Findings suggest need to reassess ideas about galaxy formation
Leonard Doublet/Swinburne University of Technology
A hefty red, dead galaxy in the early universe appears to have bulked up a bit too fast.
The galaxy, seen as it was when the universe was only 1.65 billion years old, weighs at least three times as much as the Milky Way, but has stopped making stars. Other galaxies at the time tend to be much smaller and continue to churn out stars. How such a monster was made in less than a billion years, then shut down so quickly isn’t clear, says Karl Glazebrook of Swinburne University of Technology in Australia. Finding the behemoth and possibly others like it may mean astronomers will have to rethink how galaxies are built to explain why some grow up fast, while others develop slowly, he and colleagues report in the April 6 Nature.
“The team has found an extreme galaxy, which is exciting,” says Peter Behroozi of the University of California, Berkeley, who was not involved in the study. The data, he says, offer evidence for a population of inactive galaxies early in the universe, which are extremely difficult to observe. Behroozi, however, is not convinced that the discovery warrants a rewrite of the story of galaxy formation just yet. “The galaxy is certainly not typical, but it is consistent with the broad diversity of galaxies coming out of theoretical models,” he says.
Glazebrook and colleagues used the Keck Observatory in Hawaii to study the spectrum — a catalog of light by wavelength — of the galaxy, called ZF-COSMOS-20115. The data confirmed that the galaxy did, in fact, exist in the early universe and revealed that it had stopped making stars 500 million to a billion years before its age when observed. The data also indicate the galaxy created the mass of more than a 1,000 suns each year. Typical galaxies at that time generate a mass of less than 100 suns annually. With such a rapid star formation rate, the galaxy probably grew to its monster mass in less than 100 million years, the researchers say.
Perhaps the galaxy got so massive by colliding and merging with another galaxy. Such a major merger can stir up and compress gas to high densities, which can trigger intense star formation. These events are typically short bursts, lasting less than 100 million years, compared with the typically billion-year timescales for star formation in normal galaxies. In these merger galaxies, gas gets consumed quickly because stars are forming fast. If the galaxy runs out of gas, star formation stops. Still, if this process happens and these massive galaxies exist, they are expected to be rare, says astronomer Dominik Riechers of Cornell University, who was not involved in the new study.
Such hefty galaxies should be rare because galaxies form as matter pools together and collapses due to gravity over cosmic time. When the universe was still young, very few regions of space would have had deep enough wells of gravity to form massive galaxies like this one, Riechers explains.
Glazebrook and colleagues’ earlier work, however, hints that ZF-COSMOS-20115 may not be such an oddity. In 2014, the team reported finding a bunch of dim, red objects, including ZF-COSMOS-20115, which fit the profile of massive, red and dead galaxies in the early universe (SN Online: 3/14/14). Similar studies of the other galaxies’ spectra could confirm if all of those candidates are inactive, too. If so, astronomers will have to figure out how larger numbers of deep gravity wells could develop to allow lots of galaxies to grow, merge and become red and dead just a billion years after the Big Bang, 13.8 billion years ago.
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