You can really see those jets in visible light

I don’t think the optical features are the jets - they are at the wrong orientation.

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I was thinking that since they fit within the area of the radio emissions, that maybe within there the jets were reacting with something in their path and that’s what was visible in optical light.

Optical features of jets usually come in narrow-line regions (heated gas that emits at specific wavelengths, for example H-alpha at 656 nm or oxygen [O III] at 496/501 nm) and therefore only appear in one filter. For low redshift you would expect highly ionized gas that is seen in the g-band filter (blue in legacy).

3C 321 is an example (see Evans et al. 2008). Blue patches are the regions of the galaxy ionized by the jet.


Hubble image. See also HubbleSite.

Always look for blue or green (I mean really green, not just greenish) clouds and with a 1 in a 1000 chance it will be an optical counterpart of a radio jet (see also Voorwerpje Hunt, I remember we found some possible [O III] objects in radio jet galaxies, sometimes aligned sometimes misaligned).

Jets can also trigger star-formation, see wikipedia article to NGC 541.

Any idea how being impacted by a relativistic jet COOLS intergalactic gas?

I have no idea. Here is the result of my search:

From one of the linked paper:
Fragile et al. 2017

Part of the Introduction:
"In contrast, FRI jets propagate through the ISM/IGM with energies high enough to create compression in the surrounding gas, but low enough to reduce the chance of significant turbulent heating. These jets are observed in positive feedback cases, wherein the effect of the jet serves to enhance star formation, including Centaurus A, 4C 41.17 and Minkowski’s Object "

From section 2 “Jet-Cloud interaction”
“The basic idea of jet-induced star formation (i.e. positive feedback) is that the collision of the jet with the cloud will trigger a series of shocks within the cloud. The immediate effect of these shocks will be to compress and heat the gas. Depending on how the radiative processes scale with density and temperature, the net result can be to dramatically increase the radiative efficiency within the cloud. If the temperature dependence is shallower than the density dependence, the cloud can enter a phase of runaway cooling. This process occurs most quickly in relatively over-dense regions of the original cloud. These over-dense regions then proceed to collapse at an accelerating pace. Provided some of these clumps start sufficiently close to the Jeans limit, this collapse will push them beyond this limit, such that gravitational collapse can take over and the clump will proceed to form stars. The properties of the jet and cloud are key to controlling this process. For positive feedback to be important, the initial cloud must be dense enough for some parts to be reasonably close to the Jeans limit. The temperature must also be such that any increase in temperature is met with a dramatic increase in cooling (the hydrogen cooling edge at∼104K is a good example). It also generally helps for the jet to be significantly less dense than the cloud. Finally, the jet velocity needs to be fast enough to trigger shocks in the cloud, yet not so fast that the cloud is disrupted before cooling can have much of an effect.”

In my words: The jet compresses a cloud and this compression heats the cloud to the point that it begins to emit photons (e.g. Hydrogen lines). This way the cloud cools down. Molecular clouds are usually at an equilibrium: Heat pushes outwards and gravity pulls inwards. Now that this equilibrium is disturbed the gravity can take over and the cloud can collapse and form stars. Works a bit like the compressor in a refrigerator.

Thanks for that, but I don’t quite follow the reasoning in there. The jet injects energy into the cloud, which heats it above the ambient temperature. If the cloud subsequently re-radiates that energy, it can only radiate back down to the ambient temperature again. Once at the ambient temperature there is equilibrium between incoming and outgoing radiation. You cannot get thermal energy flow against a temperature gradient, only with it.

Adiabatic expansion cools a gas, which is what happens in a refrigerator. First the gas is compressed by the compressor, which heats it, then it is passed through a heat exchanger to the outside which cools it again, and finally it is returned to the interior and the pressure is released, which cools the gas. There is no such process going on in a cloud whose only energy input is from the jet and whose only energy output is re-radiation of that energy. All other energy exchange processes will be at an equilibrium state.

My explanation would be that after the cloud releases the heat injected from the jet, it is denser than it was before but at the same ambient as its surroundings. So, the gas can become densified and then end up at the same ambient temperature as before the jet hit it - and then gravity could start to compress the gas further and lead to star formation. The idea that the jet could COOL the gas and hence promote star formation sounds physically impossible to me (but I am not an expert on thermodynamics).

I’m no expert here, but I believe “line cooling” is one of the relevant concepts, eg