An obscured quasar census with the 4MOST IR AGN survey: design, predicted properties, and scientific goals

Andonie, Carolina; Alexander, David M; Greenwell, Claire; Fotopoulou, Sotiria; Hickox, Ryan; Rosario, David J; Villforth, Carolin; Buchner, Johannes; Krogager, Jens-Kristian; Laloux, Brivael; Merloni, Andrea; Salvato, Mara; Streicher, Ole; Yan, Wei

Abstract

We present the 4MOST (4-metre Multi-Object Spectroscopic Telescope) infrared (IR) AGN survey, the first large-scale optical spectroscopic survey characterizing mid-infrared (MIR) selected obscured active galactic nuclei (AGNs). The survey targets ≈ 212 000 obscured IR AGN candidates over ≈ 10 000 deg² down to a magnitude limit of rAB = 22.8 mag and will be ≈ 100× larger than any existing obscured IR AGN spectroscopic sample. We select the targets using an MIR colour criterion applied to the unWISE catalogue from the WISE (Wide-field Infrared Survey Explorer) all-sky survey, and then apply a r − W2 ≥ 5.9 mag cut; we demonstrate that this selection will mostly identify sources obscured by NH > 10²² cm−2. The survey complements the 4MOST X-ray survey, which will follow up ∼ 1 M eROSITA (extended ROentgen Survey with an Imaging Telescope Array)-selected (typically unobscured) AGN. We perform simulations to predict the quality of the spectra that we will obtain and validate our MIR–optical colour-selection method using X-ray spectral constraints and UV-to-far-IR spectral energy distribution (SED) modelling in four well-observed deep-sky fields. We find that: (1) ≈ 80 − 87 per cent of the WISE-selected targets are AGN down to rAB = 22.1 − 22.8 mag of which ≈ 70 per cent are obscured by NH > 10²² cm−2, and (2) ≈ 80 per cent of the 4MOST IR AGN sample will remain undetected by the deepest eROSITA observations due to extreme absorption. Our SED-fitting results show that the 4MOST IR AGN survey will primarily identify obscured AGN and quasars (≈ 55 per cent of the sample is expected to have LAGN,IR > 10⁴⁵ erg s−1) residing in massive galaxies (M* ≈ 10¹⁰ − 10¹² M⊙) at z ≈ 0.5 − 3.5 with ≈ 33 per cent expected to be hosted by starburst galaxies.

Citation

Andonie, C., Alexander, D. M., Greenwell, C., Fotopoulou, S., Hickox, R., Rosario, D. J., Villforth, C., Buchner, J., Krogager, J.-K., Laloux, B., Merloni, A., Salvato, M., Streicher, O., & Yan, W. (2025). An obscured quasar census with the 4MOST IR AGN survey: design, predicted properties, and scientific goals. Monthly Notices of the Royal Astronomical Society, 539(3), 2202-2229. https://doi.org/10.1093/mnras/staf624

• Journal Article Type: Article
• Acceptance Date: Apr 10, 2025
• Online Publication Date: Apr 17, 2025
• Publication Date: 2025-05
• Deposit Date: May 20, 2025
• Publicly Available Date: May 20, 2025
• Journal: Monthly Notices of the Royal Astronomical Society
• Print ISSN: 0035-8711
• Electronic ISSN: 1365-2966
• Publisher: Royal Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 539
• Issue: 3
• Pages: 2202-2229
• DOI: https://doi.org/10.1093/mnras/staf624
• Keywords: infrared: galaxies, surveys, quasars: general
• Public URL: https://durham-repository.worktribe.com/output/3942094