[perihelions]

I'm utterly confused what's going on. They're measuring galaxies' rotations by looking at images of the subset that are spiral galaxies, and checking which direction the arms spiral. They describe their image processing algorithm in their paper [0]. (it's around figure 3)

How can local movement of stars within the Milky Way affect which way spiral galaxy arms are pointing?

[0] https://academic.oup.com/mnras/article/538/1/76/8019798.

[dvh]

You cannot decide galaxy rotation by looking at it. Consider this gallaxy:

    _______   
    \   _ B\  
    /  /_\  \ 
    \  \_/   \
     \A____  /
           \/ 

Is A side closer to us, or is B side closer to us? You can't tell by just looking at it, if the B is closer that we are seeing bottom of the galaxy and it rotates CW. If A is closer than we are seing top of the gallaxy and it rotates CCW.

[AstralStorm]

Yes you can, by gravitational lensing of another body. Works exactly by triangulation in Lorentz space. You can thank Einstein for this feature of special relativity.

(Tricky part is deciding it's another body from a picture. You would need a second JWST preferably far in the other Lagrange point. Stereoscopy solves it directly. You can )

The thing is, you need another galaxy in the way to be sure, or a black hole. Theoretically our Sun can serve. [] Or the supermassive black hole in the center of our galaxy, but the sensitivity might be a bit compromised.

And long observation time.

[] https://en.m.wikipedia.org/wiki/Solar_gravitational_lens It's a bit hard to put satellites in the right place.

[Mertax]

There is no absolute direction for a galaxy’s spin—it’s always relative to the observer’s perspective.

So I’d suspect they’re saying time and distance would need to be factored in rather than just looking at static images relative to our position today since our own spin may have caused a particular galaxy to appear to have been spinning in a different direction at another point in space-time

[tremon]

I don't understand this logic. To me, that's equivalent to saying "there's no absolute direction for which way a wheel spins, it's relative to the speed of the observer". Which makes no sense to me, because my definition of spin is measured against the axis of rotation of the object itself.

I don't see how time-intermittent frame captures from our own position affect that interpretation. Or are we using an astonomy-specific definition of spin here?

[doug-moen]

It's true, there is no absolute direction for which way a wheel spins. It's relative to the observer.

If you are standing on the side of a road, and a bicycle goes by, then you may observe the wheels to rotate clockwise, while an observer on the other side of the road will observe the same wheels to rotate counter-clockwise.

The sun is said to rotate around the centre of the milky way galaxy once every 225 million years. Over that time frame, some of the galaxies we observe will flip between clockwise and counterclockwise rotation as our viewpoint changes.

But that isn't relevant here. The Space article is too vague and handwavy to make any conclusions about the research, and should be ignored. Only the original scientific paper is worth reading: https://academic.oup.com/mnras/article/538/1/76/8019798?logi...

Section 5.2 "Physics of Galaxy Rotation" seems particularly relevant.

> due to the Doppler shift effect galaxies that rotate in the opposite direction relative to the Milky Way are expected to be slightly brighter than galaxies that rotate in the same direction relative to the Milky Way. Therefore, more galaxies that rotate in the opposite direction relative to the Milky Way are expected to be observed from Earth, and the difference should peak at around the Galactic pole. That observation is conceptually aligned with the empirical data of Fig. 10, and the observation using JADES described in Section 3.

You should read the paper for the full argument.

[jacksnipe]

But what if you, the observer, are also rotating? What if you’re rotating faster than the thing you’re observing?

[cwillu]

I don't see how that could be so prominent as to reverse the visible arms of a spiral galaxy, what am I missing?

[d1sxeyes]

It’s the same as saying you can only work out how fast something is moving in relation to something else. Your car is doing 50mph on the highway, but the earth is spinning round the sun and the sun is moving around the centre of the galaxy and the galaxy is moving compared to other galaxies and so on.

[cryptonector]

> There is no absolute direction for a galaxy’s spin—it’s always relative to the observer’s perspective.

Not entirely. The galaxy is bound by gravity and the stars rotate in the galaxy around its baricenter. We can compute how fast it must be rotating from the amount of visible matter. Enter dark matter and various complications, but still, you can tell that it's rotating and which way.

And for galaxies we see edge on we can use the difference if redshift on one end versus the other to tell which way it is turning.

[Y_Y]

I don't necessarily agree, in the presence of a universe (and under some reasonable cosmological assumptions) you can't just get rid of an observed rotation by a change of inertial frame. You can rotate along with it, but you'll produce a tell-tale fictitious force.

See e.g. https://en.wikipedia.org/wiki/Mach's_principle

[Georgelemental]

Read section 5.2 of the paper

[perihelions]

Do you or other HN commenters understand it in a satisfying way?

The author writes about the Doppler effect creating a systemic bias in brightness depending on which way the galaxies are rotating. I don't understand that argument either, but it's moot, because they state categorically that that effect would be too small to explain their results. ("This explanation is challenged by the fact that the effect of the rotational velocity have merely a mild impact on the brightness of galaxies, and therefore is not expected to lead to the dramatic difference of 50 per cent in the number of galaxies as observed through JADES.")

That's the only explanation I recognized as an explanation. Then I lost track of their argument following that. They refer to several speculative physics theories like MOND, but I don't understand them to be saying something that concretely predicts distant galaxies to appear to be rotating differently.

I'm appealing to anyone on HN who knows enough about this field to understand the meat of this argument.

[perihelions]

It's a moot point, but I *really* don't understand the Doppler-shift bias mechanism either. Help?

This one of the author's other papers they cited in this one,

https://www.mdpi.com/2073-8994/15/6/1190

I'm completely lost how they're eliding between the rotation orientation of the Milky Way galaxy, and relative linear velocities with stars in other galaxies. In the special relativity argument, where does the rotation axis of the Milky Way enter?