[Paper Review] 4MOST Consortium Survey 6: Active Galactic Nuclei

A. Merloni, D. M. Alexander+18 more|arXiv (Cornell University)|Jan 1, 2019

Astrophysical Phenomena and Observations1 references23 citations

TL;DR

This paper proposes the 4MOST AGN Survey, a large-scale spectroscopic survey using the 4MOST multi-object spectrograph to obtain redshifts and physical properties for up to one million active galactic nuclei (AGN) over 10,000 square degrees. By combining X-ray (eROSITA) and mid-infrared (WISE) selections, the survey achieves high completeness (>80–90%) across redshifts z ≲ 6, enabling unprecedented studies of SMBH evolution, AGN-host galaxy connections, and cosmological clustering.

ABSTRACT

X-ray and mid-infrared emission are signposts of the accretion of matter onto the supermassive black holes that reside at the centres of most galaxies. As a major step towards understanding accreting supermassive black holes and their role in the evolution of galaxies, we will use the 4MOST multi-object spectrograph to provide a highly complete census of active galactic nuclei over a large fraction of the extragalactic sky observed in X-rays by eROSITA that is visible to 4MOST. We will systematically follow up all eROSITA point-like extragalactic X-ray sources (mostly active galactic nuclei), and complement them with a heavily obscured active galactic nuclei selection approach using mid-infrared data from the Wide-field Infrared Survey Explorer (WISE). The X-ray and mid-infrared flux limits of eROSITA and WISE are well matched to the spectroscopic capabilities of a 4-metre-class telescope, allowing us to reach completeness levels of ~80-90% for all X-ray selected active galactic nuclei with fluxes $f_{0.5-2\ { m keV}} > 10^{-14}$ erg s$^{-1}$ cm$^{-2}$; this is about a factor of 30 deeper than the ROSAT all-sky survey. With these data we will determine the physical properties (redshift, luminosity, line emission strength, masses, etc.) of up to one million supermassive black holes, constrain their cosmic evolution and clustering properties, and explore the connection between active galactic nuclei and large-scale structure over redshifts $0 \le z \le 6$.

Motivation & Objective

• To achieve a highly complete spectroscopic census of active galactic nuclei (AGN) across a large fraction of the extragalactic sky, overcoming limitations of current AGN samples.
• To resolve key uncertainties in the co-evolution of supermassive black holes (SMBHs) and their host galaxies by measuring AGN properties across redshift, luminosity, obscuration, and environment.
• To provide a benchmark dataset for testing models of SMBH formation and growth, and their role in feedback and large-scale structure formation.
• To enable high-precision cosmological measurements using X-ray and UV luminosity ratios of quasars as standard candles out to z ≈ 6.
• To study AGN clustering and triggering mechanisms by mapping AGN spatial distribution across cosmic time and large-scale structures.

Proposed method

• Combine X-ray point sources from the eROSITA all-sky survey (f0.5–2 keV > 10⁻¹⁴ erg s⁻¹ cm⁻²) with mid-IR AGN candidates from WISE (W1–W3 colour selection) to maximize completeness across obscuration levels.
• Use the 4MOST spectrograph on a 4-metre telescope to obtain redshifts and physical properties (luminosity, emission line strength, black hole mass) for ~1 million AGN targets.
• Apply uniform, well-characterized selection functions from eROSITA and WISE to ensure statistical robustness and minimal bias in AGN sample construction.
• Implement a three-armed spectrograph (blue, green, red) with S/N thresholds of 2.1, 2.4, and 2.8 per pixel to ensure >90% spectroscopic success rate for targets down to r ≈ 23.4.
• Conduct a deep survey of the South Ecliptic Pole (300 sq deg) with higher cadence and deeper depth to enable time-domain studies of variable AGN (e.g., 'changing look' AGN).
• Integrate multi-wavelength data from HSC, LSST, Euclid, and radio surveys to classify AGN by morphology, environment, and jet activity, enhancing clustering and feedback studies.

Experimental results

Research questions

• RQ1 How do the properties of supermassive black holes (SMBHs) evolve with redshift, luminosity, and host galaxy mass across cosmic time?
• RQ2 What is the role of heavily obscured, dust-obscured quasars in AGN-driven feedback, and how do they contribute to the overall AGN luminosity function?
• RQ3 How do AGN clustering properties relate to dark matter halo mass and large-scale structure, and what do they reveal about AGN triggering mechanisms?
• RQ4 To what extent do X-ray and mid-IR AGN selection methods complement each other in achieving high spectroscopic completeness, especially at high redshift and for obscured sources?
• RQ5 Can the X-ray to UV luminosity ratio of quasars be used as a robust standard candle to constrain cosmological parameters at z > 3?

Key findings

• The 4MOST AGN survey will achieve a spectroscopic completeness of at least 90% for X-ray-selected AGN (f0.5–2 keV > 10⁻¹⁴ erg s⁻¹ cm⁻²) and 80% for deeper X-ray sources, ensuring minimal statistical uncertainty.
• The survey will target up to one million AGN over 10,000 square degrees, with a sky density of ~100–120 sources per square degree, and up to 200 per square degree in the deep South Ecliptic Pole region.
• eROSITA and WISE selection together will yield a highly complete AGN sample, including luminous, dust-obscured quasars missed by X-ray surveys due to their soft energy response.
• The survey will detect tens of thousands of quasars at z > 3 and hundreds at z > 6, enabling high-precision cosmological measurements using quasar standard candles.
• The deep survey region will enable time-domain studies of variable AGN, including 'changing look' AGN and eclipsing systems, due to repeated X-ray and optical spectroscopic observations.
• The combination of 4MOST with eROSITA and WISE will improve the number of high-luminosity AGN detections by many orders of magnitude compared to previous surveys, particularly above the break in the XLF at LX ≈ 10⁴⁴ erg s⁻¹.