Irion, Robert. "A quasar in every galaxy?"(PDF). Sky and Telescope. New Track Media. Archived from the original(PDF) on 21 March 2016. Retrieved 20 February 2014.
Irion, Robert. "A quasar in every galaxy?"(PDF). Sky and Telescope. New Track Media. Archived from the original(PDF) on 21 March 2016. Retrieved 20 February 2014.