Little Red Dots in Webb Photos Turned Out to Be Quasars

In its first year of operation, the James Webb Space Telescope (JWST) made some profound discoveries. These included providing the sharpest views of iconic cosmic structures (like the Pillars of Creation), transmission spectra from exoplanet atmospheres, and breathtaking views of Jupiter, its largest moons, Saturn’s rings, its largest moon Titan, and Enceladus’ plumes. But Webb also made an unexpected find during its first year of observation that may prove to be a breakthrough: a series of little red dots in a tiny region of the night sky.

These little red dots were observed as part of Webb’s Emission-line galaxies and Intergalactic Gas in the Epoch of Reionization (EIGER) and the First Reionization Epoch Spectroscopically Complete Observations (FRESCO) surveys. According to a new analysis by an international team of astrophysicists, these dots are galactic nuclei containing the precursors of Supermassive Black Holes (SMBHs) that existed during the early Universe. The existence of these black holes shortly after the Big Bang could change our understanding of how the first SMBHs in our Universe formed.

The research was led by Jorryt Matthee, an Assistant Professor in astrophysics at the Institute of Science and Technology Austria (ISTA) and ETH Zürich. He was joined by researchers from the MIT Kavli Institute for Astrophysics and Space Research, the Cosmic Dawn Center (DAWN), the National Astronomical Observatory of Japan (NAOJ), the Niels Bohr Institute, the Max Planck Institute for Astronomy (MPIA), the Centro de Astrobiología (CAB), and multiple universities and observatories. Their findings were published in a study recently published in The Astrophysical Journal.

Scientists have known for some time that Supermassive Black Holes reside at the center of most massive galaxies. And whereas some are relatively dormant, like the SMBH located in the center of the Milky Way (Sagittarius A*), others are extremely active and are growing at the rate of several Solar masses a year. These fast-growing black holes power particularly luminous Active Galactic Nuclei (AGNs) – or quasars – which become so bright they temporarily outshine all the stars in their disk, the brightest of which are known as quasars.

Quasars are among the brightest objects known to astronomers and can be seen at the very edge of our expanding Universe. In recent years, though, astronomers have spotted several quasars and SMBHs in the early Universe that are larger than cosmological models predict. As Matthee explained in a recent ISTA press release:

“One issue with quasars is that some of them seem to be overly massive, too massive given the age of the Universe at which the quasars are observed. We call them the ‘problematic quasars.’ If we consider that quasars originate from the explosions of massive stars–and that we know their maximum growth rate from the general laws of physics, some of them look like they have grown faster than is possible. It’s like looking at a five-year-old child that is two meters tall. Something doesn’t add up.”

Mathee and his team identified the population of little red dots while studying images taken during the EIGER and FRESCO surveys, a large and medium first-year JWST campaign in which Mathee was involved. The EIGER campaign was specifically designed to search for rare blue supermassive quasars and their environments, and not for quasars in the early Universe. However, Webb‘s Near Infrared Camera (NIRCam) can acquire emissions spectra from all objects in the known Universe. These objects had been previously observed by Hubble and mistaken for regular galaxies.

But thanks to the NIRCam’s resolution, the ISTA-led team identified them as SMBHs almost by accident. According to Mathee, this accidental discovery could have profound implications for astronomy and cosmology:

“Without having been developed for this specific purpose, the JWST helped us determine that faint little red dots–found very far away in the Universe’s distant past–are small versions of extremely massive black holes. These special objects could change the way we think about the genesis of black holes. The present findings could bring us one step closer to answering one of the greatest dilemmas in astronomy: According to the current models, some supermassive black holes in the early Universe have simply grown ‘too fast’. Then how did they form?”

The team was able to make the distinction between galaxies and small quasars thanks to NIRCam’s detection of deep-red emission lines (aka. H? spectral lines) that are produced when hydrogen atoms are heated. They also found that the lines they observed had a wide-line profile, which they used to trace the motion of the hot hydrogen gas. “The wider the base of the H? lines, the higher the gas velocity,” said Mathee. “Thus, these spectra tell us that we are looking at a very small gas cloud that moves extremely rapidly and orbits something very massive like an SMBH.”

Just as important were the redshift values they obtained for these SMBGs (Z= 4.2-5.5), which indicate these objects existed more than 12 billion years ago – roughly 1 billion years after the Big Bang. Furthermore, they observed that these SMBHs were not overly massive like those visible in nearby galaxies today. As Mathee indicated:

“While the ‘problematic quasars’ are blue, extremely bright, and reach billions of times the mass of the Sun, the little red dots are more like ‘baby quasars.’ Their masses lie between ten and a hundred million solar masses. Also, they appear red because they are dusty. The dust obscures the black holes and reddens the colors.”

Eventually, the outflow of hydrogen gas will puncture the clouds of dust and gas that surround and obscure massive black holes (“dust cocoon”), and these smaller SMBHs will evolve into much larger ones. Thus, Mathee and his team hypothesized that the little red dots are small, red versions of giant blue SMBHs in the phase that predates the “problematic quasars.” Through follow-up observations, astronomers can conduct detailed studies of these baby SMBHs, which could lead to a better understanding of how problematic quasars come to exist.

“Black holes and SMBHs are possibly the most interesting things in the Universe. It’s hard to explain why they are there, but they are there,” Mathee concluded. “We hope that this work will help us lift one of the biggest veils of mystery about the Universe.”

Further Reading: ISTA, The Astrophysical Journal