Some galaxies, like the Milky Way and our neighboring galaxy, Andromeda, have relatively slow and measured rates of star birth, with about one new star igniting each year. Other galaxies, known as starburst galaxies, forge 100s or even 1,000s of stars each year. This furious pace, however, cannot be maintained indefinitely.
To avoid burning out in a short-lived blaze of glory, some galaxies throttle back their runaway star birth by ejecting – at least temporarily – vast stores of gas into their expansive halos, where the gas either escapes entirely or slowly rains back in on the galaxy, triggering future bursts of star formation.
Up to now, astronomers have been unable to directly observe the powerful outflows in the very early universe, where such mechanisms are essential to prevent galaxies from growing too big, too fast.
New observations with the Atacama Large Millimeter/submillimeter Array, or ALMA, show – for the first time – a powerful galactic "wind" of molecules in a galaxy seen when the universe was only one billion years old. This result provides insights into how certain galaxies in the early universe were able to self-regulate their growth so they could continue forming stars across cosmic time.
"Galaxies are complicated, messy beasts, and we think outflows and winds are critical pieces to how they form and evolve, regulating their ability to grow," said Justin Spilker, who graduated with a doctorate in astronomy from the UA last year and now is an astronomer at the University of Texas at Austin.
Spilker is the lead author on a paper about the discovery that appeared in the journal Science.
"We were able to directly observe a galaxy engaging in this self-regulating cycle: first, it forms all these stars, then it blasts out the raw material so it can't form stars anymore," Spilker said.
Although not observed in this case, some of the material may rain back onto the galaxy eventually, allowing the process to begin anew.
Astronomers have observed winds with the same size, speed and mass in nearby star-bursting galaxies, but the new ALMA observation is the most distant unambiguous outflow ever seen in the early universe.
The galaxy, known as SPT2319-55, is more than 12 billion light-years away. It was discovered by the National Science Foundation’s South Pole telescope.
Previous observations of galaxies with powerful gas outflows had only shown indistinct pixels, says co-author Dan Marrone, an associate professor at Steward Observatory, making this observation the first to resolve an image of a gas-spewing galaxy at such an early age of the universe.
As new, dust-enshrouded stars form, the dust heats up and glows brightly in infrared light. But the galaxy is also launching a wind and some of it is blowing in our direction. As the infrared light passes through the wind on its journey toward Earth, the hydroxyl molecules in the wind absorb some of the infrared light at a very particular wavelength that the ALMA can observe. Essentially, the telescope detects the shadow of a hydroxyl fingerprint in the galaxy’s bright infrared light.
"That’s the absorption signature that we detected, and from that, we can also tell how fast the wind is moving and get a rough idea of how much material is contained in the outflow," Spilker said. "We look at hydroxyl because it occurs in places where molecular hydrogen – the main ingredient in star-forming material – is present."
"For most of the universe, we can't resolve these types of galaxies because the emission is at wavelengths that we can't easily observe," Marrone added.
The ALMA was able to observe this object thanks to two cosmic effects: the expansion of the universe and the gravitational bending of light. The extreme distance of the galaxy means that the ongoing expansion of space has stretched the infrared light of the hydroxyl molecules to millimeter wavelengths, where the ALMA radio telescopes can detect it. Moreover, because there happens to be another galaxy that sits almost exactly along the line of sight between Earth and SPT2319-55, the gravity of the intervening object acts as a lens.
Gravitational lensing – the bending of light due to gravity – magnifies the background galaxy to make it appear brighter, which allows the astronomers to observe it in more detail than they would otherwise be able to. Astronomers use specialized computer programs to "unscramble" the effects of gravitational lensing to reconstruct an accurate image of the more-distant object.
The lens-aided view revealed a powerful "wind" of star-forming gas exiting the galaxy at nearly 800 kilometers per second. Rather than a constant, gentle breeze, the wind is hurtling away in discrete clumps, removing the star-forming gas just as quickly as the galaxy can turn that gas into new stars.
Molecular winds are an efficient way for galaxies to self-regulate their growth, the researchers note. These winds are likely triggered by either the combined effect of all the supernova explosions that go along with rapid, massive star formation or by a powerful release of energy as some of the gas in the galaxy falls down onto the supermassive black hole at its center.
"So far, we have only observed one galaxy at such a remarkable cosmic distance, but we’d like to know if winds like these are also present in other galaxies to see just how common they are," Spilker said. "If they occur in basically every galaxy, we know that molecular winds are both ubiquitous and also a really common way for galaxies to self-regulate their growth."
The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities Inc.