Radio Galaxy Zoo Talk

Making Radio Galaxies in a Computer, continuing the discussion here

  • JeanTate by JeanTate

    Making Radio Galaxies in a Computer is a recent (August 28, 2014) blog post in the GZ blog, by twjastro. It's fascinating! 😃

    I wrote a comment, and twjastro responded, almost immediately; yay! I then wrote another, but so far no reply.

    Perhaps here, in RGZ Talk, we could continue the discussion?

    I won't copy/paste the blog post, but here are the three comments:

    [me, September 10, 2014] I find it amazing that events at such tiny scales as SMBH and its immediate environs (no bigger than ~the size of the solar system) can produce such gigantic structures as Mpc-wide jets and lobes! How do you successfully simulate physics which covers such a huge range of scales?

    A question: in the first para “The jets carry magnetic fields and high energy electrons” Surely the jets also carry at least protons (hydrogen nuclei) and alpha particles (helium-4 nuclei), and possibly highly ionized ‘metals’ too? And while ‘carry’ isn’t the right verb, the jets are also beams of very high energy neutrinos, aren’t they?

    Comment #2:

    [twjastro, September 10, 2014] Thanks for your interest. First, it is indeed very difficult to simulate the full range of scales from the size of the solar system to the size of a cluster of galaxies in a single simulation. Impossible, actually. So, instead, simulations generally focus on either the source end of the scales (how do you make a jet realistically) or what happens to a jet once it is made. The simulations I show in the blog are of the latter type. They follow events only after the scales get to kpc and bigger. We utilize the results of other simulations that zoom down to SMBH scales in order to create our jets.

    As to the composition of jets, that is still an unanswered and debated question. It probably depends actually on where you are. It is quite possible, and some simulations suggest, that at the source near the SMBH the jets consist almost entirely of electromagnetic energy along with electron/positron pairs that are products of that electromagnetic energy. Some models do begin with substantial proton loads. Then, either if there are also neutrons or the protons collide with other protons, the jets, as you suggest,would also generate a neutrino flux. The neutrinos would not generally follow the dynamics of the other components, however, once they were generated.The biggest worry about models with big ‘baryon loads’ is that if the proton load is high then the total energy requirements for the jets become very large. The minimum energy requirements for the most powerful jets are already hard to explain even if the ‘baryon load’ is small. Independent of their composition at the beginning, jets will eventually entrain ordinary matter, which, as you suggest would contain all the usual ingredients. But, again it is hard to explain the most powerful jets if this entrained gas is well mixed in. Furthermore, measurements of the so-called Faraday rotation effect in polarized radio emission puts pretty tight limits on the amount of this gas in at least the most powerful jets. It is possible and often suggested that many jets may contain fast cores devoid of anything but electromagnetic energies and electron/positron pairs, although that is not established. Some jets, and probably the most severely distorted jets are likely to contain significant amounts of entrained gas. Our simulations are intended partly to study this process, actually.

    In case the question comes up, it turns out that the annihilation rates of any electron/positron populations in the jets are too small to lead to detectable gamma rays from that process. Other sources of gamma rays will overwhelm that.

    As to terminology, the verb ‘carry’ is correct, I think, since it reflects what you find if you put yourself in the local rest frame of the jet plasma, whatever it is. So long as there is any matter of any kind in the jet, this is something you can do. The terms ‘jet’ and ‘beam’ get used interchangeably in this community. They represent different aspects of what is going on, just as you can call a stream of water coming from a hose a jet or a beam. The former term reflects the fact that it moves fast, but has internal ‘thermodynamics’, while the latter reflects the fact that it transports energy and momentum at high speed. Both are valid, but incomplete.

    And #3:

    [me, September 11, 2014] Thanks for the information-packed and lengthy reply! 😃

    I have lots, and I mean lots, of questions; perhaps I should ask on RGZ Talk?

    Anyway, here are just a couple … at the source near the SMBH the jets consist almost entirely of electromagnetic energy along with electron/positron pairs that are products of that electromagnetic energy. Yeah, this is about something other than your simulations, but still … “electromagnetic energy”? What is this? Intense electric and magnetic fields, generated by the residual charge of the SMBH? UHE gammas trapped near the event horizon/in the ergosphere?

    If the jets have only a small baryon load (lovely term!), and comprise predominantly electrons, what prevents them from being braked (probably not the best verb) by whatever is the relativistic equivalent of a Debye sphere?

    All right, three questions … when you ‘look down the barrel’ of your simulations, do you ‘see’ things which resemble BL Lac objects, SEDs, polarizations, variability and all?

    Posted