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Identifying Red Supermassive Black Holes in the Early Universe

A team of astronomers, led by Dr. Lukas Furtak and Professor Adi Zitrin from Ben-Gurion University, analyzed images from the James Webb Space Telescope and detected an extremely red, gravitationally lensed supermassive black hole in the early universe.

Obscured behind a thick veil of dust that dims its light, this ancient black hole was found to be significantly larger relative to its host galaxy than those in the more local universe, according to the astronomers’ measurements published in Nature.

Launched two years ago, the James Webb Space Telescope (JWST) has revolutionized the understanding of early galaxy formation. The telescope has revealed an abundance of brighter, more massive galaxies in the early universe than previously predicted and detected new types of celestial objects.

The group of astronomers detected what seemed to be a lensed quasar-like object from the early universe in images from the JWST. Quasars are bright, active galactic nuclei: supermassive black holes at the centers of galaxies rapidly accumulating material.

The accretion of material onto the black hole emits large amounts of radiation that outshine the host galaxy, giving it a compact, bright, star-like appearance. The JWST images in which Furtak and Zitrin identified the object were taken for the UNCOVER program. This program, led by Ivo Labbé from Swinburne University of Technology and Rachel Bezanson from the University of Pittsburgh, imaged the field of the galaxy cluster Abell 2744 to an unprecedented depth.

The cluster has a large mass that bends spacetime, creating a gravitational lens effect that magnifies the background galaxies behind it. This allows astronomers to observe even more distant galaxies that would otherwise be impossible to see.

We were very excited when JWST started sending its first data. We were scanning the data that arrived for the UNCOVER program, and three very compact yet red-blooming objects prominently stood out and caught our eyes. Their “red-dot” appearance immediately led us to suspect that it was a quasar-like object.

Dr Lukas Furtak, Postdoctoral Researcher and Lead Author, Ben-Gurion University of the Negev

Furtak and the UNCOVER group started investigating the object.

We used a numerical lensing model that we had constructed for the galaxy cluster to determine that the three red dots had to be multiple images of the same background source, seen when the Universe was only some 700 million years old.

Adi Zitrin, Professor, Astronomer and Study Lead Author, Ben-Gurion University of the Negev

Bezanson says, “Analysis of the object's colors indicated that it was not a typical star-forming galaxy. This further supported the supermassive blackhole hypothesis.”

Prof. Rachel Bezanson is from the University of Pittsburgh and is the co-lead of the UNCOVER program. Bezanson  adds, “Together with its compact size, it became evident this was likely a supermassive black hole, although it was still different from other quasars found at those early times.”

The discovery of the uniquely red and compact object was published last year in the Astrophysical Journal. But that was just the beginning of the story. The team then acquired JWST/NIRSpec data of the three images of the “red dot” and analyzed the data.

Prof. Ivo Labbé, Co-Lead of the UNCOVER program, states, “The spectra were just mind-blowing. By combining the signal from the three images together with the lensing magnification, the resulting spectrum is equivalent to approximately 1700 observing hours by JWST on an unlensed object, making it the deepest spectrum JWST has obtained for a single object in the early universe.”

Dr Furtak says, “Using the spectra, we managed to not only confirm that the red compact object was a supermassive black hole and measure its exact redshift, but also obtain a solid estimate for its mass from the width of its emission lines. Gas is orbiting in the gravitational field of the black hole and achieves very high velocities that are not seen in other parts of galaxies. Because of the Doppler shift, light emitted by the accreting material is red-shifted on one side and blue-shifted on the other side, according to its velocity. This causes emission lines in the spectrum to become broader.”

The measurement led to another surprising discovery, published in Nature: the black hole's mass appears disproportionately high relative to the mass of its host galaxy.

All the light of that galaxy must fit within a tiny region the size of a present-day star cluster. The gravitational lensing magnification of the source gave us exquisite limits on the size. Even packing all the possible stars into such a small region, the black hole ends up being at least 1 % of the total mass of the system. In fact, several other supermassive black-holes in the early Universe have now been found to show a similar behavior, which lead to some intriguing views of black hole and host galaxy growth, and the interplay between them, which is not well understood.

Jenny Greene, Professor and Lead Author, Princeton University

Astronomers do not know how supermassive black holes grow. Two possibilities are that they developed from the remnants of stars or that they formed directly from matter that collapsed into black holes in the early universe.

Zitrin says, “In a way, it's the astrophysical equivalent of the chicken and egg problem. We do not currently know which came first–the galaxy or black hole, how massive the first black holes were, and how they grew.”

Since the James Webb Space Telescope has recently detected many more “little red dots” and other active galactic nuclei, we should soon have a better understanding of these phenomena.

Journal Reference:

Furtak, J. L. et al. (2024) A high black hole to host mass ratio in a lensed AGN in the early Universe. Nature.


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