Discovery of a Vigorous Star-Forming Galaxy at the Cosmic Dawn
December 21, 2011
An international team of astronomers led by Masami Ouchi at the University of Tokyo has discovered a vigorous, star-forming galaxy that existed about 750 million years after the Big Bang. This galaxy, named GN-108036, was a remarkable source of star formation at the so-called "cosmic dawn", a very early time in cosmic history; it was generating an exceptionally large amount of stars in the calm, dark cosmos.
The team obtained the spectra of GN-108036 and calculated its distance from Earth to confirm that it is one of the most distant galaxies ever discovered. A review of archival images revealed the occurrence of remarkably energetic star formation within it; a large mass of gas, equivalent to about a hundred Suns per year, was forming stars. Although nine galaxies from about 600 to 800 million years after the Big Bang have been confirmed to date, GN-108036 was forming stars much more vigorously than the other galaxies. Team leader Ouchi commented, "A number of galaxies were actively forming stars long after the Big Bang, and some of them were comparable to or even more active than GN-108036. The significant finding about GN-108036 is that it demonstrates the existence of a vigorous star-forming galaxy when the Universe was still very cold and dark."
About 380,000 years after the Big Bang, a decrease in the temperature of the Universe caused electrons and protons to join to form neutral hydrogen. The Universe entered its "dark age" at this time. Bahram Mobasher, a team member from the University of California, Riverside, explained how the dark age ended: "It ended when gas clouds of neutral hydrogen collapsed to generate stars, forming the first galaxies, which probably radiated high-energy photons and reionized the Universe. Vigorous galaxies like GN-108036 may well have contributed to the reionization process, which is responsible for the transparency of the Universe today.”
An extensive survey of the Great Observatories Origins Deep Survey (GOODS) northern field by the Subaru Prime Focus Camera (Suprime-Cam) led to the initial discovery of GN-108036. To determine the distance to the galaxy, the astronomers observed it with the Keck Observatory's highly sensitive Deep Imaging Multi-Object Spectrograph (DEIMOS). Data reduction showed a Lyman-alpha line, which indicates bright hydrogen emissions in the ultraviolet part of the spectrum. Attenuation of neutral hydrogen in the intergalactic medium caused its expected asymmetry.
Despite the use of a splendid spectrograph on a huge telescope, it was difficult to obtain a reliable spectrum. Cosmic expansion resulted in a line shifted to a very red wavelength, nearly equal to 1 micron. In addition, many night-sky OH emission lines surrounded the line. Yoshiaki Ono at the University of Tokyo, a team member and the leading author of the article, commented on this difficulty: "We went back and repeated our observations on three different occasions over two years. We observed the feature at the same wavelength on each occasion. The asymmetric line provided undisputed evidence for the huge distance to the galaxy."
The astronomers searched for a sign of detection by using recent observations of the GOODS field with the Hubble Space Telescope's newly-installed camera, WFC3. Team member Mark Dickinson at the National Optical Astronomy Observatory remarked: "We were very excited when we confirmed the robust detection of GN-108036 in the WFC3 image. The galaxy's intense brightness in the intrinsic ultraviolet indicates that it contained many young, massive stars. In addition, we found that the galaxy has a diameter of about 5,000 light years, only five percent (1/20 th) as large as that of our own Milky Way Galaxy."
The team gathered images of the galaxy obtained with the Spitzer Space Telescope's infrared camera IRAC (Infrared Array Camera) and the Subaru Telescope's near-infrared camera MOIRCS (Multi-Object Infrared Camera and Spectrograph); they found that the galaxy was harboring surprisingly powerful star-formation. "The amount of stars born every year was more than ten times higher than those in the other galaxies at a comparable distance," explained Ono. "We're surprised to know that such a vigorous galaxy existed at a time when the Universe was only five percent of its present age."
Interestingly, several teams of astronomers have reported discoveries of compact and massive old galaxies around four billion years after the Big Bang. How they formed remains a mystery. Small, actively star-forming galaxies like GN-108036 might be the ancestors of such galaxies. Detailed investigations of the nature of GN-108036 will provide us with an important key for understanding the early phases of the formation and evolution of galaxies.
This research will be published in the January 10, 2012 issue of The Astrophysical Journal. The research is based in part on data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan; the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration (NASA); the Hubble Space Telescope, managed by NASA and the European Space Agency (ESA); and the Spitzer Telescope, managed by the Jet Propulsion Laboratory (JPL) for NASA.
The authors of the paper:
- Yoshiaki Ono (graduate student at the University of Tokyo, JSPS research fellow)
- Masami Ouchi (associate professor at the University of Tokyo)
- Bahram Mobasher (professor at the University of California, Riverside)
- Mark Dickinson (associate astronomer at National Optical Astronomy Observatory)
- Kyle Penner (graduate student at the University of Arizona)
- Kazuhiro Shimasaku (associate professor at the University of Tokyo)
- Benjamin J. Weiner (assistant astronomer at Steward Observatory)
- Jeyhan S. Kartaltepe (research associate at National Optical Astronomy Observatory, Hubble fellow)
- Kimihiko Nakajima (graduate student at the University of Tokyo)
- Hooshang Nayyeri (graduate student at the University of California, Riverside)
- Daniel Stern (NuSTAR Project Scientist at NASA, Jet Propulsion Laboratory)
- Nobunari Kashikawa (associate professor at National Astronomical Observatory of Japan)
- Hyron Spinrad (professor emeritus at University of California, Berkeley)