Using the NASA/ESA/CSA James Webb Space Telescope, astronomers have observed an extremely red, quasar-like object — named A2744-QSO1 — behind the strongly lensing galaxy cluster Abell 2744. Its colors suggest that the black hole in A2744-QSO1 lies behind a thick veil of dust obscuring much of its light. The researchers have also measured the black hole’s mass (40 million solar masses) and discovered that it was significantly more massive, compared to its host galaxy, than what has been seen in more local examples. The discovery suggests that it may represent the missing link between black hole seeds and the first luminous quasars.
“We were very excited when Webb started sending its first data,” said Dr. Lukas Furtak, a postdoctoral researcher at Ben-Gurion University of the Negev.
“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.”
“We used a numerical lensing model that we had constructed for the Abell 2744 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,” said Dr. Adi Zitrin, also from Ben-Gurion University of the Negev.
“Analysis of the object’s colors indicated that it was not a typical star-forming galaxy,” said Professor Rachel Bezanson, an astronomer at the University of Pittsburgh.
“This further supported the supermassive black hole hypothesis.”
“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 astronomers then analyzed the JWST/NIRSpec spectra of A2744-QSO1.
“The spectra were just mind blowing,” said Swinburne University of Technology’s Professor Ivo Labbé.
“By combining the signal from the three images together with the lensing magnification, the resulting spectrum is equivalent to 1,700 observing hours by Webb on an unlensed object, making it the deepest spectrum Webb has obtained for a single object in the early Universe.”
“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,” Dr. Furtak said.
“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.”
But the measurement led to yet another surprise: the black-hole’s mass seems to be excessively high compared to the host galaxy’s mass.
“All the light of that galaxy must fit within a tiny region the size of a present-day star-cluster,” said Dr. Jenny Greene, an astronomer at Princeton University.
“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.”
Astronomers do not know if such supermassive black-holes grow, for example, from stellar remnants, or perhaps from material that directly collapsed into black holes in the early Universe.
“In a way, it’s the astrophysical equivalent of the chicken and egg problem,” Professor Zitrin said.
“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 many more such ‘little red dots’ and other active galactic nuclei have recently been detected with Webb, hopefully, we will have a better idea soon.”
The team’s results appear in the journal Nature.
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L.J. Furtak et al. A high black hole to host mass ratio in a lensed AGN in the early Universe. Nature, published online February 14, 2024; doi: 10.1038/s41586-024-07184-8