Newswise — A group of EPFL scientists have discovered a method to utilize strong gravitational lensing to accurately gauge the mass of a quasar-hosting galaxy. This technique offers three times greater precision than any other approach and also allows for studying the evolution of these galaxies over cosmic time. Understanding the mass of quasar host galaxies contributes to our knowledge of early universe galaxy development and the formation of black holes. The findings have been documented in Nature Astronomy.
EPFL astrophysicist Frédéric Courbin, the senior author of the study, highlights the novel opportunity presented by gravitational lensing in obtaining reliable and precise mass estimates in the distant Universe. Traditional techniques often lack the required precision and are prone to biases, making the unprecedented precision and accuracy achieved through gravitational lensing truly groundbreaking.
Martin Millon, the lead author of the study and currently conducting research at Stanford University with the support of an SNF grant, emphasizes the significance of gravitational lensing in measuring the masses of host galaxies in the distant Universe. While previous measurements have been made, this is the first time such measurements have reached such remarkable levels of precision, all thanks to the implementation of gravitational lensing techniques.
Determining the mass of a quasar's host galaxy poses a challenge due to the nature of quasars themselves. Quasars represent luminous manifestations of supermassive black holes that consume surrounding matter and reside at the cores of their host galaxies. Measuring the weight of a quasar's host galaxy is particularly challenging for two reasons. Firstly, quasars are located at vast distances, making precise measurements a difficult task. Secondly, quasars emit intense brightness that tends to overshadow anything in their immediate vicinity, further complicating the measurement process.
Through gravitational lensing, the mass of the lensing object can be determined. Einstein's theory of gravitation provides insights into how objects in the foreground of the night sky, known as the gravitational lens, can bend light originating from objects in the background. This phenomenon gives rise to peculiar light rings, which are essentially distortions of the background object's light caused by the gravitational lens.
More than ten years ago, while cycling to the Sauverny Observatory, Courbin had a realization: he could merge quasars and gravitational lensing to determine the mass of a quasar's host galaxy. To achieve this, he needed to locate a quasar within a galaxy that also functioned as a gravitational lens.
A handful of gravitational lensing quasars observed so far
The SDSS database proved to be a valuable resource for identifying potential quasars exhibiting gravitational lensing. However, in order to confirm their findings, Courbin and his team needed to visually observe the lensing rings. In 2010, they secured observation time on the Hubble Space Telescope to study four selected candidates, out of which three exhibited lensing effects. Among the three, SDSS J0919+2720 stood out due to its distinct gravitational lensing rings, solidifying its significance in the study.
In the Hubble Space Telescope image of SDSS J0919+2720, two prominent objects in the foreground are evident, and both serve as gravitational lenses. Courbin suggests that these objects are most likely two merging galaxies. The left object is a luminous quasar embedded within a host galaxy that is too faint to be directly observed. On the right side, there is another galaxy that acts as the primary gravitational lens. Additionally, a faint companion galaxy can be seen on the far left of the image. The distinctive rings observed in the image result from the distortion of light originating from a background galaxy.
Computational lens modeling to the rescue
Through meticulous analysis of the gravitational lensing rings observed in SDSS J0919+2720, it is theoretically feasible to ascertain the masses of the two prominent objects. However, untangling the masses of these distinct entities would have been an insurmountable task without the recent advancement of a lens modeling technique based on wavelets. Co-author Aymeric Galan, who is presently affiliated with the Technical University of Munich (TUM) and supported by an SNF grant, played a crucial role in developing this innovative approach.
Galan highlights that comprehending the formation of supermassive black holes represents one of the major hurdles in astrophysics. Understanding their mass, how it relates to their host galaxies, and how it evolves over cosmic times are crucial for evaluating and confirming different theories of formation. These insights enable scientists to either reject or validate specific hypotheses regarding the origins of supermassive black holes.
Millon explains that in the nearby Universe, there is a notable observation that the most massive galaxies are accompanied by correspondingly massive black holes at their cores. This observation potentially implies that the growth of galaxies is influenced by the energy emitted by their central black holes, which is subsequently injected into the surrounding galaxy. However, to validate this theory, it is crucial to investigate these interactions not only in local environments but also in the distant Universe. By studying these phenomena across cosmic distances, researchers can gain a more comprehensive understanding of the dynamics between supermassive black holes and their host galaxies.
Gravitational lensing events are exceptionally rare, occurring in only about one out of a thousand galaxies. Considering that quasars are found in approximately one out of a thousand galaxies, the chances of encountering a quasar that also acts as a gravitational lens are estimated to be one in a million. However, the scientists are optimistic about the forthcoming Euclid mission, a joint endeavor by ESA (European Space Agency) and NASA. The mission is scheduled to be launched this summer using a Falcon-9 SpaceX rocket. With the Euclid mission, they anticipate the detection of hundreds of these rare lensing quasars, which will provide invaluable insights into the phenomenon of gravitational lensing.
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