Einstein Cross - why not a ring?

[erick]

I've just learnt of the Einstein Cross.

Just wondering why it is four separate images and not a ring?

I would imagine that, in a world of perfection, a point source of light directly behind an object with a perfectly "spherical" gravitational well should be observed as a gravitationally lensed circular ring of light. Correct? I also understand that no perfect ring has been observed as yet in visible wavelengths?

So the fact that the QSO behind galaxy PGC 69457 is seen as four images is just pointing to the imperfections of the system: The geometrical alignment? The particular geometry of the gravitational well around this galaxy? Am I on the right track?

[Robh]

Eric,

I agree with your assessment.
I think if the foreground mass was symmetrically perfect along the axis or line of sight, the more distant object would appear as a ring. But all three, the distant object, the foreground mass and the observer should be on this axis.
However, in this case, the foreground mass is a galaxy whose mass is distributed unevenly along the line of sight. Hence, the images of the distant quasar are "focussed" at varying angles away from this axis. In this case, four of the images are focussed coincidently towards our line of sight.

Regards, Rob

[pgc hunter]

To get a ring, you need a spherical mass between our point of view and the quasar. A perfect ring also requires exact alignment of the viewer, the lensing mass and the quasar.

[Robh]

The foreground mass that does the lensing does not have to be spherical to form a ring.
See ...
http://hubblesite.org/newscenter/archive/releases/2008/04/

Rob