Using the Atacama Large Millimeter/submillimeter Array (ALMA), astronomers have discovered a new kind of galaxy which, although very old — formed less than a billion years after the Big Bang — produces new stars at a rate of more than 100 solar masses per year.
An artist’s impression of a quasar and neighboring merging galaxy. The galaxies observed by Decarli et al are so distant that no detailed images are possible at present. This combination of images of nearby counterparts gives an impression of how they might look in more detail. Image credit: Max Planck Institute for Astronomy / NASA / ESA / Hubble.
The discovery could help solve a cosmic puzzle — a population of massive elliptical galaxies from when the Universe was only 1.5 billion years old, a tenth of its current age.
After first observing these galaxies in 2014, astronomers proposed that they must have been created from hyper-productive precursor galaxies, which is the only way so many stars could have formed so quickly.
But they had never seen anything that fit the bill for these precursors until now.
This newfound population could solve the mystery of how these massive galaxies came to have hundreds of billions of stars in them when they formed only 1.5 billion years after the Big Bang, requiring very rapid star formation.
A team of astronomers led by Dr. Roberto Decarli of the Max Planck Institute for Astronomy, Germany, made this discovery by accident when investigating quasars, which are supermassive black holes that sit at the center of enormous galaxies, accreting matter.
The researchers were trying to study star formation in the galaxies that host these quasars.
“We were looking for something different: for star formation activity in the host galaxies of quasars,” Dr. Decarli said.
“But what we found, in four separate cases, were neighboring galaxies that were forming stars at a furious pace, producing a hundred solar masses’ worth of new stars per year.”
“Very likely it is not a coincidence to find these productive galaxies close to bright quasars,” said team member Dr. Fabian Walter, also of the Max Planck Institute for Astronomy.
“Quasars are thought to form in regions of the Universe where the large-scale density of matter is much higher than average.”
“Those same conditions should also be conducive to galaxies forming new stars at a greatly increased rate.”
“Whether or not the fast-growing galaxies we discovered are indeed precursors of the massive galaxies first seen a few years back will require more work to see how common they actually are,” said team member Dr. Eduardo Bañados, a Carnegie-Princeton fellow at the Observatories of the Carnegie Institution for Science and the Department of Astrophysical Sciences of Princeton University.
The new ALMA observations also showed what appears to be the earliest known example of two galaxies undergoing a merger, which is another major mechanism of galaxy growth.
The observations provide the first direct evidence that such mergers have been taking place even at the earliest stages of galaxy evolution, less than a billion years after the Big Bang.
This research was presented in a paper published this week in the journal Nature.
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R. Decarli et al. 2017. Rapidly star-forming galaxies adjacent to quasars at redshifts exceeding 6. Nature 545: 457-461; doi: 10.1038/nature22358
