A detailed look at the most obscured galactic nuclei in the mid-infrared

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 36₋₇⁺⁸% 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.