JWST MIRI reveals the diversity of nuclear mid-infrared spectra of nearby type 2 quasars

¹ Instituto de Astrofísica de Canarias, Calle Vía Láctea, s/n, E-38205, La Laguna, Tenerife, Spain
² Departamento de Astrofísica, Universidad de La Laguna, E-38206, La Laguna, Tenerife, Spain
³ Centro de Astrobiología (CAB), CSIC-INTA, Camino Bajo del Castillo s/n, E-28692, Villanueva de la Cañada, Madrid, Spain
⁴ Instituto de Física Fundamental, CSIC, Calle Serrano 123, 28006 Madrid, Spain
⁵ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
⁶ Cavendish Laboratory – Astrophysics Group, University of Cambridge, 19 JJ Thomson Avenue, Cambridge, CB3 0HE, UK
⁷ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
⁸ Observatorio Astronómico Nacional (OAN-IGN)-Observatorio de Madrid, Alfonso XII, 3, 28014 Madrid, Spain
⁹ Instituto de Radioastronomía and Astrofísica (IRyA-UNAM), 3-72 (Xangari), 8701 Morelia, Mexico
¹⁰ Department of Physics, University of Oxford, Oxford OX1 3RH, UK
¹¹ School of Sciences, European University Cyprus, Diogenes street, Engomi, 1516 Nicosia, Cyprus
¹² Department of Physics & Astronomy, University of Sheffield, S3 7RH, Sheffield, UK
¹³ Telespazio UK for the European Space Agency, ESAC, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Spain
¹⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

^⋆ Corresponding author: cra@iac.es

Received: 20 December 2024
Accepted: 30 March 2025

Abstract

Type 2 quasars (QSO2s) are active galactic nuclei (AGN) seen through a significant amount of dust and gas that obscures the central supermassive black hole and the broad-line region. Here, we present new mid-infrared spectra of the central kiloparsec of five optically selected QSO2s at redshift z ∼ 0.1 obtained with the Medium Resolution Spectrometer module of the Mid-Infrared Instrument (MIRI) aboard the James Webb Space Telescope (JWST). These QSO2s belong to the Quasar Feedback (QSOFEED) sample, and they have bolometric luminosities of log L_bol = 45.5 to 46.0 erg s⁻¹, global star formation rates (SFRs) that place them above the main sequence, and practically identical optical spectra in terms of spectral shape and [OIII] luminosity, but their nuclear mid-infrared spectra exhibit an unexpected diversity in both continua and features. They show 9.7 μm silicate features going from emission (strength of S_9.7 = 0.5) to relatively strong absorption (S_9.7 = –1.0), and 18 and 23 μm silicates that are either in emission or flat (S₁₈ = [0.2,0.0] and S₂₃ = [0.1,0.0]). In addition, two of the QSO2s show absorption bands of CO, H₂O, and aliphatic grains, indicating different levels of nuclear obscuration across the sample. Their [NeV]/[NeII] ratios range from 0.1 to 2.1 and [NeIII]/[NeII] from 1.0 to 3.5, indicating different coronal line and ionizing continuum strengths. They have warm molecular gas masses of 1–4 × 10⁷ M_⊙ and warm-to-cold gas mass ratios of 1–2%, with molecular gas excitation likely due to jet-induced shocks in the case of the Teacup (J1430+1339) and to UV heating and/or turbulence in J1509+0434. Finally, they show polycyclic aromatic hydrocarbon (PAH) emission features with equivalent widths ranging from less than 0.002 to 0.075 μm, from which we measure a larger contribution from neutral molecules (PAH 11.3/6.2 = 1.3–3.4) and SFRs ≤ 3–7 M_⊙ yr⁻¹. This unprecedented dataset allowed us to start exploring the role of various AGN and galaxy properties, including ionizing continuum, obscuration, electron density, and jet-interstellar medium interactions, in some of the spectral differences listed above. Larger samples observed with JWST/MIRI are now required to fully understand the diversity of QSO2s’ nuclear mid-infrared spectra.