Death spiral: why the universe is producing fewer stars
Two new studies attempt to explain how gases crucial to star formation are being expelled from galaxies, turning fertile spiral galaxies into fallow elliptical ones.
Why isn't the universe producing as many stars as it once did, even though it still holds a respectable amount of hydrogen gas, the raw material for stars?
Researchers have pointed to enormous black holes in the centers of galaxies and dense collections of massive, hot young stars as accomplices in the relentless decline, which has been underway during the last 10 billion years. The processes are thought to propel massive galaxies like the Milky Way on an inevitable evolutionary journey from hotbeds of star formation early in the universe's history to galactic geezers – giant elliptical galaxies whose stars are ancient with no new ones in sight.
Now, two teams of astronomers have taken cosmic snapshots of these processes at work in unprecedented detail, opening important windows on the mechanisms contributing to the decline of star formation rates in galaxies.
One study, set to appear in Friday's issue of the journal Science, focused on a galaxy more than 1.5 billion light-years away as a supermassive black hole at its center was waking up following a dormant period. Jets of plasma erupting near the poles of the black hole and hurtling into space at nearly the speed of light were bulldozing massive amounts of hydrogen gas – 16 to 20 suns' worth – out of the galaxy each year.
Another team, whose results were published in July, has seen large amounts of hydrogen gas streaming from another galaxy 11.5 million light-years away. There, stellar winds and the explosions of hot young stars gathered in dense clusters near the center of the galaxy are causing nine suns' worth of hydrogen to flow out of the galaxy each year.
The findings could help answer several lingering questions.
For years, astronomers have detected elements heavier than hydrogen and helium between galaxies. Those elements require stellar fusion to form, so their existence in a place with no stars was a mystery.
"We always wondered where that comes from," says Juergen Ott, a researcher at the National Radio Astronomy Observatory's Very Large Array in Socorro, N.M.
Evidence had been building to support the idea that supermassive black holes or stellar winds might be responsible. Now researchers have watched as it happens.
At the same time, he says, computer simulations of galaxy evolution during early periods in the universe's history point to a need for some way to shut off star formation. Otherwise the galaxies astronomers see today would look much different. During their periods of activity, the black holes' bulldozing jets could be just such an "off" switch.
And some mechanism has allowed galaxies like the Milky Way to continue forming stars long after theories suggest the galaxy should have run out of gas and star formation should have ended. Researchers suspect that some gas pushed outward by the jets moves too slowly to escape its galaxy's gravity, and eventually falls back in to provide the raw material for new stars.
"We see jets. We know there's a lot of star formation going on, which pushes gas around, but actually now to directly see that we lose all that gas and to see the processes directly, that's really new, " says Dr. Ott, a member of the team that observed the action of star formation itself ejecting gas.
The team that looked at the jets of the supermassive black hole in galaxy 4C12.50 used a global network of 14 radio telescopes stretching from Hawaii to the Netherlands. The awakening black hole forms jets of hot gas, or plasma, that hurtle into space from each pole at nearly the speed of light. These jets pass through the gas in the galaxy, not only heating it and thereby preventing it from collapsing to form stars, but also pushing colder, molecular gas out of the system at speeds of more than 2 million miles an hour.
Data gathered by the team, led by Raffaella Morganti with the Netherlands Institute for Radio Astronomy, found evidence that the galaxy has undergone this process in the past.
Some researchers had proposed that heat from the accretion disks of supermassive black holes squelched star formation near galactic centers by warming the gas there, preventing it from gaining enough density to clump and form fledgling stars. "We now know, no it's not," Ott says. "It's actually the jets."
At one point, these jets were thought to be too narrow to act as an effective bulldozer. Indeed, the jets previously studied were tightly focused features. But they emanated from supermassive black holes that had been active for some time. For whatever reason, the jets from newly awakened black holes appear to be more effective at pushing gas than their more mature counterparts.
"They can affect a fairly large chunk of the surrounding medium," says Alberto Bolatto, an astronomer at the University of Maryland at College Park and the lead author of the July research paper that focused on the activity of young, hot stars in expelling a galaxy's gas.
In the end, one mechanism of gas outflows from galaxies could help offset the other.
If jets push gases out of galaxies at such high velocities that they are gone for good – depriving the galaxy of fuel for both star formation and the central black hole – then ejection by stellar wind may be a key to a galaxy's ability to recycle gas to perpetuate star formation, Dr. Bolatto suggests.
In the study he and his colleagues conducted of gas that hot, young stars eject from galaxies, "the velocities we were seeing in the gas being moved out of the central regions of the galaxy probably weren't high enough to escape the entire galaxy," Bolatto says. After a while, it could "rain back."
With initial observations of these processes in the bag, Ott says, now comes the hard part: trying to get a more precise sense of how influential these processes actually are. That means observing more examples of these processes during their different stages.
Still, if the outflows "carry the masses we seem to be measuring, they have to be important for galaxy evolution," Bolatto says, eventually leading a galaxy like 4C12.50 down the path to becoming a large, gas-deprived elliptical galaxy where star formation is largely history.