2023A&A...679L...2B

We present a study of the mid-IR (MIR) emission of quiescent galaxies (QGs) beyond the local universe. Using deep JWST imaging in the SMACS-0723 cluster field we identify a mass-limited (M_* > 10⁹ M_⊙) sample of intermediate redshift QGs (0.2 < z < 0.7) and perform modelling of their rest-frame UV to MIR photometry. We find that QGs exhibit a range of MIR spectra that are composed of a stellar continuum and a dust component that is 1-2 orders of magnitude fainter than that of star-forming galaxies. The observed scatter in the MIR spectra, especially at λ_rest > 5 μm, can be attributed to different dust continuum levels and/or the presence of polycyclic aromatic hydrocarbon (PAH) features. The latter would indicate enhanced 11.3 and 12.7 μm PAH strengths with respect to those at 6.2 and 7.7 μm, consistent with the observed spectra of local ellipticals and indicative of soft radiation fields. Finally, we augment the average UV to MIR spectrum of the population with cold dust and gas emission in the far-IR/millimetre, and construct a panchromatic UV to radio SED that can serve as a template for the future exploration of the interstellar medium of z > 0 QGs with ALMA and JWST.