Fossil Star Clusters Reveal Their Age

W. M. Keck Observatory press release

Using a new age-dating method and the W. M. Keck Observatory on Maunakea, an international team of astronomers have determined that ancient star clusters formed in two distinct epochs – the first 12.5 billion years ago and the second 11.5 billion years ago. These results are being published in Monthly Notices of the Royal Astronomical Society.

Cosmic Timeline of Globular Clusters
A cosmic timeline showing the birth of the Universe in a Big Bang 13.7 billion years ago to the present day. Credit: NASA/CXC/SAO and A. Romanowsky
Although the clusters are almost as old as the Universe itself, these age measurements show the star clusters – called globular clusters – are actually slightly younger than previously thought.

“We now think that globular clusters formed alongside galaxies rather than significantly before them,” research team leader, Professor Duncan Forbes of Swinburne University of Technology said.

The new estimates of the star cluster average ages were made possible using data obtained from the SAGES Legacy Unifying Globulars and GalaxieS (SLUGGS) survey, which was carried out on Keck Observatory’s 10-meter, Keck II telescope. Observations were carried out over years using the powerful DEIMOS multi-object spectrograph fitted on Keck II, which is capable of obtaining spectra of one hundred globular clusters in a single exposure.

DEIMOS breaks the visible wavelengths of objects into spectra, which the team used to reverse-engineer the ages of the globular clusters by comparing the chemical composition of the globular clusters with the chemical composition of the Universe as it changes with time.

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Discovery of the Most Distant Galaxy in the Cosmic Dawn

W. M. Keck Observatory press release

A team of astronomers has used the Subaru Telescope and the W. M. Keck Observatory to discover the most distant galaxy yet, at 12.91 billion light-years from the Earth. This new galaxy, dubbed SXDF-NB1006-2, is slightly farther away than the previous record holder, galaxy GN-108036, which was found last year.

To identify SXDF-NB1006-2, the team used the Subaru Telescope to observe a total of 37 hours in seven nights in two wide fields of the sky. The team, led by Takatoshi Shibuya (The Graduate University for Advanced Studies, Japan), Dr. Nobunari Kashikawa (National Astronomical Observatory of Japan), Dr. Kazuaki Ota (Kyoto University), and Dr. Masanori Iye (National Astronomical Observatory of Japan), 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, which translates into about 12.9 billion light-years. A careful investigation of the brightness variation of the objects allowed the team to narrow down the number of candidates to two.

SXDF-NB1006-2Color composite image of the Subaru XMM-Newton Deep Survey Field of galaxy SXDF-NB1006-2, credit NAOJ

Finally, the team needed 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 II Telescope, and identified one candidate for which a characteristic emission line of distant galaxies could be detected. The results are slated to be published in the June 20, 2012, edition of Astrophysical Journal.

In addition to locating the galaxy, the team’s research verified that the proportion of neutral hydrogen gas in the 750-million-year-old early Universe was higher than it is today. These findings help to decipher the early Universe during the “cosmic dawn,” when the light of ancient celestial objects and structures first appeared. 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.

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