The broad and narrow line regions (BLR and NLR) of an active galactic nucleus (AGN) are commonly described as regions of gas emitting atomic spectral lines whose width is broadened - via the Doppler effect - according to the speed with which the material is orbiting the central black hole. The gas clouds composing the BLR are orbiting with the velocities compatible with $\sim10^4 {\rm km s}^{-1}$ and are indeed the closest to the central Black Hole and its accretion disc (sub-parsec distances). The accretion disk is incidentally hotter than any star (at least a million K). The NLR is even at larger distances (order of magnitude of one to ten pc).

Now that you have the context, in a thesis I am reading these regions were described as "regions of ionised plasma" and I was wandering if this definition is correct. My first intuition was that it is not. Matter in a plasma state implies that the electrons have been stripped off the atoms. Hence, for example, any of the emission lines of Hydrogen, which are among the most commonly observed, would not be visible (the electrons are not bounded to the atoms)! But then - thinking to heavier elements - the reasoning was not so straightforward. Even ionising the atoms, electronic transitions could still occur and hence spectral lines would still be observable. Indeed one of the most-commonly observed line is the [OIII] line, from doubly-ionised Oxygen.

It is clear that the processes of ionisation and excitation are occurring at the same time (see e.g. Balmer spectroscopic lines from plasma?) and that ionisation must certainly occur within one parsec of a UV-emitting accretion disk. But I was wandering what degree of ionisation is sufficient to classify these gases (or any gas in general) as a plasma. In one of the comments of the post I am referencing, it is argued that "The term 'plasma' could be used as soon as there is any ionisation.", but I disagree with this because I think a significant part of the medium has to be ionised to display collective behaviours (see e.g. Chen's definition of plasma in "Introduction to Plasma Physics"). Or it is actually that these gases are almost fully ionised (they are actual plasma) and the minimum fraction of the atoms (e.g. H) still neutral is that responsible for the observed lines?



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