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What are Einstein rings, and how do they form?

I'm analyzing Hubble data and have found a beautiful, nearly complete arc. My supervisor suggested it might be an Einstein ring. Beyond the basic "source, lens, and observer alignment," what are the precise requirements? Does the lensing mass distribution need to be perfectly symmetric, and how critical is the exact alignment?

 

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By Krithi Answered 1 year ago

To get that textbook-perfect ring, two conditions are paramount. First, the background source (a quasar or galaxy), the foreground lens (typically a massive galaxy or cluster), and the observer must be exquisitely aligned almost perfectly collinear. Second, and this is crucial, the projected mass distribution of the lens must be highly symmetric. A singular isothermal sphere model is the classic example. Even a slight ellipticity in the lens will break the ring into distinct arcs. In practice, most 'rings' we see are slightly disturbed, telling us about the lumpiness of dark matter in the lens.

 

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