Astronomers using the European Southern Observatory's Very Large Telescope (VLT) have discovered by far the brightest galaxy yet found in the early universe and found strong evidence that examples of the first generation of stars lurk within it.
The astronomers, led by David Sobral of the University of Lisbon, Portugal, peered back into the ancient universe, to a period known as reionization that is approximately 800 million years after the Big Bang. Instead of conducting a narrow and deep study of a small area of the sky, they broadened their scope to produce the widest survey of very distant galaxies ever attempted.
Their expansive study was made using the VLT with help from the W. M. Keck Observatory and the Subaru Telescope as well as the NASA/ESA Hubble Space Telescope. The team discovered—and confirmed—a number of surprisingly bright very young galaxies containing massive blue stars: the first generation of stars.
These massive, brilliant, and previously purely theoretical stars were the creators of the first heavy elements in history—the elements necessary to forge the stars around us today, the planets that orbit them, and life as we know it. One of the galaxies, labeled CR7, was an exceptionally rare object, by far the brightest galaxy ever observed at this stage in the universe (it is three times brighter than the brightest distant galaxy known up to now).
Bahram Mobasher, a professor of physics and astronomy at UC Riverside, and his graduate students Behnam Darvish and Shoubaneh Hemmati, performed detailed spectroscopic observation of CR7 using the W. M. Keck Observatory telescopes. Through these observations they confirmed the distance of the galaxy from us and hence, the intrinsic luminosity of its constituent stars.
"We confirmed that the galaxy is more than 12 billion light years from us," Mobasher said. "Combining these with other observations, the UCR team successfully identified the presence of ions from elements that could only be produced through intense radiation."
Astronomers have long theorized the existence of a first generation of stars—known as Population III stars—that were born out of the primordial material from the Big Bang. All the heavier chemical elements—such as oxygen, nitrogen, carbon and iron, which are essential to life—were forged in the bellies of stars. This means that the first stars must have formed out of the only elements to exist prior to stars: hydrogen, helium and trace amounts of lithium.
"These elements are responsible for forming stars and galaxies and for originating life," Mobasher said. "Discovery of light from these stars confirms the longstanding theoretical predictions regarding the existence of such stars early in the history of the universe."
The Population III stars would have been enormous—several hundred or even a thousand times more massive than the Sun—blazing hot, and transient—exploding as supernovae after only about two million years. But until now the search for physical proof of their existence had been inconclusive.
Mobasher explained that during a period spanning 600 million years in the universe's early history, the universe went through "dark ages" when space was entirely dominated by hydrogen and helium atoms, making the universe opaque to radiation. Due to the high luminosity of Population III stars and their intense radiation field, the stars could ionize these elements, even those with very high ionization potential (the energy needed for radiation to kick an electron off the atom and, hence, to ionize the atom).
"Population III stars are one candidate responsible for the reionization of the universe, which ends the dark ages and makes the universe transparent around one billion years after the Big Bang," Mobasher said. "The discovery of these stars also indicates the nature of galaxies at the very early stages of their evolution."
Instruments on the VLT found strong ionized helium emission in CR7 but—crucially and surprisingly—no sign of any heavier elements in a bright pocket in the galaxy. This meant the team had discovered the first good evidence for clusters of Population III stars that had ionized gas within a galaxy in the early universe.
"The discovery challenged our expectations from the start," Sobral said, "as we didn't expect to find such a bright galaxy. Then, by unveiling the nature of CR7 piece by piece, we understood that not only had we found by far the most luminous distant galaxy, but also started to realize that it had every single characteristic expected of Population III stars. Those stars were the ones that formed the first heavy atoms that ultimately allowed us to be here. It doesn't really get any more exciting than this."
Within CR7, bluer and somewhat redder clusters of stars were found, indicating that the formation of Population III stars had occurred in waves—as had been predicted. What the team directly observed was the last wave of Population III stars, suggesting that such stars should be easier to find than previously thought: they reside amongst regular stars, in brighter galaxies, not just in the earliest, smallest, and dimmest galaxies, which are so faint as to be extremely difficult to study.
Further observations with the VLT, ALMA, and the NASA/ESA Hubble Space Telescope are planned to confirm beyond doubt that what has been observed are Population III stars, and to search for and identify further examples.
The name Population III arose because astronomers had already classed the stars of the Milky Way as Population I (stars like the Sun, rich in heavier elements and forming the disc) and Population II (older stars, with a low heavy-element content, and found in the Milky Way bulge and halo, and globular star clusters).
CR7's nickname is an abbreviation of COSMOS Redshift 7, a measure of its place in terms of cosmic time. It was inspired by the Portuguese footballer Cristiano Ronaldo, known as CR7. The higher the redshift, the more distant the galaxy and the further back in the history of the universe it is seen. CR7, whose energy is mostly in the form of ultraviolet/visible light, is located in the COSMOS field, an intensely studied patch of sky in the Sextant constellation.
(Source: University of California, Riverside)