The Close AGN Reference Survey (CARS): a comparison between sub-mm and optical AGN diagnostic diagrams

Elford, Jacob S., Davis, Timothy A. ORCID: https://orcid.org/0000-0003-4932-9379, Ruffa, Ilaria, Baum, Stefi A., Combes, Francoise, Gaspari, Massimo, McElroy, Rebecca, O'Dea, Christopher P., Omoruyi, Osase, Singha, Mainak, Tremblay, Grant R. and Winkel, Nico 2025. The Close AGN Reference Survey (CARS): a comparison between sub-mm and optical AGN diagnostic diagrams. Monthly Notices of the Royal Astronomical Society 10.1093/mnras/staf1096

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Abstract

The LIR − LHCN relation suggests that there is a tight connection between dense gas and star formation. We use data from the Close AGN Reference Survey (CARS) to investigate the dense gas - star formation relation in AGN hosting galaxies, and the use of dense gas as an active galactic nuclei (AGN) diagnostic. Our sample contains five Type-1 (unobscured) AGN that were observed with the Atacama Large Millimeter/submillimeter Array (ALMA) with the aim to detect HCN(4-3), HCO+(4-3) and CS(7-6). We detect the dense gas emission required for this analysis in 3 of the 5 targets. We find that despite the potential impact from the AGN on the line fluxes of these sources, they still follow the LIR − LHCN relation. We then go on to test claims that the HCN/HCO+ and HCN/CS line ratios can be used as a tool to classify AGN in the sub-mm HCN diagram. We produce the classic ionised emission-line ratio diagnostics (the so-called BPT diagrams), using available CARS data from the Multi Unit Spectroscopic Explorer (MUSE). We then compare the BPT classification with the sub-mm classification made using the dense gas tracers. Where it was possible to complete the analysis we find general agreement between optical and sub-mm classified gas excitation mechanisms. This suggests that AGN can contribute to the excitation of both the low density gas in the warm ionised medium and the high density gas in molecular clouds simultaneously, perhaps through X-ray, cosmic ray or shock heating mechanisms.

Item Type:	Article
Date Type:	Published Online
Status:	In Press
Schools:	Schools > Physics and Astronomy
Publisher:	Oxford University Press
ISSN:	0035-8711
Date of First Compliant Deposit:	3 July 2025
Date of Acceptance:	1 July 2025
Last Modified:	11 Jul 2025 16:00
URI:	https://orca.cardiff.ac.uk/id/eprint/179511

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