Understanding if molecular ratios can be used as diagnostics of AGN and starburst activity: The case of NGC 1068

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: butterworth@strw.leidenuniv.nl, viti@strw.leidenuniv.nl, holdship@strw.leidenuniv.nl
² Department of Physics and Astronomy, University College London, Gower St., London WC1E 6BT, UK
³ Observatorio Astronómico Nacional (OAN-IGN)-Observatorio de Madrid, Alfonso XII, 3, 28014 Madrid, Spain

Received: 21 July 2022
Accepted: 9 September 2022

Abstract

Context. Molecular line ratios, such as HCN(1−0)/HCO⁺(1−0) and HCN(4−3)/CS(7−6), are routinely used to identify active galactic nuclei (AGN) activity in galaxies. Such ratios are, however, hard to interpret as they are highly dependent on the physics and energetics of the gas, and hence can seldom be used as a unique, unambiguous diagnostic.

Aims. We used the composite galaxy NGC 1068 as a “laboratory” to investigate whether molecular line ratios between HCN, HCO⁺, and CS are useful tracers of AGN-dominated gas and determine the origin of the differences in such ratios across different types of gas. Such a determination will enable a more rigorous use of such ratios.

Methods. First, we empirically examined the aforementioned ratios at different angular resolutions to quantify correlations. We then used local thermodynamic equilibrium (LTE) and non-LTE analyses coupled with Markov chain Monte Carlo sampling in order to determine the origin of the underlying differences in ratios.

Results. We propose that at high spatial resolution (< 50 pc) the HCN(4−3)/CS(2−1) is a reliable tracer of AGN activity. We also find that the variations in ratios are not a consequence of different densities or temperature but of different fractional abundances, yielding to the important result that it is essential to consider the chemical processes at play when drawing conclusions from radiative transfer calculations.

Conclusions. From analyses at varying spatial scales, we find that previously proposed molecular line ratios, as well as a new one, have varying levels of consistency. We also determine from an investigation of radiative transfer modelling of our data that it is essential to consider the chemistry of the species when reaching conclusions from radiative transfer calculations.