Webb spotted a galaxy that was surprisingly active when the universe was just 430 million years old
Unraveling the mysteries of the early Universe is one of Webb's core missions. Finding and studying the first galaxies is an important part of his work. One of the universe's first galaxies is unusually bright, and researchers are wondering why. It looks like Webb has found the answer.
The galaxy in question is called GN-z11, and it existed when the Universe was less than half a billion years old. Hubble first noticed it in 2016 using the Spitzer Space Telescope. At that time, it was the most distant and ancient galaxy ever discovered. In the paper announcing the discovery, the authors write: “GN-z11 is luminous and young, but moderately massive, suggesting rapid accumulation of stellar mass in the past.”
They also write: “Future objects will be able to find the progenitors of such galaxies at higher redshifts and probe the cosmic epoch of the onset of reionization.” Webb is currently in the process of fulfilling such a mission. In addition, he took a closer look at the GN-z11.
The discoverers suggested that the galaxy's high luminosity might be caused by an active galactic nucleus, but were not sure. A new study based on Webb's observations shows they were right. It appears that the galaxy's luminosity comes from a supermassive black hole (SMB) at the center of the galaxy, which illuminates it while actively accumulating matter. One of the characteristic signs is the accumulation of gas near the SBS.
“We discovered extremely dense gas, which is often found in the vicinity of gas-accreting supermassive black holes,” explained principal investigator Roberto Maiolino from the Cavendish Laboratory and the Kavli Institute of Cosmology at the University of Cambridge (UK). “These were the first clear indications that GN-z11 harbors a black hole that is consuming matter.”
Scientists know that the region near the SBS is extremely hot and that gas clumps form there. The powerful gravity of the hole creates a rotating accretion disk near it, and the matter in the disk can accelerate to relativistic speeds. At these speeds, molecules collide and experience friction. This releases heat, the temperature of which can reach millions of degrees. Extreme heat drives gas outward at extremely high speeds, but it can also lead to the formation of dense clumps like what Webb discovered in GN-z11.
The clumps have no metallicity (elements heavier than helium), so they are likely to be of a pristine nature, and not contaminated by heavier elements that could only appear in subsequent generations of stars.
We have never seen the first stars of the Universe – the Population III stars. But, being the very first stars, they formed from hydrogen and helium – all that was available at the time. Finding these first stars is an important goal in astronomy, so finding similar pristine clumps is very important. The gas clumps discovered by Webb also consist only of hydrogen and helium, so they may be precursors to the formation of Population III stars.
“The fact that we don't see anything other than helium suggests that this clump must be fairly pure,” Maiolino said. “This is what was expected by theory and modeling in the vicinity of particularly massive galaxies from these eras – there should be pockets of pristine gas in the halo that could 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 Webb found evidence of this. The powerful space telescope also discovered strong winds of 800-1000 km/s-1 near the black hole – another result of the processes occurring in actively accreting black holes. (Some rare exploding galaxies can also produce powerful winds, but they have less ionization.)
Webb's NIRCam (near-infrared camera) detected an extended component indicative of a host galaxy and a central compact source whose colors match those of the accretion disk surrounding the black hole, says researcher Hannah Uebler, also of the Cavendish Laboratory and the Kavli Institute.
There is little doubt that there is a black hole and its accretion disk at the center of GN-z11. But the fact that this galaxy's extreme luminosity is due to a black hole raises interesting questions. They are related to black hole seeds and Eddington velocity.
Scientists believe black holes in the early Universe may have formed differently from stellar-mass black holes, which form when a star collapses under its own gravity. Instead, ancient black holes formed from seeds—clumps of matter massive enough to collapse directly into black holes. There can be large, intermediate and small black holes. The researchers behind these results write that the black hole “…accretes at a rate approximately five times the Eddington velocity. These properties are consistent with both heavy-nucleation scenarios and intermediate- and light-nucleation scenarios experiencing episodic super-Eddington events.” phases.”
The Eddington velocity is the rate at which a black hole must accumulate matter to reach Eddington limit. The Eddington limit is the maximum luminosity that an object can achieve as long as its external radiative force is equal to its internal gravitational force.
But black holes can exceed the Eddington limit during super-Eddington episodes. These episodes may explain the rapid emergence of SMBHs in the first billions of years of the Universe's existence. Super-Eddington episodes involve radiatively ineffective accretion and are often accompanied by powerful outflowing winds and jets.
If the researchers are right, then they have unraveled the mystery of this extremely ancient and extremely luminous galaxy. “Our find explains the high luminosity of GN-z11…” the authors write.
