Difference between images produced by gravitational lensing

[PWiz]

Around massive bodies, light bends so that we can see multiple images of the same object, such as multiple images of the same galaxy behind the massive body.
I know this seems kind of a dim question, but how can we know that the similar images we pick up are light rays from the same object that have undergone gravitational lensing or whether there is an actual cluster of galaxies that emit nearly identical light? Given an image, how can we know that two "light spots" that seem identical are from the same source or different sources that emit identical light?

 

[CWatters]

I think the spectrum of the light would be the same (because it comes from the same souce object). Perhaps unlikely you would have a cluster of galaxies in the required arc and all emitting the same spectrum?

 

[CWatters]

Google found..

http://csep10.phys.utk.edu/astr162/lect/galaxies/lensing.html

Gravitational lensing was actually discovered for quasars before it was found for galaxies. In 1979 two quasars only 6 seconds of arc apart were discovered and they were found to have identical redshifts and spectra. The probability of this happening by accident is exceedingly small, and it was postulated that this pair actually corresponds to the same quasar that is lensed so strongly by intervening mass that its image appears double as we view it from Earth.

 

[sophiecentaur]

Around massive bodies, light bends so that we can see multiple images of the same object, such as multiple images of the same galaxy behind the massive body.
I know this seems kind of a dim question, but how can we know that the similar images we pick up are light rays from the same object that have undergone gravitational lensing or whether there is an actual cluster of galaxies that emit nearly identical light? Given an image, how can we know that two "light spots" that seem identical are from the same source or different sources that emit identical light?

This is a problem with all astronomical observations. Resolving power is always limited, with optical lenses and microwave reflectors and an observation made today may suggest a single star but, when a bigger telescope is used, it is resolved into a binary system - or just two stars in nearly the same direction. A massive cosmological object is a lousy lens, in fact - but better than nothing.

 

[PWiz]

@CWatters Thanks for the link. What do you think should be the maximum separation of two observed images that are identical to one another for them to qualify as product of gravitational lensing? (i.e. at what arc length separation can we be confident that the two images are not from the same object?)
@sophiecentaur I see, your point is very valid. But I'm talking about two objects that have already been resolved by the telescope. I'm actually more interested in the procedure which can confirm that the two images are from the same/different object(s).

 

[sophiecentaur]

@CWatters Thanks for the link. What do you think should be the maximum separation of two observed images that are identical to one another for them to qualify as product of gravitational lensing? (i.e. at what arc length separation can we be confident that the two images are not from the same object?)
@sophiecentaur I see, your point is very valid. But I'm talking about two objects that have already been resolved by the telescope. I'm actually more interested in the procedure which can confirm that the two images are from the same/different object(s).

Your "two objects" are, in fact 'two images' (until proved not to be). You could still be looking at an artefact - or not. There are many examples of double images in ordinary optics and it may not be obvious what you are really looking at. There are sometimes strategies for judging whether there are two distinct ones. In your case, I guess you could just look at the spectra of the two images. If they are indistinguishable then you could conclude they are of the same object.

 

[Vanadium 50]

You do know that there have been observations of a lensed supernova, one that has appeared in multiple images of the same galaxy. That seems to me to be pretty convincing.

Otherwise, you need to argue that something like this is going on:

 

[sophiecentaur]

observations of a lensed supernova,

Not many examples of two of those going off at the same time!

 

[Vanadium 50]

Indeed. There are also predictions on when we will see an image of the same supernova in another part of the lensed image. I think the next such date is a few years from now, but it seems to me that this is awfully predictive.

 

[sophiecentaur]

Indeed. There are also predictions on when we will see an image of the same supernova in another part of the lensed image. I think the next such date is a few years from now, but it seems to me that this is awfully predictive.

That is really amazing. But shouldn't surprise me, bearing in mind the large number of light years of distance (path lengths) involved. I was thinking of the images arriving at the same time. What sort of accuracy is involved here? Imagine being able to see the very start of the Nova process in detail with a narrow field telescope.

 

[PWiz]

Your "two objects" are, in fact 'two images' (until proved not to be). You could still be looking at an artefact - or not. There are many examples of double images in ordinary optics and it may not be obvious what you are really looking at. There are sometimes strategies for judging whether there are two distinct ones. In your case, I guess you could just look at the spectra of the two images. If they are indistinguishable then you could conclude they are of the same object.

My bad, I meant two images that have been resolved, not two objects. Thanks for the answer.

 

[Vanadium 50]

I don't know (or rather, don't remember) how precise the prediction is. I assume it depends on how well we know the details of the lensing galaxy, and since it's mostly dark matter, that has to mostly come from the other lensed images.

 

[sophiecentaur]

I don't know (or rather, don't remember) how precise the prediction is. I assume it depends on how well we know the details of the lensing galaxy, and since it's mostly dark matter, that has to mostly come from the other lensed images.

Are 'they' actually pointing telescopes in any particular direction in order to catch one?