Might NGC 1073 have a non-coplanar bar and spiral arms?

  1. Spiral galaxies are usually taken to be structured lenticular objects with spiral arms depending from a bulging star-dense central hub. The whole arrangement is essentially planar. Since our view of these objects is projected upon our fixed celestial sphere, their shape has to be inferred from the various projected shapes of many similar galaxies. Three- dimensional features of a single galaxy must appear flattened in projection.

    The spectacular Hubble photo of the barred spiral NGC 1073 just released by NASA
    nevertheless looks to me as if its spiral arms might not be coplanar with its central bar, which looks a bit like a star-dense lenticular disc inclined to the plane of projection. Is this possible? Or dynamically forbidden?
  2. jcsd
  3. phyzguy

    phyzguy 2,502
    Science Advisor

    I think what you are saying is possible. You need to remember that none of these forms are static - they are all in constant motion. For most purposes the laws of Newtonian gravity are adequate to describe these systems. Simulations I have seen of various spiral galaxies shows them constantly rotating, pulsing, etc., with the spiral arms constantly evolving.
  4. This article briefly discusses how the intrinsic galactic forces [density waves radiating outward from the core]and the tidal forces involved in galactic collisions differ in the creation and deformation of spiral galaxy hub bars.

    I assume that the writer is applying that galactic "maturity" criterion as evidenced by the bar only to spiral galaxies and not claiming that eventually all galaxies become barred spiral ones when they mature.
  5. Thankyou both phyzguy and Radrook for these helpful replies, and for introducing me to the
    extensive (and for me quite formidable) literature on bar formation. I found this comment from the link to the Hubblesite illuminating:

    I also gather that tidal forces during close encounters between galaxies may trigger bar
    formation and affect bar morphology, which is of course hard to establish from our two-dimensional perspective on galaxy features.

    But is it certain that galaxy bars are always axisymmetric-like, elongated structures (aptly described as bars) lying and rotating in the plane of spiral arms? Does modelling exclude for good reasons (like conservation of angular momentum, for example) non-coplanar (perhaps lenticular) structures forming in galaxy cores?
    Last edited: Feb 5, 2012
  6. Drakkith

    Staff: Mentor

    My best guess is that anything else would result in large instability of the bar and the part of the spiral arms near the bar. However, as far as I know, the core itself consists of stars which do in fact orbit in many different types of orbits, hence the core is more spherical than the rest of the galaxy.
  7. phyzguy

    phyzguy 2,502
    Science Advisor

  8. Thanks again for the new links, phyzguy and and for your comments about possible instabilities, Drakkith.

    What I'd really like to have clarified is this: among the zoo of galaxies out there spirals with bars are a prominent peculiarity. What actual evidence is there that bars have been correctly identified as truly linear features, rather than disc-like central features seen edge-on, just not co-planar with outlying spiral arms? Perhaps there are dynamic reasons which make this too peculiar to be possible, but don't enough strange objects exist out there for tricks of our two-dimensional perspective to render first impressions (here of linearity) possibly misleading?
  9. Drakkith

    Staff: Mentor

    We see most galaxies from an oblique angle, not directly from above or below. Any sort of disc shape would be obvious.
  10. Drakkith: Of course, as you say: "We see most galaxies from an oblique angle, not directly
    from above or below". But when you claim suddenly that if bars were non-coplanar disc-like features, this would be "obvious" from oblique views, I think you're perhaps being a bit hasty. Deducing the true three-dimensional shapes of complicated objects like galaxies from the projections we see may be trickier than appears at first glimpse, as it were.

    I hasten to add that a central non-coplanar disc does seems improbable to me, but one can’t rule out possibilities just because they’re judged improbable. Instead, let's suppose that bars are indeed non-coplanar discs, like the inner parts of plane galaxies that had somehow become twisted relative to outer regions, perhaps by tidal interactions with other galaxies.

    Then, in perspectives judged to be oblique because the envelope of the spiral arms seems oval rather than circular, it might not be easy to distinguish obliquely viewed central discs as separate features of galaxies, unless they were viewed nearly edge-on, as might be the case with NGC 1073 . I’ll bet that taxonomists, being naturally conservative, would then classify all such cases as linear bars. But what you see isn’t always what you get! Or is it in NGC 1073?

    In replies here I haven’t yet heard of actual supporting observational evidence for the
    existence of true bar-like “bars”. Is there any?
  11. Drakkith

    Staff: Mentor

    I don't see how. Any disc would be able to be seen above and below the plane of the galaxy. We do not see this.

    Alright. Let's suppose that. Tidal interactions cause the bars to be warped and become non coplanar. Why isn't the rest of the galaxy affected? I've seen multiple pictures of interacting galaxies and any warping of that magnitude would greatly affect the entire galaxy, especially the outlying stars and gas at the ends of the spiral arms.

    I don't even know what you would classify as "observational evidence" since the pictures themselves aren't evidence to you. Based on the information I've looked at, the development of the bar requires certain conditions, one being that gas is accumulated in the bar via orbital resonance, which requires that it be coplanar. If it were not the gas would accumulate near the center in more of a ring shape, not a bar, if the gas accumulated at all.

    COULD there be bars out there that are not coplanar? Sure. I'd bet there are. But they are probably a result of tidal interactions with another galaxy and are very unlikely to be stable.

    However, I am far from an expert on the matter, so I could be incorrect.
    Last edited: Feb 6, 2012
  12. turbo

    turbo 7,063
    Gold Member

    We cannot choose our viewing angle for galaxies. It is possible for a spiral to exhibit "warping", especially in response to gravitational interaction with other galaxies, but there is no way to detect that in images of a galaxy that is essentially face-on.
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