A team of Norwegian and US astronomers, using data from Gemini North and the Nordic Optical Telescope (NOT), have measured the time delay in images of a quasar lensed by a foreground cluster of galaxies. The Gemini observations are the first published result obtained with the innovative Fast Turnaround (FT) mode of observing.
A distant quasar may have its light split into multiple images by a foreground galaxy cluster that acts as a gravitational lens. The light travels along different paths of differing lengths to form each of these images. Quasars themselves are intrinsically variable, so the observed fading and brightening of each image happens at different observed times. Measuring these “time delays” yields tight constraints on the mass distribution in the lensing cluster, as well as the lensing geometry, and hence cosmology.
The team monitored the redshift z=2.82 quasar SDSS J2222+2754 over the course of three years, using the NOT and Gemini+GMOS-N. They found a time delay of 48 and 722 days for two pairs of the quasar’s lensed images. The Gemini data were instrumental in refining the time delay measurements for the quasar image that leads the other image by ~ 2 years and hence predicts the behavior of other images of the quasar; continuing monitoring of the system will now allow further observations that take advantage of that 2 year peek into the future.
Under Gemini’s FT mode, users can submit proposals every month and (if accepted) receive data 1-4 months after their initial proposal idea. The mode can be used for any kind of scientifically valuable project that needs just a few hours of observing time. Since the program’s launch in January, it has been used to follow up discoveries of new solar system objects, obtain data sets needed to complete projects, and also for short, self-contained programs.