To be lensed or not to be lensed: on the nature of the alleged high-z lensed quasars J0109-5424 and P170+20

Aurora Mata-Sanchez, Emanuele P. Farina, Anniek J. Gloudemans, Debora Pelliccia, Roberto Decarli, Eduardo Banados, Silvia Belladitta, Manuela Bischetti, Sarah E. Bosman, Xander Byrne, Thomas Connor, Frederick B. Davies, Thales A. Gutcke, Weizhe Liu, Chiara Mazzucchelli, Romain A. Meyer, Silvia Onorato, Fabian Walter, Feige Wang, Jinyi Yang

First listed 2026-08-27 | Last updated 2026-08-25

Abstract

We present a detailed multi-wavelength analysis of two gravitationally lensed z>6 quasar candidates: J0109-5424 and P170+20. Using JWST/NIRSpec near-infrared data from the Aether survey, we demonstrate that J0109-5424 is in fact a FeLoBAL quasar at z=2.07, exhibiting strong iron broad absorption lines that mimic the Lyman break. From the Pabeta emission line, we estimate a black hole mass of 3.9 x 10^8 Msun and a bolometric luminosity of Lbol = 7.7 x 10^45 erg/s for this quasar. Follow-up HST/ACS WFC imaging of P170+20 with the F555W filter, probing emission below the Lyman limit of the quasar, reveals a compact source located 0.09+/-0.04 arcsec from the quasar position. PSF modelling and subtraction yield no evidence for extended emission. This is inconsistent with a foreground galaxy acting as a lens. Furthermore, we obtain Gemini-South/Flamingos 2 near-infrared observations to complement the existing optical Gemini-North/GMOS spectrum of P170+20. Our spectral analysis shows that the observed spectral shape of P170+20 is consistent with a composite quasar and M-dwarf spectrum. However, this interpretation remains uncertain, as a chance alignment between an ultracool dwarf and a quasar within 0.1 arcsec would be unlikely. Overall, our findings establish J0109-5424 as an intermediate redshift source and do not favor the lensed nature of P170+20, although this possibility cannot be conclusively ruled out, highlighting the challenges of selecting lensed quasars at high redshift.

Short digest

Mata-Sanchez et al. use JWST/NIRSpec, HST/ACS, and Gemini spectroscopy to reassess two alleged strongly lensed z>6 quasars. J0109-5424 is instead a z=2.07 FeLoBAL quasar: iron broad-absorption features imitate a Lyman break, while Paβ implies a 3.9 × 10^8 Msun black hole and Lbol = 7.7 × 10^45 erg s−1. For P170+20, unresolved F555W emission 0.09 ± 0.04 arcsec from the quasar and the lack of extended residuals argue against a foreground lensing galaxy; its broad spectral shape can be fit by a quasar plus M-dwarf composite, though the improbable sub-0.1-arcsec alignment means lensing cannot be fully excluded. The paper is a sharp caution that apparent blue flux and dropout-like breaks alone can produce persuasive but false high-redshift lens candidates.

Key figures to inspect

• Figure 1. This is the decisive reclassification figure for J0109-5424: the GMOS spectrum shows how Mg II and iron absorption can masquerade as a Lyα break, while NIRSpec reveals the Paschen lines and FeLoBAL absorption that fix its intermediate-redshift nature.
• Figure 2. The HST F555W imaging and GALFIT residuals provide the cleanest direct test of the P170+20 lens hypothesis. A lone unresolved source below the putative quasar Lyman limit, with no extended light after PSF subtraction, is inconsistent with the expected foreground lens galaxy.
• Figure 3. This broad optical-to-near-infrared spectral synthesis shows why P170+20 can be explained by a high-redshift quasar template blended with an M dwarf. It captures the paper's preferred alternative to lensing and makes clear that the conclusion rests on the full spectral shape rather than a single blue-band excess.
• Figure 4. The quasar-plus-elliptical-galaxy comparison is the key counter-model for P170+20. Its mismatch with the spectrum and the F555W flux helps explain why a normal foreground lens galaxy is disfavored, while preserving the paper's caution that the system is not conclusively resolved.