How do we tell a distorted galaxy is gravitational lensing or intrinsic shape?

[William72]

I’ve been trying to understand the concept of gravitational lensing by looking at images from Hubble and JWST, but I’m struggling to grasp how we can be so sure the distorted galaxy we see isn’t just a strangely shaped one. What exactly tells us it’s a lensing effect and not an intrinsic structure?

[Matthew_G]

In gravitational lensing you see more than an odd shape: you get multiple images or a long curved arc that sits around a foreground mass. The arc lines up with the predicted mass distribution, and a lens model can reproduce where other images should appear. The background source is usually at a higher redshift, which spectroscopy confirms, and surface brightness is conserved along the image, so it looks like a warped version of the same galaxy rather than a new object. All of that together is what tells you it’s lensing, not an intrinsic structure.

[GaryL]

I once tried a very simple mass model and, yes, it could bend light to produce a curved arc around a fake cluster in the simulation. It felt plausible, but then you worry that an actually weird galaxy could just be arranged so the bright knots line up by chance. In practice, the cross-check is that the arc sits where the model says the critical curve should be and that you can match the redshift and the relative magnifications. Still, I’m not totally convinced every time.

[Aaron.G]

Do we rely on the redshift difference to confirm lensing, or is there another tell-tale?

[Evelyn.J]

I remember staring at a JWST image and the curves reminded me of something else, a pastry crust maybe, then someone pointed out the foreground mass distribution and the predicted image symmetry. It helped to focus on that instead of just the pretty curves. The messy part was separating weak distortions from noise in cluster outskirts, because the signals get faint. Anyway, the bottom line for me was: look for consistent image configurations across different wavelengths and check against a foreground mass map; if you see a coherent set of multiple images with the same spectrum at different places, that seems convincing.