Using gravitational lensing to accurately measure the mass of a quasar galaxy

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The strong lens system SDSS J0919+2720. Credit: Nature astronomy (2023). DOI: 10.1038/s41550-023-01982-2

A team of EPFL researchers has found a way to use the phenomenon of strong gravitational lensing to determine with about three times more precision than any other technique the mass of a galaxy containing a quasar, as well as their evolution over cosmic time. Knowing the mass of quasar host galaxies provides information on the evolution of galaxies in the early universe, to build scenarios of galaxy formation and black hole development. The results are published in Nature astronomy.

“The unprecedented precision and accuracy achieved with gravitational lensing provides a new avenue for robust mass estimates in the distant Universe, where conventional techniques lack precision and are susceptible to bias,” says the EPFL astrophysicist Frdric Courbin, senior author of the study.

“The masses of host galaxies have 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,” explains Martin Millon, lead author of the study and currently at Stanford University. on an NFC grant.

Combination of gravitational lensing and quasars

A quasar is a luminous manifestation of a supermassive black hole accreting surrounding matter, sitting at the center of a host galaxy. It is generally 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 dwarf anything in their vicinity.


Illustration of the quasar lens. Credit: EPFL/Austin Peel

Gravitational lensing allows us to calculate the mass of the lensing object. Thanks to Einstein’s theory of gravitation, we know how massive foreground objects in the night sky, the gravitational lens, can bend light from background objects. The resulting strange rings of light are actually distortions of the light from the background object by the gravitational lens.

Courbin was cycling to the Sauverny Observatory more than a decade ago when he realized he could combine the two quasars and gravitational lensing to measure the mass of a quasar’s host galaxy. For this, he had to find a quasar in a galaxy that also served as a gravitational lens.

A handful of lensed quasars observed so far

The Sloan Digital Sky Survey (SDSS) database was a great place to look for lensed quasar candidates, but to be sure, Courbin had to see the lensing rings. In 2010, he and his colleagues took the time to the Hubble Space Telescope to observe four candidates, three of which exhibited lenses. Of the three, one was notable for its distinctive gravitational lensing rings: SDSS J0919+2720.

The HST image of SDSS J0919+2720 seen here shows two bright objects in the foreground each acting as a gravitational lens, “probably two galaxies about to merge,” says Courbin. The one on the left is a bright quasar within a host galaxy too faint to be observed. The bright object on the right is another galaxy, the main gravitational lens. A faint object to the far left is a companion galaxy. The characteristic rings are distorted light from a background galaxy.

Computational modeling of lenses to the rescue

By carefully analyzing the gravitational lensing rings in SDSS J0919+2720, it is possible in principle to determine the mass of the two luminous objects. Untangling the masses of the various objects would have been impossible without the recent development of a wavelet-based lens modeling technique by co-author Aymeric Galan, currently at the Technical University of Mnich (TUM), also on an SNF fellowship .






Credit: EPFL/Austin Peel

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

“In the local Universe, we observe that the most massive galaxies also harbor the most massive black holes at their centers. This could suggest that the growth of galaxies is governed by the amount of energy radiated from their central black hole and injected into the galaxy. However, for To test this theory, we have yet to study these interactions not only locally but also in the distant universe,” explains Millon.

Gravitational lensing events are very rare, with one in a thousand galaxies exhibiting the phenomenon. Since quasars are seen in about one in every thousand galaxies, a lensing quasar is one in a million. Scientists plan to detect hundreds of these lensing quasars with ESA-NASA’s Euclid mission, which will launch this summer on a SpaceX Falcon-9 rocket.

More information:
Martin Millon et al, Strong gravitational lensing of AGNs as a probe of quasarhost relationships in the distant Universe, Nature astronomy (2023). DOI: 10.1038/s41550-023-01982-2

About the magazine:
Nature astronomy

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