December 6, 2012
First Stars Born Short 750 Million Years After The Big Bang
April Flowers for redOrbit.com - Your Universe Online
The universe has had traces of heavy elements such as carbon and oxygen as far back in time as astronomers have been able to see. Elements such as these were originally churned from the explosion of massive stars. They formed the building blocks for planetary bodies, and eventually for life on Earth.
Peering back far in time, a research team from MIT, Caltech, and the University of California at San Diego (UCSD) found matter with no discernible trace of heavy elements. They analyzed light from the most distant known quasar — a galactic nucleus more than 13 billion light-years from Earth — to make this measurement.
These observations of the quasar provide a snapshot of the early universe, a mere 750 million years after the Big Bang. The team's analysis of the quasar's light spectrum showed no evidence of heavy elements in the surrounding gaseous cloud. This suggests that the quasar dates to an era nearing that of the universe's first stars. The findings of this study were published this week in the journal Nature.
“The first stars will form in different spots in the universe “¦ it´s not like they flashed on at the same time,” Robert Simcoe, an associate professor of physics at MIT, says in a statement. “But this is the time that it starts getting interesting.”
Most scientists agree on a general sequence of events during the early development of the universe. This sequence is based on numerous theoretical models. The Big Bang threw off massive amounts of matter and energy nearly 14 billion years ago, creating a rapidly expanding universe. Protons and neutrons collided in nuclear fusion reactions in the first few minutes after the explosion, forming hydrogen and helium.
Fusion stopped generating these basic elements as the universe cooled, leaving hydrogen as the dominant constituent of the early universe, while heavier elements such as carbon and oxygen would not form until the first stars appeared.
By analyzing light from more distant bodies, astronomers have attempted to identify the point at which the first stars were born. These objects are used because the farther away an object is in space, the older it is. However, scientists have only been able to observe objects that are less than about 11 billion years old until recently, and these objects all exhibit heavy elements. The presence of such elements suggests that stars were already plentiful, or at least well established, at the point in the universe's history when the object formed.
“[The astrophysics community] sort of hit this wall,” says Simcoe, an astrophysicist at MIT´s Kavli Institute for Astrophysics and Space Research. “When this [quasar] was discovered, we could sort of leapfrog further back in time and make a measurement that was substantially earlier.”
The quasar used in this study was discovered in August 2011 and is the most distant of its kind. Simcoe and his team built an infrared spectrometer in order to study such distant objects. The spectrometer was fitted onto the Magellan Telescope, a massive ground-based telescope in Chile.
In January 2012, the scientists trained the telescope on the newly discovered quasar and collected data from its light.
Incoming light was split into different wavelengths by the spectrometer. These were plotted onto a graph and examined for telltale dips in the data, correlating various wavelengths with the light given off by different chemicals.
“Each chemical has its own fingerprint,” Simcoe says. “Based on the pattern of what light is absorbed, it tells you the chemical composition.”
The "intrinsic spectrum" - the amount of light naturally given off by any such body - of the quasar was determined and compared with the observed data to search for the presence of heavy elements. Evidence of hydrogen was found, but no oxygen, sillicon, iron or magnesium was found in the light data. Confirming the absence of heavy metals, however, was a challenging task.
“It´s always hard to establish the absence of something,” Simcoe says.
The team examined every other scenario that might explain the light patterns they observed, including newborn galaxies and other matter situated in front of the quasar. This intense examination confirmed that the quasar´s light spectrum indicated an absence of heavy elements 750 million years after the Big Bang. “[The birth of the first stars] is one of these important moments in the history of the universe,” Simcoe says. “It went from looking like the early universe, which was just gas and dark matter, to looking like it does today, where there are stars and galaxies “¦ it´s the point when the universe started to resemble what it looks like today. And it´s sort of amazing how early that happens. It didn´t take long.”
John O´Meara, an associate professor of physics at St. Michael´s College, says the MIT discovery is an “impressive and important step in advancing our knowledge of the universe when it was very young.” Confirmation through analysis of many other distant quasars is necessary, though.
“Prior to this result, we have not seen regions of the universe this old and devoid of heavy elements, so there was a missing link in our understanding of how the elemental content of the universe has evolved with time,” O´Meara adds. “[This] discovery possibly provides such a rare environment where the universe had yet to form stars.”
Simcoe and his team hope to analyze other quasars from this early era to further confirm the absence of heavy elements. “If we can find things in this epoch, we can start to characterize them,” Simcoe says. “There´s always something interesting at the edge.”