Astronomers may have figured out why massive galaxies are so rare. A study published Wednesday in the journal Nature revealed that vigorous star formation ejects massive amounts of hydrogen and other gases from a galaxy, depriving would-be stars of the material they need to grow.
Using a powerful new telescope -- the Altacama Large Millimeter/submillimeter Array (ALMA) in Chile -- researchers discovered that nearby galaxy NGC 253 was spewing out gas much faster than it was taking it in. The ALMA data also offered a detailed picture of how this flow happens.
NGC 253 lies 11.5 million light-years away in the constellation Sculptor, near our own Milky Way Galaxy. The galaxy’s closeness and tilted orientation, combined with ALMA’s high resolution and sensitivity, allowed researchers to capture a detailed image of its central stellar nursery, or starburst region, said Leonard Blitz, an astronomer at
Computer simulations of the universe predict an abundance of high-mass galaxies, said study leader Alberto Bolatto, an astronomer at the University of Maryland. But in reality, such galaxies are few and far between.
Scientists believe that galactic winds, or gas outflows, might account for the discrepancy. Sure enough, the velocity data from ALMA revealed that NGC 253 was hurtling gas three times faster than gas was spilling into the galaxy's starburst region.
In earlier studies, telescopes have detected hot ionized gas flowing from NGC 253's starburst region. But that couldn't limit galaxy size on its own. Ionized gas is light and "very tenuous," so its loss would do little to hinder future star production, Bolatto said.
Over the last few years, scientists have also seen hints of a much denser, hydrogen-rich molecular gas in NGC 253. This is the type of gas that makes up a large part of the parent clouds that give birth to stars in a galaxy's central region. As the stars form, their radiation pushes against the surrounding parent cloud.
Additionally, the stars in NGC-253 tend to die young, expanding quickly until they explode as supernovae near where they were born, alongside newly forming stars. These explosions, together with the effects of star formation, may exert enough pressure to launch the molecular gas out of the galaxy core, Bolatto and his colleagues wrote in their study.
Unlike older instruments, ALMA could measure molecular gas directly, Bolatto said. The new data showed the velocity and position of the molecular wind coinciding with expanding structures in NGC 253's starburst region. To the scientists, that suggested the energy associated with star formation gives the molecular gas in parental clouds an "initial kick" toward the galaxy's outer edge, or halo.
There, the molecular gas gets a little help. The new ALMA images showed molecular gas flowing in an arc "almost perfectly aligned" with ionized gas outflows, indicating that the heftier molecular gas "hitches a ride" with the lighter ionized gas, Bolatto said. The molecular gas flows about 1,500 light-years above and below the galactic disk, he said.
The researchers estimated that the gas is traveling between 25 to 155 miles per second, but that might not be fast enough to escape NGC 253's gravitational pull. If not, the gas could remain suspended in the galactic halo for many millions of years, Bolatto said. It may even fall back into the galaxy's starburst region. Either way, the galactic winds end up stalling star formation.
Further studies will help determine whether the winds recycle or remove star-forming material, Bolatto said. If the latter, the galaxy's contents could run out in 60 million to 120 million years, the researchers estimated.
In other words, by forming stars, the galaxy is "causing its own demise," Blitz said.
The team also considered whether galactic winds from gas falling into a supermassive black hole could explain the limited star formation in NGC 253. As gas spirals into a supermassive black hole, it heats up and hurls star-forming material far from the galaxy. However, earlier measurements have indicated that NGC-253's black hole isn't actively feeding, Bolatto said.
Blitz cautioned that NGC 253 is just one example of galactic winds limiting galaxy size – how prevalent this is across the universe remains to be seen. Still, the study is "a significant data point" that will help astronomers investigate how galaxies emerge and evolve, he said.
"This is likely an important piece of the puzzle for understanding the universe in general and how galaxies came to be," Bolatto said.
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