http://arxiv.org/abs/2209.05928
Molecular line ratios, such as HCN(1-0)/HCO$^+$(1-0) and HCN(4-3)/CS(7-6) are routinely used to identify 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 a as a unique unambiguous diagnostic. We use 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 determination will allow a more rigorous use of such ratios. We first empirically examine the aforementioned ratios at different angular resolutions to quantify correlations. We then use LTE and non-LTE analyses coupled with Markov Chain Monte Carlo (MCMC) sampling in order to determine the origin of the underlying differences in ratios. We propose that at high spatial resolution (< 50 pc) the HCN(4-3)/CS(2-1) is a reliable tracer of AGN activity. Finally we 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 what chemical processes are at play when drawing conclusions from radiative transfer calculations. Upon analysis at varying spatial scales previous proposed as well as a new molecular line ratio have been shown to have varying levels of consistency. We have also determined from 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.
J. Butterworth, J. Holdship, S. Viti, et. al.
Wed, 14 Sep 22
21/90
Comments: 16 pages, 13 figures, accepted for publication
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