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

The 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 active galactic nuclei (AGN) hosting galaxies, and the use of dense gas as an AGN diagnostic. Our sample contains five Type-1 (unobscured) AGN that were observed with the Atacama Large Millimetre/submillimetre Array 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 three of the five targets. We find that despite the potential impact from the AGN on the line fluxes of these sources, they still follow the 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 ionized emission-line ratio diagnostics (the so-called BPT diagrams), using available CARS data from the Multi Unit Spectroscopic Explorer. 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 ionized medium and the high-density gas in molecular clouds simultaneously, perhaps through X-ray, cosmic ray, or shock heating mechanisms.