Exploring mass sub-structure in the Einstein Cross lensing galaxy
A gravitational lens is created when a massive foreground object (a large intervening galaxy in this case of the Einstein Cross) bends and amplifies the light from a far more distant object (a quasar in this case) directly behind it. In certain circumstances, the background object may be distorted or even multiply-imaged into two or four images. The Einstein Cross (Q2237+0305) is such a 4-image lens, where the bulge of a nearby spiral galaxy is lensing a very distant quasar. In the case of the Einstein Cross, the more distant quasar appears much brighter than the intervening galaxy and four images of the same quasar are formed. Without the intervening galaxy, the quasar image would be a single object of much fainter magnitude.
The goal of the CIRPASS near-infrared spectroscopy program on the Einstein Cross is to explore mass substructures of 10^4 to 10^7 solar masses that may be present in the dark matter halo of the lensing galaxy responsible for the multiple quasar images. From comparative modeling of a smooth-potential case versus a halo with substructure the CIRPASS investigators are predicting different gravitational signatures. Simultaneous spectra obtained across the whole object are critical. CIRPASS allows to measure simultaneously the relative intensities (with respect to the Narrow Line Region [OIII] lines) and equivalent widths (with respect to the underlying continuum) of the Broad Line Region as sampled by the H-beta line. The observations obtained on Gemini South will allow to measure the filling factor of substructures, and to estimate the level of microlensing in progress. By use of the IFU, all images are measured simultaneously and without the problematic effects of slit losses and differential refraction, endemic to longslit observations.
More details can be found at http://www.gemini.edu/sciops/instruments/cirpass/cirpassDemoScience.html.
For more information on this and other CIRPASS results, see: http://www.ast.cam.ac.uk/~optics/cirpass/World-tour/Gemini-S/results.html.