A Novel Technique Triples the Accuracy of Quasar Host Galaxy Mass Measurement Through Gravitational Lensing

Leveraging gravitational lensing, scientists from EPFL have remarkably increased the accuracy of determining the mass of galaxies that harbor quasars, thereby enriching our comprehension of galaxy evolution and black hole formation in the early universe.

EPFL scientists have devised a new approach employing strong gravitational lensing, which yields a precision that is threefold greater than any currently available method. This technique enables them to correctly assess a quasar-bearing galaxy’s mass and its progression over cosmic time.

This advancement in our understanding of the mass of galaxies that contain quasars enriches our knowledge of early universe galaxy evolution, and assists in the development of models for galaxy and black hole creation. These discoveries were recently documented in the scientific journal Nature Astronomy.

“The remarkable precision and accuracy we have achieved through gravitational lensing offer a fresh pathway to secure mass estimations in the distant Universe. Traditional techniques fail to provide the necessary precision and are prone to biases,” says Frédéric Courbin, an EPFL astrophysicist and the study’s senior author.

Martin Millon, the lead author of the study and currently at Stanford University on an SNF grant, explains, “Previously, we have measured the masses of host galaxies, but gravitational lensing allows us, for the first time, to make such measurements with extraordinary precision in the distant Universe.”

Merging gravitational lensing and quasars

A quasar is a brightly glowing manifestation of a supermassive black hole that consumes surrounding matter, located at the core of a host galaxy. The task of determining a quasar’s host galaxy’s mass is typically challenging due to the substantial distance of quasars and their intense brightness that outshines anything in close proximity.

Gravitational lensing lets us calculate the mass of the lensing object. Massive objects in the foreground of the night sky, known as gravitational lenses, can distort light from objects in the background, due to Einstein’s gravitational theory. This leads to unusual light rings that are distortions of the background object’s light caused by the gravitational lens.

Courbin had an insight over a decade ago while cycling to the Sauverny Observatory; he realized that quasars and gravitational lensing could be combined to determine a quasar host galaxy’s mass. This required finding a quasar located in a galaxy that also serves as a gravitational lens.

A few quasars acting as gravitational lenses have been spotted so far

The Sloan Digital Sky Survey (SDSS) database was an excellent resource for finding gravitational lensing quasar candidates, but Courbin needed to confirm their presence by observing the lensing rings. In 2010, he and his team used the Hubble Space Telescope to study four candidates, three of which demonstrated lensing. Among the three, SDSS J0919+2720 stood out due to its distinctive gravitational lensing rings.

The application of computational lens modeling

The analysis of the gravitationally lensed rings in SDSS J0919+2720 allows for the determination of the mass of the two bright objects… theoretically. The partition of the various object masses would not have been possible without the recent development of a wavelet-based lens modeling technique by co-author Aymeric Galan, who is currently at the Technical University of Münich (TUM) on an SNF grant.

Astrophysicist Aymeric Galan explains, “One of the greatest challenges in astrophysics is understanding the formation of supermassive black holes. Knowing its mass, comparing it to its host galaxy, and understanding its evolution over cosmic time enables us to either reject or validate certain formation theories.”

Millon adds, “In the local Universe, we observe that the most massive galaxies also host the most massive black holes at their center. This could suggest that the growth of galaxies is regulated by the amount of energy radiated by their central black hole and injected into the galaxy. However, to test this theory, we need to study these interactions not only locally but also in the distant Universe.”

Gravitational lensing events are exceptionally rare, occurring in about one in every thousand galaxies. Since quasars are found in roughly one out of every thousand galaxies, a quasar functioning as a lens is quite rare – a one-in-a-million occurrence. The scientists anticipate detecting hundreds of these lensing quasars with the upcoming ESA-NASA mission Euclid, which will be launched this summer on a Falcon-9 SpaceX rocket.

Reference: “Strong gravitational lensing by AGNs as a probe of the quasar–host relations in the distant Universe” by Martin Millon, Frédéric Courbin, Aymeric Galan, Dominique Sluse, Xuheng Ding, Malte Tewes, and S. G. Djorgovski, 1 June 2023, Nature Astronomy.
DOI: 10.1038/s41550-023-01982-2

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More about Gravitational Lensing

• EPFL (École polytechnique fédérale de Lausanne)
• Nature Astronomy
• Gravitational Lensing
• Sloan Digital Sky Survey (SDSS)
• ESA-NASA mission Euclid
• Falcon-9 SpaceX rocket