[ncmncm]

It matters enormously how ionized the gas cloud is. If it is neutral, the ion jet blasts right through, hardly noticing it: it is all, after all, "vacuum". Fully ionized, every particle in the jet interacts with every particle in the cloud.

Most such clouds are partially ionized, often to different degrees in different parts. Even a 0.01% ionized cloud behaves differently to a gas.

The proper name for such a cloud is "plasma". Many astronomers are allergic to this word, probably because the maths for plasma fluid dynamics is all just way too hard, and they steer clear, preferring literally any alternative (heating! gravitation! shocks!) over engaging.

Ordinary fluid dynamics is tricky enough to be often intractable. Plasma fluid dynamics is freaky because usually the positive ions are at least 1836 times as massive as the negative ions. Dust can be ionized, too, either direction. Then the ratio might be 6, 7, 10 orders of magnitude.

Vacuum is usually treated, in astrophysics, as infinitely conductive, and the charge carriers as massless, making static electric fields impossible, and freezing magnetic fields in place. But moving ion clouds (including jets) carry magnetic fields, and thus generate varying local electric fields. And, of course, the carriers are not really massless at all.

[antognini]

> The proper name for such a cloud is "plasma". Many astronomers are allergic to this word, probably because the maths for plasma fluid dynamics is all just way too hard

I'm an astronomer (or at least used to be) and I don't know any astronomers who are allergic to the word "plasma". MHD simulations are hard, but when it's necessary to do them, astronomers do them. I'm not exactly sure what the point of this comment is. Are you saying that astronomers are not handling the physics of jet-ISM interactions properly?

[ncmncm]

It is very rare to encounter the word "plasma", at all, in any astro article, and vanishingly rare to encounter any mention of any plasma fluid dynamics phenomenon, as such, except in solar physics. Usually we read "hot gas". There has been some progress: x-radiation is not militantly insisted to be blackbody radiation from stuff at absurdly high temperatures, anymore.

MHD is the trivial, sanitized subset of plasma fluid dynamics that rarely occurs in nature, and is hard enough to maintain even artificially. But MHD maths are easier.

I am saying that astronomers seem, from any remove, to try hard to avoid discussing things that seem to require treatment with PFD. Polar jets are embarrassing because they cannot be ignored. So, how they work is just never mentioned in the popular press.