Unlocking the mysteries of the early Universe is one of the JWST’s primary endeavours. Finding and examining some of the first galaxies is an important part of its work. One of the Universe’s first galaxies is extraordinarily luminous, and researchers have wondered why. It looks like the JWST has found the answer.
The galaxy at issue is named GN-z11, and it existed when the Universe was less than half a billion years old. The Hubble first spotted it in 2016, with help from the Spitzer Space Telescope. At the time, it was the most distant, ancient galaxy ever spotted. In the paper announcing the discovery, the authors wrote, “GN-z11 is luminous and young, yet moderately massive, implying a rapid build-up of stellar mass in the past.”
They also wrote that “Future facilities will be able to find the progenitors of such galaxies at higher redshift and probe the cosmic epoch at the beginning of reionization.” Now that the JWST is deep into its mission, that’s exactly where we find ourselves. It also took a closer look at GN-z11.
The discoverers suggested that the galaxy’s high luminosity could be caused by an active galactic nucleus (AGN) but weren’t certain. New research based on JWST observations shows that they were right. It looks like the galaxy’s luminosity comes from a supermassive black hole (SMBH) in the galaxy’s centre, lighting it up as it actively accretes matter. One of the telltale signs is a gas clump near the SMBH.
“We found extremely dense gas that is common in the vicinity of supermassive black holes accreting gas,” explained principal investigator Roberto Maiolino of the Cavendish Laboratory and the Kavli Institute of Cosmology at the University of Cambridge in the United Kingdom. “These were the first clear signatures that GN-z11 is hosting a black hole that is gobbling matter.”
Scientists know that the region near an SMBH is extremely hot and that gas clumps form near there. The hole’s powerful gravity creates a swirling accretion disk of material near it, and the material in the disk can be accelerated to relativistic speeds. At those speeds, the molecules collide and generate friction. That generates heat that can reach a temperature of millions of degrees. The extreme heat drives gas outward at extremely high speeds, but it can also drive the gas to form dense clumps like the ones JWST found at GN-z11.
The clump lacks metallicity, so it’s likely primordial in nature, uncontaminated by heavier elements that would only later be created by successive generations of stars.
We’ve never seen the Universe’s first stars, the Population III stars. But as the very first stars, they formed from hydrogen and helium, all that was available at the time. Finding those first stars is an important goal in astronomy, so finding these similarly pristine clumps is important. The gas clumps found by JWST are also made only of hydrogen and helium, so they could be precursors to the formation of Population III stars.
“The fact that we don’t see anything else beyond helium suggests that this clump must be fairly pristine,” said Maiolino. “This is something that was expected by theory and simulations in the vicinity of particularly massive galaxies from these epochs – that there should be pockets of pristine gas surviving in the halo, and these may collapse and form Population III star clusters.”
Two more pieces of evidence support the black hole hypothesis. Accreting black holes produce ionized chemical elements, and the JWST found evidence of them. The powerful space telescope also detected high winds with velocities of 800 to 1000 km/s-1 near the black hole, another result of the processes involved in actively accreting black holes. (Some rare starburst galaxies can also produce powerful winds, but they show less ionization.)
“Webb’s NIRCam (Near-Infrared Camera) has revealed an extended component, tracing the host galaxy, and a central, compact source whose colours are consistent with those of an accretion disc surrounding a black hole,” said investigator Hannah Übler, also of the Cavendish Laboratory and the Kavli Institute.
There doesn’t seem to be much doubt that GN-z11 has a black hole and its accretion disk in its center. But the fact that this galaxy’s extreme luminosity is powered by a black hole raises interesting questions. It has to do with black hole seeds and the Eddington rate.
Scientists think that black holes in the early Universe could have formed differently than stellar mass black holes, which form when a star collapses under its own gravity. Instead, these ancient black holes formed from seeds, collections of matter massive enough to collapse directly into black holes. There could be large, intermediate, and small black hole seeds. The researchers behind these results write that the black hole is “… accreting at about five times the Eddington rate. These properties are consistent with both heavy seeds scenarios and scenarios considering intermediate and light seeds experiencing episodic super-Eddington phases.”
The Eddington rate is the rate at which a black hole has to accrete matter to reach the Eddington limit. The Eddington limit is the maximum luminosity an object can reach while its outward force of radiation is equal to its inward force of gravity.
But black holes can exceed the Eddington limit during super-Eddington episodes. Those episodes may be able to explain the rapid assembly of supermassive black holes (SMBHs) in the Universe’s first billion years. Super-Eddington episodes are associated with radiatively inefficient accretion and are often accompanied by powerful outflowing winds and jets.
If the researchers are correct, then they’ve figured out the mystery behind this extremely ancient and extremely luminous galaxy. “Our finding explains the high luminosity of GN-z11…,” the authors write.
Note: The research on the pristine gas clump in GN-z11’s halo has been accepted for publication in Astronomy & Astrophysics. The results of the study of GN-z11’s black hole were published in the journal Nature on 17 January 2024