In addition, not everyone believes Livermore’s results. Rychard Bouwens, an astrophysicist at Leiden University in the Netherlands, argues in a paper submitted to The Astrophysical Journal that Livermore didn’t properly subtract the light from the galaxy clusters that make up the gravitational lens. As a result, he said, the distant galaxies aren’t as faint as Livermore and colleagues claim, and astronomers have not found enough galaxies to conclude that stars ionized the universe.
Supremacy of Supermassive Black Holes
If stars couldn’t get the job done, perhaps supermassive black holes could. Beastly in size, up to a billion times the mass of the sun, supermassive black holes devour matter. They tug it toward them and heat it up, a process that emits lots of light and creates luminous objects that we call quasars. Because quasars emit way more ionizing radiation than stars do, they could in theory reionize the universe.
The trick is finding enough quasars to do it. In a paper posted to the scientific preprint site arxiv.org last month, astronomers working with the Subaru Telescope announced the discovery of 32 quasars that are about a 10th as bright as ones identified before. With such faint quasars, the astronomers should be able to calculate just how much ultraviolet light these supermassive black holes emit, said Michael Strauss, an astrophysicist at Princeton University and a member of the team. The researchers haven’t done the analysis yet, but they expect to publish the results in the coming months.
The oldest of these quasars dates back to around a billion years after the Big Bang, which seems about how long it would take ordinary black holes to devour enough matter to bulk up to supermassive status.
This is why another recent discovery is so puzzling. A team of researchers led by Richard Ellis, an astronomer at the European Southern Observatory, was observing a bright, star-forming galaxy seen as it was just 600 million years after the Big Bang. The galaxy’s spectrum — a catalog of light by wavelength — appeared to contain a signature of ionized nitrogen. It’s hard to ionize ordinary hydrogen, and even harder to ionize nitrogen. It requires more higher-energy ultraviolet light than stars emit. So another strong source of ionizing radiation, possibly a supermassive black hole, had to exist at this time, Ellis said.
One supermassive black hole at the center of an early star-forming galaxy might be an outlier. It doesn’t mean there were enough of them around to reionize the universe. So Ellis has started to look at other early galaxies. His team now has tentative evidence that supermassive black holes sat at the centers of other massive, star-forming galaxies in the early universe. Studying these objects could help clarify what reionized the universe and illuminate how supermassive black holes formed at all. “That is a very exciting possibility,” Ellis said.
All this work is beginning to converge on a relatively straightforward explanation for what reionized the universe. The first population of young, hot stars probably started the process, then drove it forward for hundreds of millions of years. Over time, these stars died; the stars that replaced them weren’t quite so bright and hot. But by this point in cosmic history, supermassive black holes had enough time to grow and could start to take over. Researchers such as Steve Finkelstein, an astrophysicist at the University of Texas at Austin, are using the latest observational data and simulations of early galactic activity to test out the details of this scenario, such as how much stars and black holes contribute to the process at different times.
His work — and all work involving the universe’s first billion years — will get a boost in the coming years after the 2018 launch of the James Webb Space Telescope, Hubble’s successor, which has been explicitly designed to find the first objects in the universe. Its findings will probably provoke many more questions, too.