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

Abstract

We present the 4MOST (4-metre Multi-Object Spectroscopic Telescope) infrared (IR) AGN surv e y, the first large-scale optical spectroscopic surv e y characterizing mid-infrared (MIR) selected obscured active galactic nuclei (AGNs). The surv e y targets ≈212 000 obscured IR AGN candidates o v er ≈10 000 de g 2 down to a magnitude limit of r AB = 22 . 8 mag and will be ≈100 ×larger than an y e xisting 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 −W 2 ≥5 . 9 mag cut; we demonstrate that this selection will mostly identify sources obscured by N H > 10 22 cm −2 . The surv e y complements the 4MOST X-ray surv e y, which will follow up ∼1 M eROSITA (extended ROentgen Surv e y 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-observ ed deep-sk y fields. We find that: (1) ≈80 −87 per cent of the WISE -selected targets are AGN down to r AB = 22 . 1 −22 . 8 mag of which ≈70 per cent are obscured by N H > 10 22 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 surv e y will primarily identify obscured AGN and quasars ( ≈55 per cent of the sample is expected to have L AGN , IR > 10 45 erg s −1 ) residing in massive galaxies ( M _ ≈10 10 −10 12 M _) at z ≈0 . 5 −3 . 5 with ≈33 per cent expected to be hosted by starburst galaxies.