On unveiling buried nuclei with JWST: A technique for hunting the most obscured galaxy nuclei from local to high redshift

¹ Centro de Astrobiología (CAB), CSIC-INTA, Camino Bajo del Castillo s/n, E-28692 Villanueva de la Cañada, Madrid, Spain
² Department of Physics, University of Oxford, Keble Road, Oxford, OX1 3RH UK
³ School of Sciences, European University Cyprus, Diogenes street, Engomi, 1516 Nicosia, Cyprus
⁴ Instituto de Física Fundamental, CSIC, Calle Serrano 123, E-28006 Madrid, Spain
⁵ Universidad de Alcalá, Departamento de Física y Matemáticas, Campus Universitario, E-28871 Alcalá de Henares, Madrid, Spain
⁶ Department of Space, Earth and Environment, Osala Space Observatory, Chalmers University of Technology, SE-439 92 Onsala, Sweden
⁷ School of Physics, H.H. Wills Physics Laboratory, Tyndall Avenue, University of Bristol, Bristol, BS8 1TL UK
⁸ Department of Physics, Oliver Lodge Building, University of Liverpool, Oxford Street, Liverpool, L69 7ZE UK
⁹ Chinese Academy of Sciences South America Center for Astronomy, National Astronomical Observatories, CAS, Beijing, 100101 People’s Republic of China
¹⁰ Cosmic Dawn Center (DAWN), Copenhagen, Denmark

^⋆ Corresponding author; igbernete@gmail.com

Received: 1 October 2024
Accepted: 17 February 2025

Abstract

We analyze JWST NIRSpec+MIRI/MRS observations of the infrared (IR) polycyclic aromatic hydrocarbon (PAH) features in the central regions (∼0.26″ at 6 μm; ∼50–440 pc depending on the source) of local luminous IR galaxies. In this work, we examine the effect of nuclear obscuration on the PAH features of deeply obscured nuclei, predominantly found in local luminous IR galaxies, and we compare these nuclei with “normal” star-forming regions. We extend previous work to include shorter wavelength PAH ratios now available with the NIRSpec+MIRI/MRS spectral range. We introduce a new diagnostic diagram for selecting deeply obscured nuclei based on the 3.3 and 6.2 μm PAH features and/or mid-IR continuum ratios at ∼3 and 5 μm. We find that the PAH equivalent width ratio of the brightest PAH features at shorter wavelengths (at 3.3 and 6.2 μm) is impacted by nuclear obscuration. Although the sample of luminous IR galaxies used in this analysis is relatively small, we find that sources exhibiting a high silicate absorption feature cluster tightly in a specific region of the diagram, whereas star-forming regions experiencing lower extinction levels occupy a different area in the diagram. This demonstrates the potential of this technique to identify buried nuclei. To leverage the excellent sensitivity of the MIRI imager on board JWST, we extend our method of identifying deeply obscured nuclei at higher redshifts using a selection of MIRI filters. Specifically, the combination of various MIRI JWST filters enables the identification of buried sources beyond the local Universe and up to z ∼ 3, where other commonly used obscuration tracers such as the 9.7 μm silicate band, are out of the spectral range of MRS. Our results pave the way for identifying distant deeply obscured nuclei with JWST.