Unveiling faint X-ray AGN populations in the NewAthena era: Insights from cosmological simulations

Nuno Covas, Israel Matute, Stergios Amarantidis, José Afonso, Giorgio Lanzuisi, Andrea Comastri, Stefano Marchesi, Ciro Pappalardo, Rodrigo Carvajal, Polychronis Papaderos

First listed 2025-04-14 | Last updated 2025-06-13

Abstract

Recent observations expanded our understanding of galaxy formation and evolution, yet key challenges persist in the X-ray regime, crucial for studying Active Galactic Nuclei (AGN). These limitations drive the development of next-generation observatories such as ESA's NewAthena. Now in phase B (preliminary design), the mission requires extensive testing to ensure compliance with its scientific goals, particularly given the uncertainties surrounding high redshift AGN. This work leverages the IllustrisTNG cosmological simulation to build an X-ray AGN mock catalogue and assess the performance of NewAthena's WFI. We created a Super Massive Black Hole (SMBH) light cone, spanning 10 deg2, with corrections to account for the limited resolution of the simulation and X-ray properties derived in post-processing. The resulting catalogue reveals a 5* overabundance of faint AGN compared to current X-ray constraints, an inconsistency potentially resolved by invoking a higher Compton-thick (CTK) fraction and intrinsic X-ray weakness, as suggested by recent JWST findings. An end-to-end survey simulation using SIXTE predicts 250000 AGN detections, including 20,000 at z > 3 and 35 in the Epoch of Reionization (z > 6); notably, only AGN with LX > 43.5 erg/s are detectable at z > 6. The analysis also forecasts a significant population of detectable CTK AGN, even beyond z > 4. These findings suggest X-ray observations will, for the first time, probe a significant AGN population in the EoR, offering new insights into SMBH growth. They also provide key input for refining NewAthena's mission design and optimizing its survey strategy.

Short digest

From a TNG300-based 10 deg^2 SMBH light cone with post-processed X-ray properties, the authors run end-to-end NewAthena/WFI simulations via SIXTE to stress-test faint and high‑z AGN performance. The mock predicts a 5× excess of faint AGN versus current X‑ray constraints, plausibly reconciled by a higher Compton‑thick fraction and intrinsic X‑ray weakness hinted by recent JWST work. The simulated survey yields ~250,000 AGN (≈20,000 at z>3) and ~35 at z>6, with only log L_X ≳ 43.5 detectable in the EoR and a significant CTK component out to z>4. These results argue NewAthena can directly probe early SMBH growth while informing instrument trades and survey tiling.

Key figures to inspect

• LogN–logS (or number counts) from the mock versus current X-ray constraints to visualize the reported 5× overabundance at the faint end and where discrepancies emerge.
• Redshift–luminosity plane with NewAthena/WFI detection limits, highlighting the z>6 locus showing the log L_X ≳ 43.5 threshold and the predicted ~35 EoR detections.
• Obscuration (N_H) distribution versus redshift from the catalogue, isolating the CTK fraction evolution and the subset remaining detectable beyond z>4.
• Survey design and yield figure from the SIXTE runs (area–depth tiling), showing how total detections (~2.5×10^5) and z>3 counts (~2×10^4) change with exposure strategy.
• Sky-coverage/sensitivity curve for the WFI setup used, clarifying the impact of the broader PSF and effective area changes in NewAthena on completeness at the faint end.