A Detailed Look at the Most Obscured Galactic Nuclei in the Mid-Infrared

Abstract

Context. Compact Obscured Nuclei (CONs) are 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 model Spitzer spectra by decomposing the continua into nuclear and star-forming components from which we then measure the nuclear optical depth, ${\tau }_N$, of the $9.8\mu$m silicate absorption feature. We also use Spitzer spectral maps to investigate how PAH EW ratios vary with aperture size for objects hosting CONs. Results. We find that the nuclear optical depth, ${\tau }_N$, 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 ratios 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 ${\tau }_N$. Our analysis of the Spitzer spectral maps showed that the efficacy of the PAH EW ratios to isolate 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 to identify CONs in $36_{-7}^{+8}\%$ of ULIRGs and $17_{-3}^{+3}\%$ of LIRGs, consistent with previous work. Conclusions. We confirm mid-IR spectra to be a powerful diagnostic of CONs where the increased sensitivity of JWST will allow identification of CONs at cosmic noon revealing this extreme but hidden phase of galaxy evolution.

Accepted for publication in A&A. 23 pages, 15 figures, 3 tables

References in corpus (16)

• Mid-IR Galaxy Classification Based on Silicate Obscuration and PAH Equivalent Width
• Near-Infrared and Star-forming properties of Local Luminous Infrared Galaxies
• Vibrationally Excited HCN in the Luminous Infrared Galaxy NGC 4418
• CANDELS: Elevated Black Hole Growth in the Progenitors of Compact Quiescent Galaxies at z~2
• A high angular resolution view of the PAH emission in Seyfert galaxies using JWST/MRS data
• PAH features within few hundred parsecs of active galactic nuclei
• CON-quest: Searching for the most obscured galaxy nuclei
• Resolving the AGN and host emission in the mid-infrared using a model-independent spectral decomposition
• The hidden heart of the luminous infrared galaxy IC 860 I. A molecular inflow feeding opaque, extreme nuclear activity
• Polycyclic Aromatic Hydrocarbons in Seyfert and star-forming galaxies
• A new look at local ultraluminous infrared galaxies: the atlas and radiative transfer models of their complex physics
• The silicate absorption profile in the ISM towards the heavily obscured nucleus of NGC 4418
• A proto-pseudobulge in ESO 320-G030 fed by a massive molecular inflow driven by a nuclear bar
• Extinction in the 11.2 micron PAH band and the low L_11.2/L_IR in ULIRGs
• A systematic study of silicate absorption features in heavily obscured AGNs observed by Spitzer/IRS
• The Galaxy Evolution Probe

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• On unveiling Buried Nuclei with JWST: a technique for hunting the most obscured galaxy nuclei from local to high redshift
• Mid-infrared extinction toward the Galactic center
• An Imaging and Spectroscopic Exploration of the Dusty Compact Obscured Nucleus Galaxy Zw~049.057
• MICONIC: JWST/MIRI-MRS reveals heavily reprocessed PAH emission in the circum-nuclear disc of Centaurus A
• A systematic study on the properties of aromatic and aliphatic hydrocarbon dust in active galactic nuclei with AKARI near-infrared spectroscopy
• JWST MIRI color classification of mid-infrared selected galaxies: MIRI color classification toward cosmic noon