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James Webb Space Telescope reveals that most galaxies rotate clockwise

(www.smithsonianmag.com)
352 points instagraham | 3 comments | 31 Mar 25 10:31 UTC | HN request time: 0.634s | source
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like_any_other ◴[31 Mar 25 11:09 UTC] No.43533575[source]▶
> Due to an effect called the Doppler shift, astronomers expect galaxies rotating opposite to the Milky Way’s motion to appear brighter

How does this work? The page it links to doesn't explain why rotation would matter.

Edit: To clarify - one side of the galaxy would be moving towards us, and one away from us, no matter which direction it spins in, so this should not affect the average brightness of the entire galaxy.

The original paper (https://academic.oup.com/mnras/article/538/1/76/8019798?logi...) links to a few papers discussing this, among them https://www.mdpi.com/2073-8994/15/6/1190 It doesn't answer my question (or if it does, I didn't understand it), but it gives a magnitude for the expected effect on brightness - 0.6%. I do not think that would explain the 1:2 ratio of observed spin directions.

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1. tomr_stargazer ◴[31 Mar 25 13:27 UTC] No.43534819[source]▶
I think this may be a subtle reference to what is usually called "relativistic beaming" or "Doppler beaming" [0], though I normally associate this effect with matter that is moving quite close to the speed of light.

[0]: https://en.wikipedia.org/wiki/Relativistic_beaming

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2. like_any_other ◴[31 Mar 25 14:27 UTC] No.43535465[source]▶
perihelions [1] phrased it best - a galaxy spinning clockwise is just the mirror image of one spinning counter-clockwise. So why should mirroring change brightness? I don't really understand why Doppler beaming would cause this (though one of the papers did also mention it).

The other question is - what does the rotation of our own galaxy have to do with it? Let's keep the Solar system as is, and mirror the rest of the Milky Way around it, so that it is now spinning the opposite direction. Why should this affect the apparent brightness of other galaxies? Especially since the Solar system is effectively moving in a straight line, on the scale of a human lifetime.

[1] https://news.ycombinator.com/item?id=43535285

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3. lostmsu ◴[31 Mar 25 20:51 UTC] No.43539828[source]▶
This is only obvious when you yourself are not moving on a curved trajectory.

In GR curved trajectories lead to "weird" observations. For instance, I'd expect the side of the remote galaxy that is closer (visually, for you as an observer) to the center of your galaxy to be gravilensed slightly more than the other side. Because the effect is non-linear, it does not just compensate when sides are reversed.

P.S. Not claiming this has any significant effect on the described phenomenon, just that mirror symmetry does not apply.