2023A&A...669A..87D

Context. Compact obscured nuclei (CONs) represent an extreme phase of galaxy evolution where rapid supermassive black hole growth and/or compact star-forming activity is completely obscured by gas and dust.
Aims. We investigate the properties of CONs in the mid-infrared and explore techniques aimed at identifying these objects, such as through the equivalent width (EW) ratios of their polycyclic aromatic hydrocarbon (PAH) features.
Methods. We modelled Spitzer spectra by decomposing the continua into nuclear and star-forming components, from which we then measured the nuclear optical depth, τ_N, of the 9.8 μm silicate absorption feature. We also used Spitzer spectral maps to investigate how PAH EW ratios vary with aperture size for objects that host CONs.
Results. We find that the nuclear optical depth, τ_N, strongly correlates with the HCN-vib emission line in the millimetre for CONs, with a Pearson correlation coefficient of 0.91. We find the PAH EW ratio technique to be effective at selecting CONs and robust against highly inclined galaxies, where strong dust lanes may mimic a CON-like spectrum by producing a high τ_N. Our analysis of the Spitzer spectral maps shows that the efficacy of the PAH EW ratios in isolating CONs is reduced when there is a strong star-forming component from the host galaxy. In addition, we find that the use of the inferred nuclear optical depth is a reliable method for identifying CONs in $ 36^{+8}_{-7}\% $ of ultra-luminous infrared galaxies and $ 17^{+3}_{-3}\% $ of luminous infrared galaxies, consistent with previous work.
Conclusions. We confirm mid-infrared spectra to be a powerful diagnostic of CONs. The increased sensitivity of JWST will allow the identification of CONs at cosmic noon, revealing this extreme but hidden phase of galaxy evolution.