One hundred million years or so after the Big Bang, the first stars ignited. Their appearance started the transition between two periods in the early universe: an era that astronomers call the Dark Ages, when only a fog of neutral hydrogen permeated space, and a time of re-ionization, when gravity started to collapse denser regions of gas into massive, hot, and fast-burning stars emitting intense radiation that stripped the nearby hydrogen of its electrons. Light could pass freely in these growing pockets of transparency, creating the visible universe we see today.
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Slowly, the first galaxies formed, and the hot stars and gas turned them into fast-paced stellar nurseries. These early stars were made almost entirely from hydrogen and helium, but inside their cores they began to forge the heavier elements—including carbon, oxygen, nitrogen, and iron—before their short lives ended in supernovas. As heavy elements were released, they started to form molecules—dust—that blocked the visible light from these so-called starburst galaxies, hiding much of this universe-defining era of re-ionization from view.
That’s the generally accepted theory at least. But the origin of most of the objects we see in the universe today is still a mystery. Astronomers can’t yet say for sure exactly how the first stars formed, when heavy elements and molecules began to appear, and when smaller stars like our sun showed up. But now they have a new tool to help answer those questions.
The Atacama Large Millimeter/Submillimeter Array (ALMA) is designed to see through the dust of heavy elements to observe these early star-forming galaxies.
“Of all the energy produced since the Big Bang, half of it has been absorbed and re-emitted by dust,” says astronomer Joaquin Vieira of the California Institute of Technology in Pasadena. “Half hasn’t been absorbed, and we can see that as starlight…. But half of the history of the universe is obscured by dust, and that is what ALMA has just opened up.”
Located among the many telescopes that take advantage of the clear skies over the north Chilean desert, ALMA is an array of 66 dishes either seven or 12 meters in diameter. Each dish has a series of receivers tuned to a part of the spectrum where light has a wavelength in the millimeter range: radio and microwaves. Installed on the three-mile-high Chajnantor Plateau, the observatory began in 1995 as an idea that turned into a $1.3 billion project funded through a collaboration of groups in North America, Europe, and east Asia, with the cooperation of Chile. It was dedicated last March and has already produced observations that have revised theories of the early universe.
When ALMA “sees through” the dust shielding distant galaxies, it’s actually detecting reprocessed light that started out as starlight. That starlight, emitted at mostly optical and ultraviolet wavelengths, was absorbed by the dust, which warmed up and radiated the heat outward as infrared light. As it traveled through the expanding universe, the light was stretched further, or redshifted, into the wavelengths that ALMA is optimized to detect.
ALMA uses interferometry: With a supercomputer, the dishes work in precise synchrony, so that the ensemble operates like one giant dish. The more distant a galaxy, the more narrow the angle of resolution must be for a telescope to resolve details within it to see stars forming. The massive array can be configured in multiple combinations; in its largest configuration, it creates a virtual telescope nearly 10 miles in diameter, called the long baseline. Just as a bigger mirror in an optical telescope will collect finer detail, the long baseline of the full array will give ALMA extraordinary angular resolution.
During the observatory’s early science testing run in 2011 and 2012, Jacqueline Hodge from the Max-Planck Institute for Astronomy in Germany pinpointed the locations of more than 100 early starburst galaxies. Her team worked from a map made by the nearby Atacama Pathfinder Experiment (APEX), a single 12-meter-dish radio observatory, which identified 126 distant galaxies with blobs of light indicating star-forming regions. Using ALMA, Hodge found that many were multiple galaxies that were blurred together, and she identified their locations with 200 times more accuracy—using only a quarter of ALMA’s array. Once the entire array is online this fall, Hodge’s team plans to go back and increase the precision of their results even further.
Now astronomers can start to probe details within the galaxies. “We have a hard time understanding how a galaxy can literally produce that many stars,” Hodge says. “Some of these things are producing 3,000-solar-mass stars per year,” compared to a galaxy like the Milky Way, which today produces only a few sun-like stars annually. “Our computer models are straining to produce these types of galaxies.