Maunakea, Hawaii - A relic cloud of gas, orphaned after the Big Bang, has been discovered in the distant universe by astronomers using the world's most powerful optical telescope, the W. M. Keck Observatory on Maunakea, Hawaii.
The discovery of such a rare fossil, led by PhD student Fred Robert and Professor Michael Murphy at Swinburne University of Technology, offers new information about how the first galaxies in the universe formed.
"Everywhere we look, the gas in the universe is polluted by waste heavy elements from exploding stars," says Robert. "But this particular cloud seems pristine, unpolluted by stars even 1.5 billion years after the Big Bang."
"If it has any heavy elements at all, it must be less than 1/10,000th of the proportion we see in our Sun. This is extremely low; the most compelling explanation is that it's a true relic of the Big Bang."
The results will be published in the journal Monthly Notices of the Royal Astronomical Society. A preprint of the paper, "Exploring the origins of a new, apparently metal-free gas cloud at z = 4.4," is available online at http://arxiv.
Robert and his team used two of Keck Observatory's instruments - the Echellette Spectrograph and Imager (ESI) and the High-Resolution Echelle Spectrometer (HIRES) - to observe the spectrum of a quasar behind the gas cloud.
The quasar, which emits a bright glow of material falling into a supermassive black hole, provides a light source against which the spectral shadows of the hydrogen in the gas cloud can be seen.
"We targeted quasars where previous researchers had only seen shadows from hydrogen and not from heavy elements in lower-quality spectra," says Robert. "This allowed us to discover such a rare fossil quickly with the precious time on Keck Observatory's twin telescopes."
The only two other fossil clouds known were discovered in 2011 by Professor Michele Fumagalli of Durham University, John O'Meara, formerly a professor at St. Michael's College and now the new Chief Scientist at Keck Observatory, and Professor J. Xavier Prochaska of the University of California, Santa Cruz; both Fumagalli and O'Meara are co-authors of this new research on the third fossil cloud.
"The first two were serendipitous discoveries, and we thought they were the tip of the iceberg. But no one has discovered anything similar - they are clearly very rare and difficult to see. It's fantastic to finally discover one systematically," says O'Meara.
"It's now possible to survey for these fossil relics of the Big Bang," says Murphy. "That will tell us exactly how rare they are and help us understand how some gas formed stars and galaxies in the early universe, and why some didn't."
This research was funded by an Australian Research Council Discovery Project grant and Professor Fumagalli's contribution was partially funded by a European Research Council grant.
The Echellette Spectrograph and Imager (ESI) is a medium-resolution visible-light spectrograph that records spectra from 0.39 to 1.1 microns in each exposure. Built at UCO/Lick Observatory by a team led by Prof. Joe Miller, ESI also has a low-resolution mode and can image in a 2 x 8 arc min field of view. An upgrade provided an integral field unit that can provide spectra everywhere across a small, 5.7 x4.0 arc sec field. Astronomers have found a number of uses for ESI, from observing the cosmological effects of weak gravitational lensing to searching for the most metal-poor stars in our galaxy.
The High-Resolution Echelle Spectrometer (HIRES) produces spectra of single objects at very high spectral resolution, yet covering a wide wavelength range. It does this by separating the light into many "stripes" of spectra stacked across a mosaic of three large CCD detectors. HIRES is famous for finding exoplanets. Astronomers also use HIRES to study important astrophysical phenomena like distant galaxies and quasars, and find cosmological clues about the structure of the early universe, just after the Big Bang.
ABOUT W. M. KECK OBSERVATORY
The W. M. Keck Observatory telescopes are the most scientifically productive on Earth. The two, 10-meter optical/infrared telescopes atop Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems. The data presented herein were obtained at Keck Observatory, which is a private 501(c) 3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.