APOD: Galaxy Einstein Ring (2016 Apr 20)

[APOD Robot]

Galaxy Einstein Ring

Explanation: Can one galaxy hide behind another? Not in the case of SDP.81. Here the foreground galaxy, shown in blue in an image taken by the Hubble Space Telescope, acts like a huge gravitational lens, pulling light from a background galaxy, shown in red in an image taken in radio waves by the Atacama Large Millimeter Array (ALMA), around it, keeping it visible. The alignment is so precise that the distant galaxy is distorted into part of a ring around the foreground galaxy, a formation known as an Einstein ring. Detailed analysis of the gravitational lens distortions indicate that a small dark satellite galaxy participates in the deflections, bolstering indication that many satellite galaxies are quite dim and dominated by dark matter. That small galaxy is depicted by a small white dot on the left. Although spanning only a few arcseconds, the featured Einstein ring is really tens of thousands of light years across.

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[Guest]

How do we know this is a satellite of the lensing galaxy and not some other blob of dark matter either closer or further out?

[Guest]

Using modern computer techniques such as ray tracing, is it possible to remove the distortion and obtain a direct picture of the lensed object?

[Chris Peterson]

Using modern computer techniques such as ray tracing, is it possible to remove the distortion and obtain a direct picture of the lensed object?

Probably not. A gravitational lens doesn't produce an image like a conventional lens; even if the lens were perfectly characterized, there are normally multiple sources that could produce the same sort of ring. But we have no way of perfectly characterizing the lens, so that leaves even more possibilities.

[Chris Peterson]

Although spanning only a few arcseconds, the featured Einstein ring is really tens of thousands of light years across.

What exactly does that mean? The Einstein ring isn't a physical structure, so talking about its size is ambiguous at best, and nonsensical at worst (like saying a rainbow is a mile across).

Perhaps the intent here is to suggest that if this ring were a physical structure, at the distance of the lensing galaxy, it would be tens of thousands of light years across?

[Boomer12k]

Although spanning only a few arcseconds, the featured Einstein ring is really tens of thousands of light years across.

What exactly does that mean? The Einstein ring isn't a physical structure, so talking about its size is ambiguous at best, and nonsensical at worst (like saying a rainbow is a mile across).

Perhaps the intent here is to suggest that if this ring were a physical structure, at the distance of the lensing galaxy, it would be tens of thousands of light years across?

I am thinking that since the galaxy that is in front is probably tens of light years across, and the ring is larger than that structure... then one can deduce the ring is tens of thousands of LY also....as it goes around that galaxy's edges...and seemingly larger...

Interesting phenomena...
:---[===] *

[Chris Peterson]

Although spanning only a few arcseconds, the featured Einstein ring is really tens of thousands of light years across.

What exactly does that mean? The Einstein ring isn't a physical structure, so talking about its size is ambiguous at best, and nonsensical at worst (like saying a rainbow is a mile across).

Perhaps the intent here is to suggest that if this ring were a physical structure, at the distance of the lensing galaxy, it would be tens of thousands of light years across?

I am thinking that since the galaxy that is in front is probably tens of light years across, and the ring is larger than that structure... then one can deduce the ring is tens of thousands of LY also....as it goes around that galaxy's edges...and seemingly larger...

But the ring isn't larger than the galaxy. The ring doesn't exist, so it has no physical size at all. It merely subtends a larger angle.

[Ann]

Usually the foreground galaxy is yellow and the lensed galaxy is blue. That is because foreground galaxies massive enough to serve as lenses typically have little or no star formation, while the lensed galaxies that existed in the distant typically had copious star formation. I would guess that this general rule applies for the galaxies in the Einstein Ring as well, even though they have been colored differently.

I like the Einstein Ring. It is quite satisfying to see old Albert's model of the universe being confirmed again and again. (Not that it isn't still possible that we may have to re-write the laws of physics and the model of the universe all over again, to explain things like dark matter and dark energy.)

Ann

[geckzilla]

Using modern computer techniques such as ray tracing, is it possible to remove the distortion and obtain a direct picture of the lensed object?

Probably not. A gravitational lens doesn't produce an image like a conventional lens; even if the lens were perfectly characterized, there are normally multiple sources that could produce the same sort of ring. But we have no way of perfectly characterizing the lens, so that leaves even more possibilities.

I thought this was done rather routinely now? I don't think anyone claims to have perfect characterization of a lensing structure, but the techniques are good enough to have predicted one of Refsdal's images.

[alter-ego]

How do we know this is a satellite of the lensing galaxy and not some other blob of dark matter either closer or further out?

That's a good question. Y. Hezaveh's paper details the analysis method and probabilities of various mass subhalos that lead to distortions in the Einstein Ring:

The proximity of SDP.81 to a galaxy cluster has the potential
to cloud our predictions for its abundance of substructure.
In principle, small subhalos unbound to SDP.81 but within the
cluster could project into the strong lensing region. For the
current configuration, we have calculated the expected number
of subhalos from such a cluster, at the measured distance
(using the same model described in the previous section), and
find that these subhalos are subdominant to those expected
from the halo of SDP.81. However, this underscores the importance
of determining reliable estimates of lens halo masses
and nearby environments before deriving any bounds on cosmological
models from observed lensing systems.

