[Pecos_Bill (OP)]

I have turned from computer solitare to classifying photos of galaxies on "galaxy zoo".

There is a gismo to say that a photo contains evidence of lensing of light -- in my opinion..

I often so mark photos which show 2, 3 or even  4 symmetrical and similar instances around a central object.

Is that plausibly lensing as I have seen in "Einsteins cross" or should it just be an arc near the image -- as described on the gismo/choice?

[evan_au]

It is possible for gravitational lensing to distort a very distant (point-like) object into several images which also are point-like.

But gravitational lenses are not like well-designed camera lenses - they tend to distort extended (closer) objects, often into arcs.

It is conceivable that an isolated elliptical galaxy (if it had a spherical dark matter halo) could approximate a camera lens; but galaxies tend to occur in clusters, so the light you see from a distant object is likely to take different paths through a cluster of overlapping gravitational lenses. This would cause significant distortion of the distant galaxy image.

It may be possible to see if the distorted fragments you see surrounding a galaxy are images of the same distant object by looking at the red-shift of each fragment. But this requires a lot more information than you get with the survey images on Galaxy Zoo. Measuring the red shift requires a lot more light (ie much longer exposures on a bigger telescope), so it is impractical at present.

If you mark potential lensing (and this is a consensus view of a number of members who review the same image), investigators can come back later for a closer look - for example to try and analyse the mass distribution of the intermediate object (including any dark matter halo).