In a preprint submitted to arXiv in April 2026, lead author T. Mkrtchyan and collaborators deliver a catalog of 6,104 high-redshift quasar candidates at 4.5 < z < 7, carefully selected through spectral energy distribution (SED) fitting rather than relying solely on traditional color cuts. The team compiled a multi-band photometric dataset merging optical imaging from DELVE DR2 and DECaLS DR10, near-infrared data from VHS DR5 plus a VIKING field, mid-infrared AllWISE detections, and Gaia DR3 astrometry. After applying morphological filters to exclude extended sources and performing SED modeling against both quasar and brown-dwarf templates, they ranked objects using χ², Bayesian Information Criterion (BIC > 0), and an F-test threshold (>10), retaining only candidates detected in at least seven bands. This methodology produced a sample size substantially larger than most prior southern surveys. Initial spectroscopic validation with NTT/EFOSC2 and Palomar/NGPS confirmed three z > 5 quasars out of six observed targets, yielding a 50% success rate in that small subsample.

This work must be understood as a preprint, not yet peer-reviewed, and carries clear limitations: photometric redshifts can suffer from aliasing, dust reddening, or confusion with rare cool stars, and the confirmation fraction, while encouraging, is based on very few objects. Nonetheless, its value lies in addressing a long-standing observational asymmetry. For decades, the majority of z > 5 quasars have come from northern facilities such as SDSS and Pan-STARRS. Bañados et al. (2016, arXiv:1608.03279) used Pan-STARRS1 data to discover dozens of luminous z ≈ 6 quasars, yet the southern sky remained comparatively blank. The new Mkrtchyan catalog directly corrects this imbalance, providing an all-sky-capable resource that future large-scale structure analyses can exploit.

The deeper significance emerges when synthesized with two other strands of research. First, measurements of the Lyman-α forest in high-z quasar spectra have been central to mapping the timeline of cosmic reionization—the epoch roughly 13 billion years ago when ultraviolet photons from the first galaxies and accreting black holes ionized the pervasive neutral hydrogen. Studies such as the 4MOST Consortium's own design papers (arXiv:1910.08217) explicitly list high-redshift quasars as key targets precisely because their bright continua backlight the intergalactic medium. Second, recent JWST results have revealed surprisingly massive, already-grown galaxies and active nuclei at z > 7, intensifying the tension in black-hole seed formation models. A larger, cleaner sample of quasars at z = 5–7 will allow statisticians to measure the bright-end slope of the luminosity function across both hemispheres, testing whether spatial clustering or environmental factors accelerated early supermassive black hole growth.

Previous media coverage of high-redshift quasars has disproportionately celebrated individual record holders—the brightest, most distant single objects—while underplaying the need for uniform, statistically complete samples. By contrast, this catalog’s strength is its deliberate design for the forthcoming 4MOST/ChANGES survey, which will obtain hundreds of spectra and convert these candidates into science-ready data. The analysis also highlights an under-appreciated pattern: SED fitting, when combined with multi-wavelength coverage including mid-infrared, substantially reduces contamination compared with color-only methods used in early SDSS work. This methodological advance, paired with the southern focus, supplies exactly the targets needed to probe reionization topology and to cross-correlate with JWST-detected faint galaxies.

Ultimately the Mkrtchyan catalog is not merely an incremental list; it represents infrastructure for the next decade of early-universe astronomy. If even 30 % of the 6,104 candidates prove real, the community will gain hundreds of new probes of the cosmic dawn—enough to map patchy reionization, constrain black-hole assembly histories, and anchor simulations of first-galaxy feedback. The southern sky, long the poorer cousin in high-redshift studies, may soon become the richest.