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I know Hubble's galaxy classification scheme and the bifurcation from elliptical galaxies into two types of spiral ones. I am also aware of different theories on how spiral galaxies form e.g. through collision with another one. I was always wondering in how much any kind of galaxy morphological classification actually relates with the dynamics of a galaxy. The following paragraph of the Wikipedia article seem to support my uncomfort:

To this day, the Hubble sequence is the most commonly used system for classifying galaxies, both in professional astronomical research and in amateur astronomy. Nonetheless, in June 2019, citizen scientists through Galaxy Zoo reported that the usual Hubble classification, particularly concerning spiral galaxies, may not be supported, and may need updating.

Differently put: Would a lone elliptical galaxy eventually become a spiral one? And if so, how to define the transition point? For me, it does not appear obvious how to set a threshold after which density fluctuations within an elliptical galaxy deserve to be called spiral arms. More technical: Which confidence band around the average radial density distribution of a given galaxy has to be breached in order for the galaxy to be called spiral?

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    $\begingroup$ What a shame, a typo in the bounty description: "Up to now ..." Is there any way that could be fixed? Reading meta.stackexchange.com/questions/112188/… seems like I should find somebody who could do that for me... $\endgroup$
    – B--rian
    Feb 10, 2021 at 8:13

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To start, if I'm reading your question correctly you've got the general galaxy evolution model backwards, at least as far as morphology goes. At a very high level the picture goes like this: most if not all (large) galaxies form as spirals, then at varying points in their lives they merge with other large galaxies (either ellipticals or other spirals) and the product becomes an elliptical galaxy. This happens faster and more often in dense environments like galaxy clusters, and takes longer to occur for more isolated galaxies like the Milky Way (which will merge with Andromeda in a few billion years).

Question on isolated spirals

If what you were asking about is isolated spirals, well no I don't think an isolated spiral would ever turn into an elliptical galaxy by itself (at least not on life-age-of-the-Universe timescales). It may eventually start to resemble an elliptical galaxy (in that its stellar halo may grow) depending on how much it gets harassed by other small dwarf galaxies. No galaxy is truly isolated, there is always a hierarchy of smaller galaxies nearby and they gravitationally interact with the host galaxy. For example the Milky Way has the Sagittarius dwarf galaxy, and the Large and Small Magellanic clouds which will merge with us soon (it's basically happening now for Sagittarius, and the Magellanic clouds will be a few billion years). None of these events will be impactful enough to turn the Milky Way into a true elliptical, that will only occur with it merges with Andromeda.

Question on morphology versus internal dynamics

To specifically answer your other question on how morphology relates to the internal dynamics of a spiral galaxy: it most definitely does, but the relationship is complicated. When you see a spiral galaxy with a central bar, or spiral arms, these features are being generated by dynamical instabilities within the galaxy. This could happen thanks to a number of causes, which may operate from within the galaxy itself, or from outside the galaxy. One of the most common means of exciting a bar or spiral arms in a disk galaxy is by interactions with smaller dwarf satellite galaxies that orbit around the larger spiral galaxy and gravitationally perturb it. If this part of the answer sounds like a cop out, well it sort of is. Exactly how disk perturbations like bars and spiral arms are started, how they work, what the heck they do to the galaxy, these are all truly bleeding edge research topics that astronomers are working on right now.

Hope that helps!

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  • $\begingroup$ First of all, welcome to astronomy SE! Yes indeed, that helps! If you don't mind, I would edit in some subsections to increase readability. $\endgroup$
    – B--rian
    Feb 12, 2021 at 10:13
  • $\begingroup$ I would also appreciate if you could refer or edit in a picture of your favorite galaxy-evolution chart, to show how I got things the wrong way. $\endgroup$
    – B--rian
    Feb 12, 2021 at 10:13
  • $\begingroup$ There may be charts out there, but none that I'm aware of that I think are particularly useful to share. Eshaya's answer brings up the key point, which is that a diagram like the Hubble sequence was only ever meant to classify the obvious morphological properties of galaxies, not their evolutionary paths. Indeed, I think many astronomers would be extremely hesitant to create a diagram that shows how galaxies evolve because it can be so complicated and varied. As far as Wikipedia goes I think this would be a good resource: en.wikipedia.org/wiki/Galaxy_formation_and_evolution $\endgroup$
    – astrolane
    Feb 12, 2021 at 19:30
  • $\begingroup$ Also, thanks for the welcome! Seems like a fun community. Added the headers. Also I would note that for my answer about morphology versus dynamics, I'm biased towards spirals based on my background. Someone who works on ellipticals would be able to tell you all about their morphology-dynamics relation. But briefly the overall triaxial shape of an elliptical would be strongly dependent on the way in which it formed, as well as the environment in which is lives. $\endgroup$
    – astrolane
    Feb 12, 2021 at 19:38
  • $\begingroup$ I like both answers. Since I could not decide which one is the better, you received the bounty, and I checked the shorter answer by @eshaya. Hope that is ok for both of you. $\endgroup$
    – B--rian
    Feb 13, 2021 at 20:28
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I think there are a few misconceptions floating around here. The Hubble Sequence is not a sequence in time. Hubble did not mean to imply that galaxies flow from one side to the other in the sequence (He may have thought it was a possibility though). It is just meant for classification. As it turns out a small fraction of galaxies have changed class, such as an occasional major merger of two spirals, orbiting the right way, can form an elliptical. But, the vast majority of ellipticals (most at the cores of clusters) formed that way very early on. A few spirals may have been converted to S0 galaxies by gas stripping as they fell into clusters. But, most spirals started out as spirals, and probably so for most S0s. The angular momentum per gram, a conserved quantity, is quite different for ellipticals, S0s and spirals.

I do agree that a given spiral galaxy can sometimes have a bar and sometimes not.

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