Astronomers think that the universe began 13.7 billion years ago at the Big Bang. The exteme temperature and density of this fireball decreased rapidly as its volume increased. Hot cosmic plasma composed mainly of protons and electrons recombined to form neutral hydrogen atoms within 380,000 years after the Big Bang — this was the beginning of the cosmic “Dark Ages.” From then on, the gas continued to cool and fluctuated in density. About 200 to 500 million years after the Big Bang, the dense parts of neutral hydrogen clouds contracted under their own gravity, and the first stars and galaxies formed. The radiation from this first generation of stars started to heat and reionize the hydrogen in nearby space, eventually leading to the reionization of the entire universe. This was the era of “cosmic reionization” or the “cosmic dawn.” The current team focused their research on identifying the exact epoch of the cosmic dawn in an effort to answer major astronomical questions about the history of our universe.
How did the team design research to explore such an ancient, extremely distant time? The group’s first steps were to conduct a survey of distant galaxies and measure their number and brightness. Because light from the distant universe takes time to reach Earth, identification of more distant galaxies allows astronomers to trace farther back in time and locate the epoch of the cosmic dawn. However, neutral hydrogen in intergalactic space dimmed the light from galaxies before the cosmic dawn and made them more difficult to observe. Because the team needed to search a vast area for objects in the far distant universe, it used the prime focus camera mounted on the Subaru Telescope (Suprime-Cam) for its initial surveys. Suprime-Cam captures images of objects in a wide field of view from the large 8.2-meter primary mirror of the Subaru Telescope and is well-known for discovering faint, far distant galaxies and then measuring the amount of neutral hydrogen in the early universe. The use of Suprime-Cam was even more compelling with the 2008 installation of new detectors with a sensitivity about twice as high as their predecessors, particularly in the red wavelengths.
Armed with the most sensitive eyes in the world, the researchers could carry out surveys for extremely distant galaxies — beyond redshift 7, where the majority of energy output from galaxies is detected in red wavelengths. To fine-tune their survey even more, a team led by Iye constructed a new special filter named NB1006 through which they could selectively identify the light of distant galaxies at a redshift of nearly 7.3.
The team used Suprime-Cam, complete with its new highly sensitive detectors, and attached the NB1006 filter to observe two specifically designated regions of the sky for detailed study: the Subaru Deep Field and the Subaru XMM-Newton Deep Survey Field. After a total of 37 hours in seven nights of observations in these wide fields, the team carefully processed the images they had obtained. Shibuya measured the color of 58,733 objects in the images and identified four galaxy candidates at a redshift of 7.3. A careful investigation of the brightness variation of the objects allowed the team to narrow down the number of candidates to two.
Then it was necessary for the team to make spectroscopic observations to confirm the nature of these candidates. They observed the two galaxy candidates with two spectrographs — the Faint Object Camera and Spectrograph (FOCAS) on the Subaru Telescope and the Deep Imaging Multi-Object Spectrograph (DEIMOS) on the Keck Telescope — and identified one candidate for which a characteristic emission line of distant galaxies could be detected.
The current team found that the proportion of neutral hydrogen was increasing in the far distant universe. They concluded that about 80 percent of the hydrogen gas in the ancient universe, 12.91 billion years ago at a redshift of 7.2, was neutral.
In sum, this careful research plan and procedures, including the appropriate removal of contaminations that could lead to false results, resulted in the successful discovery and confirmation of the most distant galaxy ever discovered: SXDF-NB1006-2. In addition, the findings gave the astronomers confidence that they were observing an object during the last phase of the cosmic dawn.
Although finding just one galaxy at a critical epoch is exciting by itself, it is not a sufficient sample to characterize the entire epoch. Precise measurement of the number of galaxies during the cosmic dawn requires surveys of even wider fields. The scheduled 2012 installation of Subaru’s new instrument, Hyper Suprime-Cam (HSC), will allow researchers to observe a field of view seven times greater than that of Suprime-Cam and open the door to a huge galaxy sample beyond redshift 7. Observations with HSC are steps in the direction of uncovering the dark periods of the universe and understanding the physical properties and formation of the first stars and galaxies. Shibuya summarized the team’s future intent and hopes: “By conducting an extremely wide HSC survey for distant galaxies beyond redshift 7, we will find the mechanisms of the cosmic reionization in a variety of ways, not just by investigating their number and brightness.”
“We have been pushing the limits of 8-10-meter-class telescopes to detect distant galaxies,” said Iye. “The 30-meter mirror of the TMT will be able to gather up to 10 times more light than current large telescopes and detect faint light from galaxies up to a redshift of 14. The day is not so far off when the mysteries of the Dark Ages of the universe and the physical properties of the first galaxies will be revealed.”