Weigh quasar galaxies with precision

June 13, 2023

(Nanowerk News) A research team from EPFL has found a way to use the phenomenon of strong gravitational lensing to determine with precision – about 3 times more accurate than other techniques – the masses of galaxies containing quasars, as well as their evolution in cosmic time. Knowing the masses of quasar host galaxies provides insight into the evolution of galaxies in the early universe, to construct scenarios of galaxy formation and black hole development.

The results are published in Natural Astronomy (“Strong gravitational lensing by AGN as an investigation of quasar-host relationships in the distant universe”).


“The unprecedented precision and accuracy achieved with gravitational lensing provides new avenues for obtaining robust mass estimates in the distant universe, where conventional techniques lack precision and are prone to bias,” said EPFL astrophysicist Frédéric Courbin, senior author of the study. the.

“The mass of host galaxies has been measured in the past, but thanks to gravitational lensing this is the first time the measurement has been so precise in the distant universe,” said Martin Millon, lead author of the study and current at Stanford University. on SNF grants.

Combines gravitational lensing and quasars

Quasars are luminous manifestations of matter-accumulating supermassive black holes around them, sitting at the center of their host galaxies. It is usually difficult to measure how heavy a quasar’s host galaxy is because quasars are very distant objects, and also because they are so bright that they illuminate everything around them. Gravitational lensing quasar. (Image: EPFL/Austin Peel, CC-BY-SA)

Gravity lensing allows us to calculate the mass of the lensed object. Thanks to Einstein’s theory of gravity, we know how a massive object in the foreground of the night sky – a gravitational lens – can bend light coming from background objects. The result is a strange ring of light, which is actually the distortion of light from background objects by gravitational lensing.

Courbin was cycling to the Sauverny Observatory just over a decade ago, when he realized he could combine the two – quasars and gravitational lensing – to measure the mass of the quasar’s host galaxy. To do this, he must find a quasar in the galaxy that also acts as a gravitational lens.

A number of gravitational lensing quasars have been observed so far

The Sloan Digital Sky Survey (SDSS) database is a great place to look for gravitational lensing quasar candidates, but to be sure, Courbin has to look at the lensing rings. In 2010, he and colleagues used the Hubble Space Telescope to observe 4 candidates, 3 of which showed lensing. Of the three, one stands out for its gravity lensing ring characteristics: SDSS J0919+2720.

The HST image of SDSS J0919+2720 shows two bright objects in the foreground each acting as gravitational lenses, “possibly two galaxies in the process of merging,” Courbin explained. The one on the left is a bright quasar, within its parent galaxy that is too faint to observe. The bright object on the right is another galaxy, a prime gravitational lens. The faint object on the far left is a companion galaxy. The characteristic rings are the distorted light emanating from the background galaxy.

Computational lens modeling to the rescue

By carefully analyzing the gravitationally lensed rings at SDSS J0919+2720, the masses of the two bright bodies can be determined… in principle. Decomposition of the masses of various objects would not have been possible without the recent development of a wavelet-based lens modeling technique by co-author Aymeric Galan, currently at the Technical University of Münich (TUM), also on an SNF grant.

“One of the biggest challenges in astrophysics is understanding how supermassive black holes form,” explains Galan. “Knowing its mass, how it compares to its host galaxy, and how it evolves through cosmic time, is what allows us to discard or validate certain theories of formation.”

“In the local Universe, we observe that the most massive galaxies also host the most massive black holes at their centers. This could show that galaxy growth is governed by the amount of energy emitted by its central black hole and injected into the galaxy. However, to test this theory, we still need to study these interactions not only locally but also in distant universes,” Millon explained.

Gravitational lensing events are extremely rare, with one galaxy in a thousand revealing the phenomenon. Since quasars are visible in about one every thousand galaxies, those that act as lenses are one in a million. Scientists hope to detect hundreds of these lensing quasars with ESA-NASA’s Euclid mission, which will launch this summer on SpaceX’s Falcon-9 rocket.

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