Why are there 4 dots in Einstein rings?

[Buckethead]

TL;DR Summary

Images of gravitational lensing often reveals 4 of the distant quasar (in addition to the center quasar) in the image. Why the specific number?

I was looking at the images of Einstein rings in a recent press release.

https://hubblesite.org/contents/media/images/2020/05/4613-Image?news=true

And in these images as well as others I have seen in the past, i.e.

https://www.space.com/28744-cosmic-lens-4-supernova-views-photo.html

there are many that show 4 images of the distant quasar instead of some other number and more interestingly, the images themselves are often undistorted instead of the expected arcs. There are many exceptions of course to both these observations. But especially in the link above, this phenomenon is unmistakable. Any particular reason for this?

 

[Vanadium 50]

Are there more than two examples? Or are you seeing the same two examples multiple times?

 

[phinds]

Are there more than two examples? Or are you seeing the same two examples multiple times?

Four times, maybe ?

 

[Motore]

This:
https://spacetelescope.org/images/heic0404b/

 

[MikeeMiracle]

I believe this is related to the optics inside the telescope causing that effect, you see something similar looking at stars sometimes and there can be more than 4 depending on your telescope construction.

 

[Motore]

It has nothing to do with the telescope.

From the site I posted above:

Gravitational lenses produce different shaped images depending on the shape of the lensing body. If the lens is spherical then the image appears as an Einstein ring (in other words as a ring of light) (top); if the lens is elongated then the image is an Einstein cross (it appears split into four distinct images) (middle), and if the lens is a galaxy cluster, like Abell 2218, then arcs and arclets (banana-shaped images) of light are formed (bottom).

 

[Buckethead]

Are there more than two examples? Or are you seeing the same two examples multiple times?

The first link show images of 6 different lenses and the second link is another image taken in 2015

 

[Buckethead]

This:
https://spacetelescope.org/images/heic0404b/

Thanks for the link. This sounds definitive, but I'm skeptical that this could be accurate. An oblong lens will not separate images into n number of images due to its symmetry. It should display a ring as in the first example or if offset, an arc, just a different size or shape. If you look at the examples, the images of the distant quasar are pinpoint like. It seems the only way you can get this is if the shape of the lens is also a crosslike shape. Since these images are suppose to reveal DM, that would indicate that the DM is crosslike shaped (along with the galaxy in the center or else there would be 4 dots and a ring. (or something like that). There is something very unusual going on here.

 

[Ibix]

I can see complete rings in several of those images - notably the top-middle one in the first link. Obvious explanation is that the intensity of lensed rays varies around the ring, so you see bright parts and can't see the dimmer parts. For an off-axis lens this isn't completely daft - you'd expect symmetry along the line through the lens and lensed object but not necessarily rotational symmetry.

 

[Motore]

This sounds definitive, but I'm skeptical that this could be accurate.

Here is a good explanation:
https://physics.stackexchange.com/q...nsing-account-for-einsteins-cross/14631#14631

 

[Buckethead]

I can see complete rings in several of those images - notably the top-middle one in the first link. Obvious explanation is that the intensity of lensed rays varies around the ring, so you see bright parts and can't see the dimmer parts. For an off-axis lens this isn't completely daft - you'd expect symmetry along the line through the lens and lensed object but not necessarily rotational symmetry.

Yes, I see the rings now that I am looking more carefully. I'm still uncomfortable with this explanation however. For one thing, the first link shows very small galaxies (I think just clusters of starts) and so symmetry would be stressed. In addition, the bright spot are very distinct, not bright smear areas for example and the rings are also a very distinct intensity along their path. The 4th picture (bottom left) is also unusual as you still have the distinct dot, yet one is way off base. Also the 2nd picture( top center) show a very small amount of offset in the top dot. All these asymetries statistically speaking it seems would lead to very random images. Like a smear on one of the stars and maybe a distinct dot on the opposing star with the other 2 stars somewhere in the middle. It's just all these images are showing more organization for such a random set than just an axial symmetry might offer. I am however not opposed to your idea, there are after all only so many explanations available here and I'm no expert in optics.

 

[Buckethead]

Here is a good explanation:
https://physics.stackexchange.com/q...nsing-account-for-einsteins-cross/14631#14631

Great link! A bit complex and I need to read it more slowly, but at first glance it seems this was thought out quite well. Good analogy with the funhouse mirror. Ibex, seems you were on the right track.