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
  • $\begingroup$ @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

protected by Community Sep 23 '18 at 4:36

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