An extremely metal-poor Lyman $α$ emitter candidate at $z=6$ revealed through absorption spectroscopy

Dominika Ďurovčíková, Anna-Christina Eilers, Robert A. Simcoe, Louise Welsh, Romain A. Meyer, Jorryt Matthee, Emma V. Ryan-Weber, Minghao Yue, Harley Katz, Sindhu Satyavolu, George Becker, Frederick B. Davies, Emanuele Paolo Farina

First listed 2025-05-02 | Last updated 2025-07-08

Abstract

We report the discovery of a Lyman $α$ emitter (LAE) candidate in the immediate foreground of the quasar PSO J158-14 at $z_{\rm QSO}=6.0685$ at a projected distance $\sim29\ {\rm pkpc}$ that is associated with an extremely metal-poor absorption system. This system was found in archival observations of the quasar field with the Very Large Telescope/Multi-Unit Spectroscopic Explorer (VLT/MUSE) and was previously missed in searches of absorption systems using quasar absorption line spectroscopy as it imparts no detectable metal absorption lines on the background quasar spectrum. The detected Ly$α$ emission line at a redshift of $z_{\rm LAE}=6.0323$ is well aligned with the outer edge of the quasar's proximity zone and can plausibly cause its observed damping wing if it is associated with a proximate sub-damped Ly$α$ absorption system with a column density of $\log {N_{\rm HI} / {\rm cm}^{-2}} \approx 19.7$. A $>10$ hour medium-resolution spectrum of the quasar observed with the Magellan/Folded-port InfraRed Echellette (FIRE) and VLT/X-Shooter spectrographs reveals a metallicity constraint of ${\rm [Z/H]} < -3$. Such low metallicity makes this system an extremely metal-poor galaxy candidate and provides an exciting site to study possible signatures of Population III stars.

Short digest

Archival VLT/MUSE data reveal a Lyα emitter at z=6.0323 just 29 pkpc in front of the quasar PSO J158-14 (z=6.0685), previously missed because it leaves no metal absorption in the quasar spectrum. The Lyα line sits at the edge of the quasar proximity zone, and a proximate sub-DLA with log N_HI≈19.7 reproduces the observed damping wing. Deep (>10 hr) FIRE+X-shooter spectroscopy yields [Z/H] < −3 after ionization-correction modeling, marking this source as an extremely metal-poor galaxy candidate. This system offers a rare window on near-pristine gas and potential Population III signatures during reionization.

Key figures to inspect

• Fig. 1: Inspect the PSF-subtracted MUSE SNR map and spectrum to see the LAE’s position relative to PSO J158-14 and its Lyα line at z=6.0323; compare the sub-DLA damping-wing fit to homogeneous-IGM models and note the absence of other lines in the MUSE range.
• Fig. 2: Use the HST F555W and F850LP non-detections to rule out a low-z interloper and confirm that the measured Lyα flux is consistent with a z≈6 dropout source.
• Fig. 3: Examine the continuum-normalized FIRE+X-shooter J-band stacks at the LAE redshift; the lack of low-ionization metal lines against marked foreground absorbers underpins the stringent metallicity limits.
• Fig. 4: Check the stacked absorption limits versus CLOUDY-based ionization corrections; [Z/H] < −3 remains robust and is only weakly affected by quasar radiation, reinforcing the EMP interpretation.