Some galaxies should be much more active than they actually are. Among them was a population of massive galaxies in the very early universe, when the popular thing to do was to make a lot of stars. However, these galaxies were instead “quiescent” with little action going on inside them. Astronomers have finally confirmed the reason why they were like that – they are simply running on empty.
The galaxies in the study, published in the journal Nature, emitted the light we see between 10 and 12 billion years ago. This was the epoch when most of the stars in the universe formed, the so-called peak of star-formation – and yet these quiescent galaxies are the exception.
These galaxies somehow went through their whole reserve of cold gas, the crucial ingredient to make stars. It might be counterintuitive, but clouds of gas need to be cold to turn into bright hot stars. By being cold, the gas more easily condenses and then collapses into a big gravitational ball of stuff. But if the gas is too hot, it spreads out. This ultimately leads the galaxies to stop producing new stars.
“The most massive galaxies in our universe formed incredibly early, just after the Big Bang happened, 14 billion years ago,” lead author Kate Whitaker, professor of astronomy at University of Massachusetts Amherst said in a statement. “But for some reason, they have shut down. They’re no longer forming new stars.”
Observations with the Hubble Space Telescope and the Atacama Large Millimeter/submillimeter Array (ALMA) identified six of these galaxies magnified by a phenomenon called gravitational lensing. A foreground galaxy cluster warps space-time so much that the light of these distant galaxies comes through much clearer. This allowed for better observations, and the revelation that these galaxies have run out of gas.
It is unclear what happened to them. They got to their impressive size very quickly, and then either consumed all the cold gas available or pushed the remaining gas away. It is also possible that something was stopping the galaxy from getting “refueled”, so no more gas was falling in.
"Did a supermassive black hole in the galaxy's center turn on and heat up all the gas? If so, the gas could still be there, but now it's hot. Or it could have been expelled and now it's being prevented from accreting back onto the galaxy. Or did the galaxy just use it all up, and the supply is cut off? These are some of the open questions that we'll continue to explore with new observations down the road," Whitaker pondered.
The study of dead galaxies has the apt backronym of REQUIEM, standing for Resolving QUIEscent Magnified Galaxies At High Redshift.