These two objects represent a new distant class of quadruple, or cross-shaped, gravitational lenses which might eventually provide astronomers with a powerful new "magnifying glass" for probing a variety of characteristics of the universe.
The two gravitational lenses were discovered in about 100 fields of sky imaged by Hubble Space Telescope's Wide Field Planetary Camera 2. The first cross-shaped lens was discovered serendipitously by Eric Ostrander while processing HST images for the Medium Deep Survey, a Hubble key project led by Richard Griffiths. A second fainter and smaller lens was identified a few weeks later by Myungshin Im. Each configuration is in the form of four faint blue images situated symmetrically around a much brighter red elliptical galaxy. The distinctive cross-like pattern around an elliptical galaxy makes them unambiguous quadruple lens candidates, even before spectroscopic observations, which are typically used to confirm lenses.
Hubble's high resolution allows astronomers to extend the search to much fainter, and hence much farther lenses, than those few examples ground-based telescopes have uncovered relatively nearby. Hubble can explore a larger volume of space which could provide enough examples of this rare cross type of lensing to allow astronomers to address a variety of fundamental cosmological questions.
A gravitational lens is produced by the enormous gravitational field of a massive object which bends light to magnify, brighten and distort the image of a more distant object. Depending on the alignment between the objects and the mass distribution of the foreground lens, the more distant object can be smeared into arcs or split into pairs, triples, or even quadruple images.
Credit: Kavan Ratnatunga (Johns Hopkins University, Baltimore, MD)