SCIENCE BLOG - Paper: Outflow densities and ionisation mechanisms in the NLRs of the prototypical Seyfert galaxies NGC 1068 and NGC 4151

Luke Holden - 8th June 2023

I'm pleased to announce that the second first-author paper of my PhD has been accepted for publication in MNRAS. Here, we used archival HST/STIS optical spectroscopy to investigate the energetics and ionisation conditions of the AGN-driven outflows in the inner (r<160pc) narrow line regions of the prototypical Seyfert galaxies NGC 1068 and NGC 4151.

The pre-print of the paper is available on arXiv, here.

Our aim here was to robustly determine crucial outflow properties - which have significant effects on derived outflow energetics (and hence the impact on host galaxies) - and compare our results to those of other methods that have been applied to these objects in the past. Principally, we measure high electron densities (log(ne[cm^-3])>3.5), and subsequently calculate modest outflow energetics. This is consistent with the results of previous complex multi-component photoionisation modelling, and reinforces the need for electron density diagnostics that are sensitive to a wide range of densities.

Furthermore, we use multiple diagnostic diagrams to investigate the ionisation of the warm ionised (~10,000K) and high-ionisation (>100eV) gas, which appears to be photoionised in both objects. Despite this, we note that the outflows have similar spatial extents to the radio structure, which would be expected if they are accelerated by the AGN radio jet. We highlight that this demonstrates the need for care when using ionisation mechanisms as proxies for outflow acceleration mechanisms. Moreover, we interpret our results for NGC 1068 in the context of the ionisation structures of the outflowing clouds, which we argue further supports the use of higher critical-density diagnostics when measuring the true impact of AGN-driven outflows.

Finally, we combined our results with those for the Seyfert 2 IC 5063 (presented in my last paper), we show that, despite all three objects being Seyfert galaxies, the ionisation conditions and outflow properties vary significantly.

In the future, I hope to expand on this work by performing similarly detailed studies of the inner regions of a larger sample of nearby Seyfert galaxies. Ideally, this would include multi-wavelength, multi gas-phase observations that quantify the full impact of the narrow line region outflows. In fact, I am grateful to have recently had a VLT/Xshooter proposal accepted to perform such observations of NGC 1068, which will likely form the next stage of this project.