The evolution of obscured AGN across cosmic time -- A large quasar survey for the 2040s

Tanya Urrutia, Darshan Kakkad, Paula Sánchez-Sáez, Mojtaba Raouf, Swayamtrupta Panda, Sarah E. I. Bosman, Francisco Pozo Nunez, Annagrazia Puglisi, Sophia Flury, Dragana Ilic, Andjelka B. Kovacevic, Mamta Pandey-Pommier, Giustina Vietri, Sarath Satheesh-Sheeba, Francesco Salvestrini, Susanna Bisogni, Eduardo Bañados, Ana Monreal Ibero, Sabine Thater, Pratika Dayal, Filippo D'Ammando, José Afonso, Paramita Barai, Valentin Ivanov

First listed 2025-12-20 | Last updated 2025-12-20

Abstract

We propose a large quasar demographic optical multi-object spectroscopic (MOS) survey targeting over 50 million AGN candidates up to the highest redshifts possible in the optical (z~6.5), with repeat visits, using a variety of selection criteria available by 2040. A large MOS survey combining all AGN selection methods is the only way to unify a diverse range of different obscured AGN populations within a single, variability- and spectroscopy-based framework, rather than as disjoint classes selected by different methods.

Short digest

White paper proposing a 2040s optical MOS program to repeatedly spectroscopically observe >50 million quasar/AGN candidates out to z≈6.5 by combining radio, IR, X-ray, variability, and astrometric selections. The unified, variability+spectroscopy framework is designed to fold in dust-reddened systems (e.g., LRD-like, red/FeLoBAL quasars), radio-selected sources, and CL AGN to measure how the obscured fraction depends on luminosity and redshift from Cosmic Noon into reionization. The plan leverages Euclid/Roman imaging, SKA-Wide radio identifications, LSST variability, and NewAthena X-rays, and argues for a WST-class 10–15 m, R≈1500, 20–30k-fiber facility achieving S/N>5 to 25 mag even for NH≈10^24 cm⁻2. The payoff is a bias-corrected, time-resolved census of early black-hole growth and obscuration evolution across cosmic time.

Key figures to inspect

• Projected completeness across the luminosity–redshift plane comparing single-method selections (radio/IR/X-ray/variability/astrometry) versus the combined strategy, to see how obscured classes (LRDs, red quasars, FeLoBALs, Type 2) are recovered uniformly.
• Survey yield maps from Euclid, SKA-Wide, LSST, and NewAthena: expected surface densities and total counts per redshift bin, including the >100k red-quasar candidates from early Euclid and the >50k SKA EoR candidates, to gauge where the 50M total comes from.
• Instrument performance for a WST-like facility: S/N versus magnitude at R≈1500 and exposure time, showing reach to r≈25 and detectability of broad emission/absorption features under heavy extinction (NH≈10^24 cm⁻2).
• Cadence and multi-epoch plan: expected numbers of changing-look events and BLR re-emergence detections versus revisit spacing, illustrating how time-domain spectroscopy constrains obscuration physics.
• Target selection and cross-match flow: decision tree integrating radio/IR/X-ray/variability/astrometry inputs with de-duplication and prioritization, clarifying how diverse AGN candidates enter a single spectroscopic program.