Needless to say this was an extremely complex numerical analysis. Yes, the mass and position of the most probable subhalo was backed out from the ring's distortions. I would say lower resolution, course reductions are routine. Using a super-computer ensemble to extract that small of a dark matter blob is not routine, but certainly the methodology is the same, just a lot more and bigger matrices.

[hearborist]

The ring is not perfectly concentric with the lensing galaxy in the center of the ring. Is this because the distant galaxy is not centered behind the lens? Does this also result in the incomplete ring?

[Guest]

While looking at the image, I noticed a smaller 'red' arc just 'inside' the satellite galaxy, between the satellite and the primary lensing galaxy. It made me wonder if it is possible that these two galaxies may be acting as 2 elements of a 'diffraction grating' thereby causing the manifestation of the inner arc mentioned above. I remember an exam question from back in my high school days about the 'diffraction of an elephant running between two palm trees', or something along those lines.

[heehaw]

Thanks! Magnificent image!

[newguy]

Should you be getting a prism effect even with the Einstein rings?

[bystander]

http://asterisk.apod.com/viewtopic.php?t=35833

[Chris Peterson]

Using modern computer techniques such as ray tracing, is it possible to remove the distortion and obtain a direct picture of the lensed object?

Probably not. A gravitational lens doesn't produce an image like a conventional lens; even if the lens were perfectly characterized, there are normally multiple sources that could produce the same sort of ring. But we have no way of perfectly characterizing the lens, so that leaves even more possibilities.

I thought this was done rather routinely now? I don't think anyone claims to have perfect characterization of a lensing structure, but the techniques are good enough to have predicted one of Refsdal's images.

What's done with raytracing isn't the reconstruction of the background galaxy, but treating the background galaxy as a point source and using it to reconstruct the dark matter distribution that produces the lens. This approach doesn't lead to closed, unique solutions, however. So while we have plausible models of the lensing bodies, they may be incorrect solutions.

[Chris Peterson]

Should you be getting a prism effect even with the Einstein rings?

No. In normal optics, prism effects are caused by dispersion- the fact that different wavelengths of light travel at different speeds through a physical material. Gravitational lenses don't produce dispersion- a photon coming from a source will follow the same path (geodesic) around the lensing body regardless of its wavelength (energy).

[Asterhole]

It makes my head hurt just thinking about it...

[Fred the Cat]

That some galaxies have more dark matter than others somehow surprised me.

That the smallest ones have the most dark matter appears to be quite profound.

I guess we are back to that same confounding principle- size does matter. There's proof though that "big" is an illusion that won't last.

[aqwalung]

Fascinating image!
1. I'm still kind of lost on how the amount of dark matter is being calculated, but wondering what role, if any, is played by the magnetic characteristics of each of the three galaxies in respect to the gravitational lensing properties or the dark matter calculations?
2. Does the color of the gravitationally lensed background galaxy directly correspond to it's red shift value used to determine it's distance?

- Thanks

[Chris Peterson]

1. I'm still kind of lost on how the amount of dark matter is being calculated, but wondering what role, if any, is played by the magnetic characteristics of each of the three galaxies in respect to the gravitational lensing properties or the dark matter calculations?

Any magnetic fields are irrelevant. They would have no effect at all on dark matter, and the only way we might see them affecting the light we see would be in polarization effects, which are not detectable in this data.

2. Does the color of the gravitationally lensed background galaxy directly correspond to it's red shift value used to determine it's distance?

I think the colors are arbitrary- a monochrome HST image mapped to blue for the foreground galaxy, a monochrome radio image mapped to an orange pseudocolor palette for the lensed galaxy.

[Guest]

Curious that the quoted size is a few arc seconds yet the Hubble image is highly pixellated. With Hubble's resolution of 0.05 arc seconds, it surely shouldn't appear like this. Are the merged images on vastly different scales?

[kkh]

"... the featured Einstein ring is really tens of thousands of light years across."
How big is a rainbow really?

[geckzilla]

Curious that the quoted size is a few arc seconds yet the Hubble image is highly pixellated. With Hubble's resolution of 0.05 arc seconds, it surely shouldn't appear like this. Are the merged images on vastly different scales?

The pixel size for the WFC3/IR channel is 0.13 arcsec. Does it make more sense now?

[Chris Peterson]

Curious that the quoted size is a few arc seconds yet the Hubble image is highly pixellated. With Hubble's resolution of 0.05 arc seconds, it surely shouldn't appear like this. Are the merged images on vastly different scales?

The pixel size for the WFC3/IR channel is 0.13 arcsec. Does it make more sense now?

Actually, it still isn't obvious what's going on. Here's the synthesized ring image combined with the actual IR/F160W Hubble image:

Here we can see the actual pixels that correspond to the visible light image of the lensing galaxy. The image seen in today's APOD apparently consists of a mathematical fit of the two datasets, resulting in synthetic pixels (of irregular dimension) that don't directly correlate to either of the original datasets.