Quasars are distant galactic nuclei generating spectacular amounts of energy by matter accretion onto their central supermassive black holes. The precise geometry and origin of this huge activity are still largely unknown, and direct spatial resolution of the emitting regions from such distant monsters is not currently possible. However, the study of rare cosmic phenomena called “double quasars” is helping to understand the accretion physics in the vicinity of supermassive black holes, as well as the structure and composition of galaxies, and the large scale properties of the universe as a whole. Double quasars are thus unique tools to investigate several poorly-known details of our universe.
When a massive galaxy lies close to the straight line joining the Earth and a distant quasar, its gravitational field produces two images, A and B, of the background quasar. This monster with two faces is a double quasar (top panel of figure below), which is caused by the unlikely alignment of a galaxy and an active galactic nucleus, and representing a confirmation on cosmological scales of the General Theory of Relativity that Albert Einstein presented 100 years ago (on 25 November 1915) to the Prussian Academy of Sciences. Quasars are also variable objects, so a prominent variation in the quasar light output (e.g., the luminosity decrement in the top panel of the figure) is seen in both images with a time delay (due to different paths to the Earth), and possibly distorted by gravitational effects of stars in the lensing galaxy (gravitational microlensing effects). In general, a long-timescale fluctuation of the quasar luminosity will be observed as two parallel flux variations in the A and B images.
As the search for new double quasars is a task of great importance in cosmology, we initially selected candidates in the Sloan Digital Sky Survey III (SDSS-III) database containing more than 100,000 quasars. These two-component candidates consisted of a confirmed quasar (A component) and a neighbour quasar-like source without spectroscopic identification (B component). We then designed an efficient plan to find the wanted objects from a list of 13 suspects (candidates). Deep optical imaging at the Maidanak Astronomical Observatory (Uzbekistan) in June 2015 allowed us to detect evidence for the presence of a lensing galaxy in three out of the 13 candidates. In addition, as expected for double quasars, the two components of the three targets varied in parallel on a 10-year timescale. After selecting one of these superb candidates (SDSS J1442+4055), optical spectroscopy with the Liverpool Telescope on 29 August 2015 confirmed its double quasar nature, since the spectra of A and B were basically identical. Regarding the targets that did not show evidence for a lensing galaxy, one of them (SDSS J2153+2732) displayed significant anti-parallel variations in the flux of its two components. These observations are better explained by a scenario involving two different distant quasars, i.e., a quasar pair (bottom panel of figure). Very recent optical spectroscopy through the “fast-track” service programme with the Nordic Optical Telescope fully supported that we observe two quasars instead of two images of the same quasar (there are evident discrepancies between the spectra of both components).
A main conclusion of our work is that a selection of superb candidates (i.e., candidates showing evidence for the existence of a lensing galaxy, as well as parallel flux variations on a long timescale) leads to an impressive efficiency in discovering double quasars. While the SDSS-III BOSS quasar lens survey has recently gotten a significant success with standard searching techniques and large spectroscopic programmes, we are following a different way for discovering two-face monsters. We are conducting a coordinated project to identify superb candidates in the SDSS-III and PanSTARRS1 databases, trying to efficiently find unique tools to study the universe and saving large amounts of spectroscopic observing time. Our collaboration includes teams of astronomers based at institutes of the National Academy of Sciences of Ukraine, the National Tsing Hua University of Taiwan and the Universidad de Cantabria in Spain. Another international collaboration is also using variability to search for lensed quasars in the Dark Energy Survey.
Feature image credit: ‘Artists concept of a double black hole’ by NASA, ESA, and G. Bacon (STScI). CC BY 2.0 via Hubble ESA Flickr.