GALAXY'S LIGHT PUSHES BACK DARK AGES OF THE UNIVERSE
"This galaxy is forming stars at a time speculated to be in the 'Dark Ages' of the universe when galaxies begin to 'turn on'," said University of Hawaii professor Esther Hu, who led the team.
According to the generally accepted picture, the universe started with the Big Bang some 14-16 billion years ago. As the universe expanded and cooled over the next half million years, the glowing plasma of which composed it, recombined into atoms of neutral gas -- mostly hydrogen and some helium. The glow from this era of recombination, observed as the cosmic microwave background radiation, proves useful in the study of the large-scale geometry of the universe. Over the next nearly half billion years, termed the 'Dark Ages', the cold gas began to assemble into the first galaxies. The Dark Ages ended as the light from the newly formed galaxies and quasars reionized and changed the character of the surrounding neutral gas.
The earliest probes of the Universe, known as 'quasars', existed as extremely luminous distant objects powered by black holes. To capture an early snapshot of galaxies, typically a thousand times fainter in their infancy, researchers concentrate on a bright hydrogen emission line dubbed 'Lyman alpha' that gets strongly excited during star formation. Since a large fraction of the light from early forming galaxies emerges in this line, distant galaxies can look prominent viewed through filters which only pass wavelengths near the Lyman alpha emission but appear faint or undetected when seen through other filters.
The method of discovering distant galaxies by searching for objects identified as "Lyman alpha emitters" by the sharp increase in their detectability in narrow bandpass filters proves very successful, and team members had previously used one of the giant Keck 10-m telescopes to find the most distant galaxy previously known, an object whose light took some 15.3 billion years to reach us.
In order to reach fainter and yet more distant galaxies in the present work, Hu and her colleagues used a gravitational lens, in the form of a massive cluster of galaxies to further amplify the light. According to Einstein's theory of general relativity, very massive objects can bend and focus light in much the same way as a magnifying glass. The astronomers used the cluster Abell 370, 6 billion light years away and whose core contains the mass of several hundred galaxies, to magnify light from a galaxy behind the cluster to a distance of 15.5 billion light years away.
The discovery images made with the 10-m Keck Mk.I telescope, confirmed with spectra obtained later on the same telescope, showed a strong Lyman alpha emission line.
"It's significant that you can see the line," said Peter Capak, a University of Hawaii graduate student and team member. "If only a few galaxies had turned on by this point the emission would have been smothered by the surrounding hydrogen gas and the light would never have made it out to us."
Len Cowie, another Hawaii astronomer and team member added, "The fact that this is a galaxy, and not a quasar, is also important. When the first galaxies form, it's like turning on lights to clear out a fog bank. Quasars are really bright though rare, so they can make large clear cavities around themselves, but the fact that light from the fainter but much more numerous galaxies is getting out means that a significant amount of early star formation has already taken place and much of the general fog has already dissipated."
The newly discovered galaxy shows a 'redshift' – light seen coming from an object proportionally increased in wavelength – to the measurement of 6.56, and samples the universe when it forms at about 780 million years old. This view comes from about 50 million years earlier than the view supplied by the most distant quasar (redshift = 6.28), and 80 million years earlier than the speculated period of reionization (redshift ~6.1).
Since most of the light from these galaxies redshifted to infrared wavelengths, the team followed up their discovery with infrared images on the Subaru 8.3-m Telescope, also on Mauna Kea, to estimate the star formation rate -- finding that 40 times the mass of the Sun turns into new stars each year.
"You want to catch galaxies in their infancy and see how they develop," commented Hu. "Scaling the age of the Universe to a person's lifetime, we're showing you baby pictures. The last snapshot showed a toddler just past his fourth birthday. This one is three and a half."
"This is good news for the Next Generation Space Telescope planned for launch in the next decade," she concluded. "It means that there should be plenty of these distant galaxies bright enough to observe, using a large telescope with good infrared detectors, above the strong airglow of our atmosphere."
So, do we exist in the 'Dark Ages' of the Universe?
NOTES:
10-m: 'the 10-metre band': a portion of the shortwave radio spectrum internationally allocated to amateur radio and amateur satellite use on a primary basis.
~6.1: approximately 6.1Infrared: Infrared (IR) light exists as electromagnetic radiation with a wavelength longer than that of visible light.
SOURCE:
IMAGE SOURCE:: http://www.ifa.hawaii.edu/~cowie/z6/z6.html
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