AGN obscuration in optical and X-rays: Host properties and the interplay of nuclear and galactic gas and dust in a combined SDSS–XMM sample

¹ Instituto de Física de Cantabria (CSIC–Universidad de Cantabria), Avenida de los Castros, 39005 Santander, Spain
² European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
³ Department of Astronomy, University of Geneva, ch. d’Ecogia 16, 1290 Versoix, Switzerland
⁴ INAF–Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monteporzio Catone, Italy
⁵ Department of Physics, Gustaf Hällströmin katu 2, 00014 University of Helsinki, Finland
⁶ IRAP, Université de Toulouse, CNRS, UPS, CNES, 9 Avenue du Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 5 December 2025
Accepted: 11 March 2026

Abstract

We investigate the link between optical obscuration and X-ray absorption in active galactic nuclei (AGNs) by combining X-ray spectroscopy from 4XMM-DR11 with SDSS DR16Q spectroscopy. Bayesian X-ray spectral fits were obtained within the XMM2Athena project, and host-galaxy properties were derived via CIGALE spectral energy distribution (SED) fitting. Our final sample comprises 241 X-ray AGNs at z < 1.9. For 172 sources (∼70%), the optical broad-line (BL) or narrow-line (NL) classification agrees with their X-ray obscuration based on N_H, but two mismatched populations emerge. Eleven BL AGNs show signs of X-ray absorption (BLAbs) and elevated gas-to-dust ratios compared to BL AGN, consistent with dust-free or host-scale absorbers. Conversely, 58 NL AGN appear unobscured in X-rays (NLUnabs) and low gas-to-dust ratios. Nearly half are assigned type 1 properties by SED fitting, suggesting diluted or intrinsically weak BL regions, host contamination, or variability. Optical line diagnostics support this picture: NL AGNs show higher Balmer decrements than NLUnabs, indicating stronger extinction and different ionization conditions. Host diagnostics further reinforce the contrasts: at z < 0.8, NLUnabs show star-formation rates and accretion efficiencies that are comparable to BL AGNs, whereas NL AGNs reside in more quiescent hosts with lower star formation and less efficient black-hole growth. BLAbs match BL AGNs in terms of host and accretion properties, with their peculiarity lying in excess X-ray absorption. These findings demonstrate that obscuration arises not only from orientation but also from multi-scale distributions of gas and dust. Identifying such mismatched populations will be crucial for interpreting AGN demographics in ongoing and upcoming surveys such as Euclid and VRO/LSST.