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

Journal Article · Thu Apr 17 00:00:00 UTC 2025 · Monthly Notices of the Royal Astronomical Society

DOI: https://doi.org/10.1093/mnras/staf624 · OSTI ID:2563511

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 $$\approx 212\,000$$ obscured IR AGN candidates over $$\approx 10\,000 \rm \: deg^2$$ down to a magnitude limit of $$r_{\rm AB}=22.8 \, \rm mag$$ and will be $$\approx 100 \times$$ 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 $$r-W2\ge 5.9 \rm \: mag$$ cut; we demonstrate that this selection will mostly identify sources obscured by $$N_{\rm H}>10^{22} \rm \: cm^{-2}$$. The survey complements the 4MOST X-ray survey, which will follow up $$\sim 1\,\rm M$$  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) $$\approx 80-87{{\ \rm per\ cent}}$$ of the WISE-selected targets are AGN down to $$r_{\rm AB}=22.1-22.8 \: \rm mag$$ of which $$\approx 70{{\ \rm per\ cent}}$$ are obscured by $$N_{\rm H}>10^{22} \: \rm cm^{-2}$$, and (2) $$\approx 80{{\ \rm per\ cent}}$$ 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 ($$\approx 55{{\ \rm per\ cent}}$$ of the sample is expected to have $$L_{\rm AGN,IR}>10^{45} \rm \: erg \: s^{-1}$$) residing in massive galaxies ($$M_{\star }\approx 10^{10}-10^{12} \rm \: M_{\odot }$$) at $$z\approx 0.5-3.5$$ with $$\approx 33{{\ \rm per\ cent}}$$ expected to be hosted by starburst galaxies.

• Sponsoring Organization: USDOE
• OSTI ID: 2563511
• Journal Information: Monthly Notices of the Royal Astronomical Society, Journal Name: Monthly Notices of the Royal Astronomical Society Journal Issue: 3 Vol. 539; ISSN 0035-8711
• Publisher: Oxford University PressCopyright Statement
• Country of Publication: United Kingdom
• Language: English