contaminated by a proximate damped Ly$\alpha$ absorber (DLA) associated with a complex of low-ionization metal absorbers detected by JWST/NIRSpec at $z>7.3$. It finds that only an extremely metal-poor DLA ($[\mathrm{O}/\mathrm{H}] \sim -3.5$) could reproduce the observed spectral profile without invoking a significant intergalactic medium (IGM) damping wing, challenging the interpretation of the IGM reionization signal.

ABSTRACT

The red damping wing from neutral hydrogen in the intergalactic medium is a smoking-gun signal of ongoing reionization. One potential contaminant of the intergalactic damping wing signal is dense gas associated with foreground galaxies, which can give rise to proximate damped Ly$α$ absorbers. The Ly$α$ imprint of such absorbers on background quasars is indistinguishable from the intergalactic medium within the uncertainty of the intrinsic quasar continuum, and their abundance at $z\gtrsim7$ is unknown. Here we show that the complex of low-ionization metal absorption systems recently discovered by deep JWST/NIRSpec observations in the foreground of the $z=7.54$ quasar ULAS~J1342$+$0928 can potentially reproduce the quasar's spectral profile close to rest-frame Ly$α$ without invoking a substantial contribution from the intergalactic medium, but only if the absorbing gas is extremely metal-poor ($[{ m O}/{ m H}]\sim-3.5$). Such a low oxygen abundance has never been observed in a damped Ly$α$ absorber at any redshift, but this possibility still complicates the interpretation of the spectrum. Our analysis highlights the need for deep spectroscopy of high-redshift quasars with JWST or ELT to "purify" damping wing quasar samples, an exercise which is impossible for much fainter objects like galaxies.

Motivation & Objective

• To assess whether the observed damping wing in the high-redshift quasar ULAS J1342+0928 could originate from a proximate damped Ly$\alpha$ absorber (DLA) rather than the intergalactic medium (IGM).
• To evaluate the impact of metal absorption systems at $z>7.3$ detected by JWST/NIRSpec on the interpretation of the IGM damping wing signal.
• To determine the required metallicity and H I column density of a DLA that could reproduce the quasar’s spectral profile near rest-frame Ly$\alpha$ without invoking IGM neutral hydrogen.
• To emphasize the need for high-resolution spectroscopy to 'purify' damping wing quasar samples and reduce systematic uncertainties in reionization studies.

Proposed method

• Used the measured equivalent widths of Mg II $\lambda\lambda 2796,2803$ and O I $\lambda 1302$ lines from Christensen et al. (2023) to estimate metal column densities for three absorbers at $z=7.368$, $7.443$, and $7.476$.
• Applied metallicity-dependent scaling to convert observed metal column densities into H I column densities, assuming a solar abundance ratio for O/Mg.
• Reconstructed the quasar’s spectral profile by injecting DLA models into the observed spectrum, using fiducial parameters derived from marginalized likelihoods of the absorption systems.
• Reanalyzed the IGM damping wing signal using the same framework as Davies et al. (2018a), now including the DLA contributions, to assess how the inferred IGM neutral fraction changes.
• Simulated future high-resolution observations with ELT/MICADO ($R=20,000$) and compared them to existing JWST/NIRSpec data to assess detectability of kinematic structure in the absorbers.
• Assessed the robustness of the results under uncertainty in saturation and kinematic modeling, particularly for saturated lines.

Experimental results

Research questions

• RQ1 Can the observed damping wing in ULAS J1342+0928 be explained by a proximate DLA rather than the intergalactic medium?
• RQ2 What metallicity and H I column density are required for a DLA to reproduce the spectral profile near Ly$\alpha$ without invoking IGM neutral hydrogen?
• RQ3 How does the presence of a DLA with extreme metal-poor characteristics affect the inferred IGM neutral fraction in reionization studies?
• RQ4 To what extent can future high-resolution spectroscopy with ELT/MICADO resolve the kinematic structure of these high-redshift absorbers and distinguish them from IGM damping wings?
• RQ5 What are the implications of undetected, extremely metal-poor DLAs for the reliability of IGM damping wing measurements in high-redshift quasars?

Key findings

• The complex of low-ionization metal absorbers at $z>7.3$ in ULAS J1342+0928 can reproduce the quasar’s spectral profile near rest-frame Ly$\alpha$ without requiring a significant contribution from the intergalactic medium.
• A DLA with an oxygen abundance of $[\mathrm{O}/\mathrm{H}] \sim -3.5$ is required to match the observed profile, which is far more metal-poor than any DLA observed at any redshift to date.
• Such a low-metallicity DLA would shift the inferred IGM neutral fraction to near-zero values, effectively ruling out a substantial IGM damping wing contribution.
• The stringent metallicity requirement implies that only an exceptionally rare and extreme DLA could mimic the IGM signal, but such a scenario remains unobserved and thus uncertain.
• High-resolution spectroscopy with future instruments like ELT/MICADO is essential to resolve the kinematics of these absorbers and distinguish them from IGM damping wings.
• Without such high-resolution data, the contamination risk from proximate DLAs remains a significant systematic uncertainty in reionization studies using quasar damping wings.