Infrared spectra of the central region of five quasars observed with JWST show clear differences, especially in the silicate band, which reveals the presence of hotter or cooler dust around the black hole. Credit: ESA/Hubble & NASA and C. Ramos Almeida. Composition: G. Pérez Díaz (SMM, IAC).
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 Lbol = 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 S9.7 = 0.5) to relatively strong absorption (S9.7 = –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 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.
