The search for quasars at z>6 is a hunt for rare objects that illuminate the first billion years of the Universe. They are scarce, but false candidates abound: ultracool dwarfs and quasars with strong absorption lines can masquerade as distant sources. Gravitational lensing further distorts the picture, boosting brightness and biasing black hole mass estimates. Only one lensed quasar at z>6 has been confirmed so far: J0439+1634. Every new candidate is valuable but requires careful scrutiny. This work stands apart: neighboring studies in the archive mostly address lensing at lower z or other selection methods, with almost no direct analogues to this kind of vetting.
For J0109–5424, a JWST/NIRSpec spectrum in the near-infrared (2.87–5.27 μm) from the Aether survey was used. The Paβ line was modeled with a Lorentzian, yielding precise redshift and black hole parameters. For P170+20, an HST/ACS image in the F555W filter (below the quasar's Lyman limit) was obtained to search for a foreground object. PSF modeling with GALFIT tested the point-like nature of the source. Spectra from Gemini/GMOS, Flamingos-2, and LBT/LUCI were combined and compared with quasar and brown dwarf templates. The probability of chance alignment was estimated from the sky density of ultracool dwarfs.
J0109–5424 turned out to be an FeLoBAL quasar at z=2.0741±0.0002. The sharp break at 8604 Å is a blend of Mg II and iron absorption lines, not a Lyman break. From the Paβ line, the black hole mass is 3.9×10⁸ M⊙, the bolometric luminosity is 7.7×10⁴⁵ erg/s, and the Eddington ratio is 0.16. The outflow velocity is at least 2500 km/s. P170+20 shows a compact source 0.09±0.04″ from the quasar with no extended emission. The spectrum is best fit by a sum of a quasar at z=6.39 and an M dwarf. The chance coincidence probability is about 4%—low but not negligible. Lensing is not ruled out, but unlikely.
This work shows that even carefully selected lensed quasar candidates at high z can be false positives. Unlike neighboring studies that mostly search for new lenses or study confirmed cases, here two specific objects were directly tested with JWST and HST. This is the first demonstration that FeLoBAL quasars can mimic a Lyman break at z>6 in optical surveys. For P170+20, a new scenario is proposed—chance alignment with an ultracool dwarf—which had not been systematically considered before.
For P170+20, the decisive test will be measuring the proper motion of the presumed M dwarf. If it lies in the Milky Way halo at ~2.5 kpc, the shift would be 0.2″ in ~9 years. Additional HST epochs or high-resolution adaptive optics would separate the components. For J0109–5424, it would be interesting to study the population of FeLoBAL quasars at z~2 that contaminate distant quasar samples. Machine learning could be trained to distinguish such spectra from genuine Lyman breaks.
The results will affect selection strategies for high-z quasars in current and future surveys (Euclid, LSST). They will also refine the statistics of lensed quasars and constraints on the luminosity function.
Obtain a second epoch image of P170+20 to search for proper motion. Conduct detailed modeling of FeLoBAL quasar spectra to create reliable rejection criteria.
This work directly addresses the problem of sample purity in high-z quasar surveys: how to distinguish a true Lyman break from imitations. It also touches on black hole mass estimates, which can be inflated if lensing is not accounted for. For P170+20, the nature of the compact source—star or galaxy—remains an open question.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
A multiwavelength analysis of two gravitationally lensed quasar candidates at z>6 is presented. For J0109–5424, JWST/NIRSpec spectroscopy revealed an FeLoBAL nature at z=2.07, mimicking a Lyman break. For P170+20, HST imaging and Gemini/LBT spectra showed a point source at 0.09″, consistent with a chance alignment with an M dwarf, but not ruling out lensing.
Where its strength is
This work demonstrates the importance of detailed scrutiny of high-z lensed quasar candidates, showing how FeLoBAL quasars can mimic a Lyman break, and proposing a chance alignment scenario with an ultracool dwarf.
Where it could go next
Worth trying
Why this may matter
Understanding the nature of false candidates is crucial for the reliable selection of quasars at z>6 and for estimating their abundance, which impacts studies of the early Universe and the growth of supermassive black holes.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.54, against a usual 0.64. It looks like few have drilled nearby.
Among the hundreds of known quasars at z>5.5, only one is confirmed as gravitationally lensed — J0439+1634 with a magnification of 50. And FeLoBAL quasars are so red that they are mistaken for objects at the edge of the observable Universe.