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.
Andonie, Carolina, et al. "An obscured quasar census with the 4MOST IR AGN survey: design, predicted properties, and scientific goals." Monthly Notices of the Royal Astronomical Society, vol. 539, no. 3, Apr. 2025. https://doi.org/10.1093/mnras/staf624
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 (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). https://doi.org/10.1093/mnras/staf624
Andonie, Carolina, Alexander, David M., Greenwell, Claire, et al., "An obscured quasar census with the 4MOST IR AGN survey: design, predicted properties, and scientific goals," Monthly Notices of the Royal Astronomical Society 539, no. 3 (2025), https://doi.org/10.1093/mnras/staf624
@article{osti_2563511,
author = {Andonie, Carolina and Alexander, David M. and Greenwell, Claire and Fotopoulou, Sotiria and Hickox, Ryan and Rosario, David J. and Villforth, Carolin and Buchner, Johannes and Krogager, Jens-Kristian and Laloux, Brivael and others},
title = {An obscured quasar census with the 4MOST IR AGN survey: design, predicted properties, and scientific goals},
annote = {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.},
doi = {10.1093/mnras/staf624},
url = {https://www.osti.gov/biblio/2563511},
journal = {Monthly Notices of the Royal Astronomical Society},
issn = {ISSN 0035-8711},
number = {3},
volume = {539},
place = {United Kingdom},
publisher = {Oxford University Press},
year = {2025},
month = {04}}
Monthly Notices of the Royal Astronomical Society, Journal Name: Monthly Notices of the Royal Astronomical Society Journal Issue: 3 Vol. 539; ISSN 0035-8711