JWST finds that accreting black holes may be to blame for reports suggesting the first galaxies were too big for their britches.
JWST's near-infrared camera captured this galaxy-filled field as part of the Cosmic Evolution Early Release Scientific Survey. Some of the most distant galaxies, which formed a billion years after the Big Bang, appear brighter than they would otherwise be because they contain accreting supermassive black holes. Credit: NASA, ESA, CSA, Steve Finkelstein (UT Austin)
The cosmos may not be broken after all. Shortly after the James Webb Space Telescope (JWST) began its science mission in 2022, astronomers discovered a half-dozen galaxies near the edge of the universe that appeared much more massive than anyone expected (see “Too Big, Too Soon” in the September issue). 2023 Astronomy).
The prevailing theory held that the first galaxies were relatively small clouds of gas, stars and dust that slowly grew into the majestic spirals and modest ellipticals that populate today's universe. But based on the amount of light these six galaxies shed into space, researchers estimated that each possessed at least 10 billion solar masses of material and one appeared to be 10 times more massive, or nearly the size of the Milky Way.
The observations implied that these galaxies, all less than a billion years after the Big Bang, were converting almost 100 percent of their gas into stars. Cosmologists couldn't understand how this was possible, at least within the standard model of cosmology known as Lambda Cold Dark Matter. But the researchers, led by Ivo Labbé of Swinburne University of Technology in Melbourne, Australia, stressed that their results were preliminary and that more detailed future observations were needed to rule out other possibilities.
A new study puts one of those alternatives front and center. Some primitive galaxies are discovered to have much less mass than they initially appeared. In these objects, not all the light comes from the stars. Instead, a significant portion radiates from an accretion disk surrounding a centrally located supermassive black hole.
A supermassive black hole can increase a galaxy's light output by 10 times or more. The black hole's intense gravity pulls in surrounding stars and gas clouds, tearing them apart in the process. As the material spirals toward the beast's event horizon (the point of no return where not even light can escape), it forms an accretion disk. The gas in the disk moves at speeds close to the speed of light and is heated to millions of degrees by friction. Large amounts of light radiate from the disk and make its host galaxy appear much more massive than it actually is. The same phenomenon drives the quasars and other active galaxies we see closer to home.
Katherine Chworowsky, a graduate student at the University of Texas at Austin, led a team that analyzed 118 massive galaxies (those weighing more than 10 billion solar masses) in the distant cosmos. The galaxies reside in 10 areas imaged by JWST within a small region known as the Extended Groth Strip, which lies near the boundary between Ursa Major and Boötes. “We are still seeing more galaxies than predicted (at these early times),” Chworowsky said in a press release, “although none of them are massive enough to 'break' the universe.” They report their results in the September issue of The astronomical magazine.
These supermassive black holes appear small and reddish in JWST images, and scientists have started calling them “little red dots.” The objects' spectra reveal fast-moving hydrogen gas, a characteristic of accretion disks. When the team removed these objects from their analysis, the first remaining galaxies were within theoretical predictions. “So the bottom line is that there is no crisis in terms of the standard model of cosmology,” said co-author Steven Finkelstein of the University of Texas at Austin.
The new results appear to have solved the biggest puzzle posed by the initial observations (that the first galaxies appeared to be too massive), but cosmologists are not out of the woods yet. Chworowsky's team also discovered about twice as many massive galaxies in their sample than the standard model predicts. Chworowsky suggests that galaxies in the early universe may have been more efficient at turning gas into stars than those that exist today.
Related: JWST discovers black holes even heavier than expected
The idea is not as strange as it might seem at first glance. Stars form in clouds of gas that cool enough for gravity to overcome the internal pressure of the gas. In the nearby universe, this process proceeds at a snail's pace: the Milky Way creates only a few new stars each year. But the higher densities and low abundance of elements heavier than helium in early cosmic times could well increase this rate significantly.
None of these discoveries would be possible without JWST. Although galaxies primarily emit ultraviolet and visible light, the expansion of the universe shifts this radiation toward the near-infrared part of the electromagnetic spectrum. Hubble sees ultraviolet, visible, and a bit of near-infrared, but it simply can't detect the most distant objects because their light has been redshifted out of its wavelength domain. And the smaller, infrared-sensitive Spitzer Space Telescope, which stopped operating in 2020, did not have the resolution to separate many of the closely spaced galaxies at these distances. Fortunately, JWST has the sensitivity, resolution, and wavelength coverage to reveal the details of these galaxies.
