@article{10902/38356,
year = {2025},
month = {5},
url = {https://hdl.handle.net/10902/38356},
abstract = {X-rays provide a robust method in identifying active galactic nuclei (AGN). However, in the high-redshift Universe (z >/ 3.0), their space density is relatively low and due to the small areas covered by X-ray surveys, the selected AGN are poorly sampled. Deep optical and infrared (IR) data are essential for locating counterparts and determining redshifts. In this work, we leverage the XMM-Newton 4XMM-DR11 serendipitous catalogue (1240 deg2), alongside the extensive optical Dark Energy Survey (DES, 5000 deg2) and the near- IR VISTA Hemisphere Survey (VHS) to select one of the largest high-redshift X-ray AGN samples to date. Our analysis is focussed on the overlapping area of these surveys, covering about 185 deg2. In addition, we aspire to compare the properties of the X-ray AGN with optically selected QSOs. For sources without spectroscopic data (⁓80%), we estimated the photometric redshifts using both spectral energy distribution (SED) fitting and machine-learning algorithms. Among the ⁓65 000 X-ray sources in the 4XMM-DES-VHS area, we ended up with 833 z >/ 3.5 AGN (11% having spec-z information) with a high level of reliability, along with a fraction of outliers of eta </ 10%. The sample completeness is ⁓90%, driven by the depth of DES data. Only ⁓10% of the X-ray selected AGN are also optical QSOs and vice versa. Our findings indicate an observed absorbed fraction (logNH [cm-2] >/ 23) of 20-40% for the X-ray AGN, significantly higher than that of optical QSOs. X-ray AGN exhibit fainter observed optical magnitudes and brighter mid-IR magnitudes than optical QSOs. Their median rest-frame SED shapes differ notably with optical QSOs being dominated by AGN emission in the UV-optical wavelengths. While the median SEDs of X-ray AGN suggest extinction in the UV-optical range, individual sources exhibit a wide range of spectral shapes, indicating significant diversity within the population. This analysis supports the notion that X-ray-selected and optically selected AGN represent distinct and complementary populations.},
organization = {The authors are grateful to the anonymous referee for a careful reading and helpful feedback. E.P., A.R., I.G., A.A. and S.M. acknowledge financial support by the European Union’s Horizon 2020 programme “XMM2ATHENA” under grant agreement No 101004168. The research leading to these results has received funding (E.P. and I.G.) from the European Union’s Horizon 2020 Programme under the AHEAD2020 project (grant agreement n.871158). N.W. acknowledges support from the Centre national d’études spatiales (CNES) for this work. F.C. acknowledges funding from grant PID2021-122955OB-C41 funded by MCIN/AEI/10.13039/501100011033 and by ERDF
A way of making Europe. This research made use of Astropy, a communitydeveloped core Python package for Astronomy (http://www.astropy.org, Astropy Collaboration 2018). This publication made use of TOPCAT
(Taylor 2005) for table manipulations. The plots in this publication were produced using Matplotlib, a Python library for publication quality graphics (Hunter 2007). Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA member states and NASA.},
publisher = {EDP Sciences},
publisher = {Astronomy and Astrophysics, 2025, 697, A123},
title = {Identification of high-redshift X-ray active galactic nuclei in the 4XMM-DR11 serendipitous catalogue using DES data: comparative analysis with optically selected QSOs},
author = {Pouliasis, E. and Ruiz Camuñas, Ángel and Georgantopoulos, I. and Akylas, A. and Webb, N.A. and Carrera Troyano, Francisco Jesús and Mateos Ibáñez, Silvia and Nebot, A. and Watson, M.G. and Pineau, F.X. and Motch, C.},
}