Possible Contamination of the Intergalactic Medium Damping Wing in ULAS J1342+0928 by Proximate Damped Ly<i>α</i> Absorption

Abstract The red damping wing from neutral hydrogen in the intergalactic medium is a key predicted signature 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 ≳ 7 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 ([O/H] ∼ −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 the Extremely Large Telescope to “purify” damping wing quasar samples, an exercise that is impossible for much fainter objects like galaxies.