(via APOD: 2013 January 2)
The Einstein Cross Gravitational Lens
Image Credit & Copyright: J. Rhoads (Arizona State U.) et al., WIYN, AURA, NOAO, NSF
What happens when a bright quasar is situated right behind the massive nucleus of a foreground galaxy? A duplication effect is created as light from the quasar is bent by relativistic gravitational lensing, creating an Einstein Cross. The relative position and brightness of the quasar can be changed by the mass distribution in the galaxy, even creating delays of entire days as the light moves around.
This example of light being lensed around a massive elliptical galaxy comes from the Subaru Telescope:
PG 1115+080: A Gravitational Cloverleaf
Credit: CISCO, Subaru 8.3-m Telescope, NAOJ
In the case of the Einstein Cross here, though, the galaxy is a spiral, and, interestingly, the intervening dust in the galaxy itself does not significantly shift the light of the quasar through scattering. Amusingly, this configuration was noted because a sky survey found a low redshift galaxy whose nucleus seemed to be exactly like that of a high redshift quasar - based on the spectroscopy. Also of interest, the individual stars in the galaxy can create noticeable shifts in the brightness of the quasar through microlensing effects. Overall, they are much smaller than the effect of the whole galaxy, but they do create fluctuations that can be detected.