APOD: The Einstein Cross Gravitational Lens (2017 Dec 17)

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APOD: The Einstein Cross Gravitational Lens (2017 Dec 17)

Post by APOD Robot » Sun Dec 17, 2017 5:08 am

Image The Einstein Cross Gravitational Lens

Explanation: Most galaxies have a single nucleus -- does this galaxy have four? The strange answer leads astronomers to conclude that the nucleus of the surrounding galaxy is not even visible in this image. The central cloverleaf is rather light emitted from a background quasar. The gravitational field of the visible foreground galaxy breaks light from this distant quasar into four distinct images. The quasar must be properly aligned behind the center of a massive galaxy for a mirage like this to be evident. The general effect is known as gravitational lensing, and this specific case is known as the Einstein Cross. Stranger still, the images of the Einstein Cross vary in relative brightness, enhanced occasionally by the additional gravitational microlensing effect of specific stars in the foreground galaxy.

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Re: APOD: The Einstein Cross Gravitational Lens (2017 Dec 17)

Post by JohnD » Sun Dec 17, 2017 11:31 am

Why/how is the light from the source beyond focussed into four distinct images?
My 'image' of how it works would cause a smeared ring of light around the source of gravity, like that seen in simulations of black holes against a starfield.
http://www.gettyimages.co.uk/detail/ill ... /680803733
Clearly Einstein said it would be a Cross of images, but that implies a non-homogenous gravity field, does it not?
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Re: APOD: The Einstein Cross Gravitational Lens (2017 Dec 17)

Post by isoparix » Sun Dec 17, 2017 1:20 pm

Is that a Poisson spot (http://www.princeton.edu/~rvdb/images/Q ... nSpot.html) we see before us, in the centre of the image? If not, what is it?

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Re: APOD: The Einstein Cross Gravitational Lens (2017 Dec 17)

Post by JohnD » Sun Dec 17, 2017 1:57 pm

Fascinating, isoparix - never heard of a Poisson spot before!
I can't explain why there is a Poissson spot, but I'm sure it's not due to gravitational lensing, which anyway produces a ring of light around the shadow of the obstructing object, not an Einstein Cross.
An other exampe of gravitational lensing, smearing the images, not producing a structured one: https://www.youtube.com/watch?v=BkBNf_nFuhM

John

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Re: APOD: The Einstein Cross Gravitational Lens (2017 Dec 17)

Post by Chris Peterson » Sun Dec 17, 2017 2:50 pm

JohnD wrote:Why/how is the light from the source beyond focussed into four distinct images?
My 'image' of how it works would cause a smeared ring of light around the source of gravity, like that seen in simulations of black holes against a starfield.
http://www.gettyimages.co.uk/detail/ill ... /680803733
Clearly Einstein said it would be a Cross of images, but that implies a non-homogenous gravity field, does it not?
I don't think Einstein suggested a cross, but rather, multiple images.

In the case of four images (which are, I think, very short arc segments), the lensing field is oblong, like a galaxy. And in all cases, the field will have at least some degree of inhomogeneity. And on top of all that, you have the fact the there's never perfect alignment between the source, lens, and observer.
Chris

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Re: APOD: The Einstein Cross Gravitational Lens (2017 Dec 17)

Post by JohnD » Sun Dec 17, 2017 3:31 pm

Thank you, Chris, but ...'oblong' ? A rectangular solid, with faces that have equal opposite sides? I suppose that might focus four images!
Would a classic spiral galaxy have an 'oblongish' gravity field? Equal on either side of the poles, and either side of the disc, but different in either position?
But, this page by Louis Marmet addresses my question, and says, " If the mass is concentrated in a small volume, only two images are produced by the gravitational lens (and if the quasar is centered on the lensing mass, a ring appears)" http://www.marmet.org/cosmology/einsteincross/ Marmet explains the second pair by supposing that the lensing mass is not a point source, "but the lens would need to have a very complex shape."

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Re: APOD: The Einstein Cross Gravitational Lens (2017 Dec 17)

Post by geckzilla » Sun Dec 17, 2017 4:29 pm

isoparix wrote:Is that a Poisson spot (http://www.princeton.edu/~rvdb/images/Q ... nSpot.html) we see before us, in the centre of the image? If not, what is it?
The foreground galaxy's nucleus.
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Re: APOD: The Einstein Cross Gravitational Lens (2017 Dec 17)

Post by Ann » Sun Dec 17, 2017 8:22 pm

I'm going to bore everyone to tears by pointing out that the lensed quasar images are all brilliantly white with the barest hint of blue in them. All the other (blurred) point sources in this image are yellowish or reddish.

The fact that these very distant and highly redshifted quasar images look white or even blue-white says something about how "hard" and short wave-rich the radiation of the quasar still is, let alone how hard and short wave-rich it was at its point of origin.

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Re: APOD: The Einstein Cross Gravitational Lens (2017 Dec 17)

Post by NCTom » Mon Dec 18, 2017 12:57 pm

I know we have moved on to other pics, but I have searched all the links I can find and cannot find an estimated distance to the lensing galaxy 2237+030. Anybody have such data? Also is there any way in a lensing situation such as the Einstein Cross, the distance to the QSO can be estimated?

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Re: APOD: The Einstein Cross Gravitational Lens (2017 Dec 17)

Post by Chris Peterson » Mon Dec 18, 2017 2:42 pm

JohnD wrote:Thank you, Chris, but ...'oblong' ? A rectangular solid, with faces that have equal opposite sides?
Oblong- longer along one axis than the other.
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Re: APOD: The Einstein Cross Gravitational Lens (2017 Dec 17)

Post by Chris Peterson » Mon Dec 18, 2017 2:47 pm

NCTom wrote:I know we have moved on to other pics, but I have searched all the links I can find and cannot find an estimated distance to the lensing galaxy 2237+030. Anybody have such data? Also is there any way in a lensing situation such as the Einstein Cross, the distance to the QSO can be estimated?
It's in the link to the source. The quasar is z=1.7, and the lensing galaxy is z=0.04. Depending on the cosmological values used, that puts the quasar at a light travel time of about 9.9 Gly, and the lensing galaxy at a light travel time of about 0.55 Gly.
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Re: APOD: The Einstein Cross Gravitational Lens (2017 Dec 17)

Post by neufer » Mon Dec 18, 2017 2:51 pm

Chris Peterson wrote:
JohnD wrote:
Thank you, Chris, but ...'oblong' ? A rectangular solid, with faces that have equal opposite sides?
Oblong- longer along one axis than the other.
Thereby, symmetry breaking an Einstein Ring into 4 pieces as defined by the 2 axes.
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