The largest galaxies in 93 galactic clusters near the Milky Way stopped forming stars 13 billion years ago, a new study reveals. The stagnant behemoths appear to challenge the leading theory that such large objects grew up gradually over time from the merger of smaller objects. Black holes lurking at their centres may be responsible for the arrested development.
Astronomers using a 3.5-metre telescope at Kitt Peak National Observatory in Arizona, US, and a 4-metre telescope at Cerro Tololo Inter-American Observatory in La Serena, Chile, made the discovery after studying the spectra of about 4100 "red" galaxies within about a billion light years of the Milky Way. Red galaxies show few new stars, which tend to make galaxies appear blue.
The survey found that the largest galaxies had already stopped forming stars about 13 billion years ago, when the Universe was only about 700 million years old. Smaller galaxies, on the other hand, gave birth to their last stars more recently - between two billion and eight billion years ago.
"The largest galaxies in the Universe formed very early on and they have been coasting along without forming many new generations of stars," says team member Nicholas Suntzeff, based in La Serena, and associate director for science at the US National Optical Astronomy Observatory, Arizona. This goes against models of "hierarchical clustering".
"From hierarchical modelling, we would have expected to see more star formation later on and we didn't," says Suntzeff.
John Huchra, an astronomer at the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts, US, says the new research does pose questions for current theories. But he says "it doesn't mean the hierarchical picture is completely wrong, just that the merging process that formed the largest galaxies must have essentially finished 10 billion years ago".
And there may be a relatively straightforward explanation. Large galaxies probably have colossal black holes at their cores. Gas falling into these black holes soon after the galaxies were born may have spawned powerful outpourings of energy, creating what astronomers call "active" galaxies. This would have heated gas left over in the galaxies to tens of millions of degrees and allowed it to escape into space before it could condense into new stars.
"I do believe that supermassive black holes are to blame for the fact that these galaxies all appear to be dead," says Lars Hernquist, an astronomer at the CfA.
The process may have been particularly speedy in the galaxies surveyed in this study. They all lie in 93 galactic clusters - crowded swarms of hundreds of galaxies. "Everything happens faster where there is more mass," Suntzeff told New Scientist.
The cluster galaxies probably formed stars, and lost their gas, more quickly than their counterparts in less dense regions. The Milky Way is still forming stars today: it only has a few dozen galactic neighbours in its so-called Local Group.
But Suntzeff says the discovery that even the universe's smaller galaxies are now dying also raises other, more philosophical questions. "Thinking not as an astronomer, I find this behaviour curious - we are living in a time in the universe when galaxies are dying out," he says. "Is it just coincidence? What is our future?"
The research, led by Jenica Nelan of Yale University in New Haven, Connecticut, US, will appear in the 10 September issue of the Astrophysical Journal.
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