Possible small distant lensing galaxy
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JasonJason
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AlexandredOr
I think it's rather an overlap..
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JasonJason
How cam you tell it's an overlap?
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AlexandredOr
The 2 galaxies are on the quasi same line of sight. They aren't at the same distance, in this case the blue must be more distant. Its the definition of an overlap. As I don't believe in a lens here... Don't know if I'm clear enough....
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JasonJason
I still don't understand how you determine the distance without having reference to pixel data to determine distances? The blue could then in fact be closer couldn't it?
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Budgieye
moderator
Blue is tricky.
Blue can be nearby, as a small star-forming galaxy is blue. They tend to be larger in the images.
Blue can also be halfway across the universe, as star-forming galaxies have a supply of UV light that gets redshifted to blue. They tend to be small in the images.
It is still only a guess. But it looks too fuzzy to be a lensed galaxy.
Thank you for sending a zoomed-in image, it makes viewing much easier!
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Tom_Collett
scientist
in response to JasonJason's comment.
Good question, and the answer is pretty much "we make an educated guess".
Basically blue equals hot and since color is defined by the difference in brightness at different frequencies, redshifting is broadly irrelevant to the colors. Imagine an object emits 10 units of UV light, 8 of Blue, 6 of Red etc. We call color the difference in brightness at different wavelengths, so the Blue-Red color would be 2, and the pixel would be blue in the image. Now let's redshift it a bit so that the UV becomes blue, the blue goes to red etc. The new Blue-Red color would still be 2 (even though that's the UV-Blue color in the rest frame). If we redshift the object even further, it will eventually appear red, because there is a break in the emission spectrum (Not much gets emitted beyond the far ultraviolet), but that doesn't tend to happen to the sources likely to be found in spacewarps because they would be super-faint due to the distance.
Stars are of course in our own galaxy (too faint to see individually otherwise), but quasars look like blue stars, and are very distant - but are much rarer than stars. This isn't a star or quasar because it's fuzzy.
That galaxy looks like it would act as a lens if a background source was in the position shown, but as @budgieye says the source looks faint and fuzzy and most importantly doesn't look distorted into the typical shapes of lenses. Lensing bends and stretches things into arcs unless the source is very small - like a quasar (like in the training images). Since the source doesn't look like it's been lensed, it's probable that the source is in front of the edge-on spiral galaxy.
We need more than just blue light to say it's lensing - and there are sometimes lenses where the source isn't red, although these are rare.
For this system, the colors are good, but it's the lack of distortion in the blue light, that implies a chance alignment of two galaxies rather than a lens.
Hope that helps.
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JasonJason
in response to Tom Collett's comment.
Thank you both for the thorough and enlightening explanation, I am sure it will help others also.
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