Radio Galaxy Zoo Talk

If the Milky Way galaxy (SgrA*) were the host of a giant ...

  • JeanTate by JeanTate

    ... what would it look like, in the radio sky?

    I wrote an OT post in the Possible giant/ host (?) thread; here's part of it:

    And wildly speculating, would we have already detected our own galaxy's doublelobes, if they were ~1 Mpc giants? If they were faded relics?

    ivywong quickly set me straight about one mistake in my thinking (interferometric observations may not be able to detect diffuse emission over a large part of the sky, but 'big dish' observations certainly can*), but that turned my curiousity to the question of what jets and doublelobes would look like, from here on Earth, if SgrA* - the center of our galaxy, where the super-massive black hole lives - were the source?

    I'm sure this is the sort of thing some university professors set as homework assignments for their students, so it's not like the answers haven't already been worked out, but I thought it might be cool to have a go at working it out myself (or ourselves, if anyone would like to join in).

    A simple model to start with: a pair of linear jets, cross-sectional radius a few pc (say), ending in a pair of pancake-shaped lobes, orthogonal to the jets. Size of the lobes? Something typical for giants; radius 10 kpc? For fun, make the lobes as far from us as the biggest giant seen so far is from its host; what's that? ~ 4 Mpc? From there it's just turning the handle, I think, to describe the appearance on the sky, in terms of (apparent) size and shape.

    Estimating intensity may be a bit trickier ... the simplest assumption is that the jets and lobes are isotropic emitters, and we can just pick typical values from known giants (including that the jets are all but invisible).

    But - as raynorris points out in his excellent How do we know the radio emission from galaxies is synchrotron? thread - jets and lobes emit synchrotron (radio) radiation, and that is most certainly not isotropic! 😮 Further, none of our observations of the lobes in giants is from the perspective we'd view any Milky Way lobes: we'd be seeing 'the back' of the lobes, a viewpoint almost impossible for giants.

    So, for the lobes, perhaps start with isotropy, then dial it back, to 10% in the reverse direction, 1%, ... ?

    *kinda obvious really; after all, the CMB is, to a high degree, a completely uniform background, yet those AT&T guys detected it perfectly well (even if, at one point, they wondered whether it might be pigeon droppings!)

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  • ivywong by ivywong scientist, admin

    Perhaps this will open a can of worms but If the jets are being emanated close to the speed of light, these jets would also be relativistically dimmed if we were looking from behind it. ....

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  • JeanTate by JeanTate in response to ivywong's comment.

    Indeed. I was thinking to put questions like that off for a while ... although the first part of the jets may not be quite so invisible, because we'd be looking at them at only a modest angle, since we're - what? - ~8 kpc from their source?

    Anyway, seeing how obvious CenA is in the gorgeous CHIPASS image Ivy posted, and recalling that surface brightness remains constant with distance*, hypothetical ~Mpc distant doublelobes created by SgrA* would be more-or-less circular patches centered (more-or-less) on the galactic poles, shining about as brightly as the lobes of CenA appear to us. True, structure within the lobes would be much more obvious, perhaps even making them not so obviously huge ~circular patches, but to 0th order ...

    It also occurred to me that there's probably already a lot of information on how radio emission from lobes deviates from isotropy ... provided there is a way to robustly estimate 'viewing angle' towards at least a big subset of the tens of thousands doublelobes already cataloged. On average, intrinsic asymmetry in lobes should ~zero out, so distance-corrected apparent relative luminosity, as a function of viewing angle, might be estimatable. And an extrapolation to ~end-on viewing not too much of a stretch. I suppose this has already been investigated, and a nice plot is almost certainly sitting in some paper or three ...

    *caveats apply, obviously, but are quite minor for this exercise

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