Are Compact Obscured Nuclei the Same As Compton Thick AGN?

Compton-thick active galactic nuclei (CT AGN) are growing supermassive black holes (SMBHs) that are obscured by a significant amount of gas and dust that blocks optical and ultraviolet light, as well as low-energy X-rays. Compact obscured nuclei (CONs) are energetic regions at the centers of some galaxies that are also hidden behind large amounts of gas and dust; however, in CONs we do not yet have direct evidence for SMBH growth (though it is likely to be occurring). CT AGN are generally diagnosed by observing X-rays, particularly at higher energies. CONs are identified by having bright emission from HCN molecules that are in vibrationally excited states (“HCN-vib”). These sound like similar phenomena, but are they same? If CONs are powered by SMBH growth then they would count as CT AGN, but what about the reverse?

In “Searching for Compact Obscured Nuclei in Compton Thick AGN” (Johnstone+) we assemble new ALMA data and literature reports of HCN-vib observations of CT AGN to see if these CT AGN also have luminous HCN-vib emission. In short we do not find evidence for the bright HCN-vib emission in the CT AGN population. This suggests that, even if CONs are determined to be CT AGN, CT AGN are not generally the same population as CONs. In the paper we also explore a simple evolutionary model where CONs drive outflows that reduce the obscuring gas columns enough for the systems to become visible as CT AGN in the X-rays, but the CT AGN of our study did not show the well-developed outflows that we expect from such a scenario.

The paper has been accepted for publication in the Astrophysical Journal.