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In a recent paper (Cosmic clocks: A Tight Radius - Velocity Relationship for HI-Selected Galaxies by Meurer, et al.), it was noted in the conclusion that:

[This] implies a constant orbital time of ∼1 Gyr at [the outermost] radius [of disc galaxies].

Given what we knew about disc galaxies and dark matter up to this point, is this an unexpected conclusion? My sense is that it is strange that all disc galaxies have a constant rotation regardless of their size, but my intuition may be wrong. I know that dark matter speeds up the rotation of the outer parts of disc galaxies, but isn't it strange that they converge on this value?

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    $\begingroup$ The result is being discussed in itself in the paper.. $\endgroup$ – AtmosphericPrisonEscape Mar 15 '18 at 19:19
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    $\begingroup$ @AtmosphericPrisonEscape Maybe it went over my head, but I can't see where they specifically discuss whether this rate is expected. If they did, I think there's still room for an answer that summarizes the relevant parts of the paper to show how it says that. $\endgroup$ – called2voyage♦ Mar 15 '18 at 19:24
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    $\begingroup$ At least the reporting on this is weird. Some sites show animations of galaxies rotating as if they were solid spoked wheels. That's just not right: en.wikipedia.org/wiki/Galaxy_rotation_curve These guys may be talking about some limit at the outer edge. $\endgroup$ – Wayfaring Stranger Mar 16 '18 at 16:22
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    $\begingroup$ @WayfaringStranger Yes, those animations are weird. To be clear, we're talking about the rate of rotation of the outer edge, like you said. $\endgroup$ – called2voyage♦ Mar 16 '18 at 16:23
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    $\begingroup$ Scientists will often search for an invariant as an outcome of a scaling law behavior, and invariably someone will attach a meaning to one if found. Sometimes it turns out to be helpful or even powerful, but sometimes it doesn't mean anything (e.g. The Matrix 1, 2). $\endgroup$ – uhoh Mar 18 '18 at 1:02
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It is not weird.

The comment is about a certain class of galaxy, and not just a class but a stage of galaxy formation and development within that class.

We start with a fairly constant distribution of matter within the universe.

Stars for when this matter clumps together. This clumping is going to have a fairly even distribution.

Stars then clump together to form structures that we identify as galaxies.

The distribution of this clumping is also going to have a fairly constant distribution.

This carries on up the chain to clusters of galaxies, and clusters of cluster and so on.

Going back to stars, we identify main sequence stars. These are stars that start of with a mass and composition that is very common and then have common life cycles. They take so long to burn up certain percentages of their hydrogen, so long burning combinations of hydrogen and helium and then so long going up the periodic table forming lighter elements and in the process becoming the different types of stars we define as being on the main sequence.

For galaxies it is similar.

Spiral galaxies need to be of a certain size and angular momentum to generate their signature spiral structure. They did not start out as spiral galaxies. And they will not remain spiral galaxies. They are simply how galaxies of a certain size and angular moments appear at certain stages in their development.

This means that the outer edge angular velocities of these types of galaxies (for a specific definition of their outer edge) are going to be similar.

For some information on the development of spiral galaxies and other galaxy types:

https://astronomy.swin.edu.au/cosmos/S/Spiral+Galaxy+Formation

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    $\begingroup$ There isn't much in that link you have posted which supports your argument. Specifically, where do you obtain your key point that "Spiral galaxies need to be of a certain size and angular momentum to generate their signature spiral structure."? I note that the angular velocity is a roughly linear function of radius in most spirals and that spiral arms exist at a wide range of radii. $\endgroup$ – Rob Jeffries Dec 8 '19 at 15:43

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