Cosmic eye peers into

distant galaxy
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: October 09, 2008

Using a technique known as gravitational lensing to magnify a distant galaxy, astronomers have peered into the heart of a young star-forming region in the distant Universe as it appeared only two billion years after the big bang.

"This is the most detailed study there has been of an early galaxy,” says Mark Swinbank of Durham University. “Effectively we are looking back in time to when the Universe was in its very early stages.”

The Cosmic Eye is so called because the foreground galaxy, which is 2.2 billion light years from Earth, appears in the centre of an arc created by the distant galaxy – giving it the appearance of a human eye. Image: Stark et al.

The distant galaxy is located 11 billion light years from Earth and is nicknamed the Cosmic Eye because of the appearance of a foreground galaxy, which is 2.2 billion light years from Earth, in the centre of an arc created by the distant galaxy. It was first identified using the Hubble Space Telescope, and then followed up with the ten metre Keck telescope on Hawaii, which is equipped with laser-assisted guide star adaptive optics to correct for blurring in the Earth's atmosphere.

By coupling the telescope with the magnifying effect of the gravitational field of a foreground galaxy – a technique called gravitational lensing – the team of astronomers were able to study the distant star system now magnified by eight times its original size. This allowed the scientists to determine the galaxy's internal velocity structure and compare it to present day star systems such as the Milky Way, providing a step in the evolutionary journey of galaxy formation.

"Gravity has effectively provided us with an additional zoom lens, enabling us to study this distant galaxy on scales approaching only a few hundred light years,” says Dan Stark of Caltech, who led the research. "This is ten times finer sampling than previously. As a result for the first time we can see that a typical-sized young galaxy is spinning and slowly evolving into a spiral galaxy much like our own Milky Way."

A red-green-blue colour scematic of the rotating galaxy after correcting for lens distortion. The blue area shows the galaxy moving towards the viewer with the red area furthest away. The green area is the centre of the galaxy. The nature of the spinning galaxy shows that it is already evolving into spiral galaxy like the Milky Way. Image: Stark et al.

Key spectroscopic observations were made with the OSIRIS instrument on the Keck and combined with data taken at millimeter wavelengths by the Plateau de Bure Interferometer (PdBI) located in the French Alps, which is sensitive to the distribution of cold gas that has yet to collapse to form stars.

"Remarkably the cold gas traced by our millimetre observations shares the rotation shown by the young stars in the Keck observations,” says Swinbank. "The distribution of gas seen with our amazing resolution indicates we are witnessing the gradual build up of a spiral disc with a central nuclear component."

This breakthrough demonstrates how important angular resolution has become in ensuring progress in extragalactic astronomy and provides a taste of what astronomers will be able to see in the distant Universe once projects such as the planned European Extremely Large Telescope (E-ELT) and the Thirty Metre Telescope (TMT) come online. When completed in the latter half of the next decade, TMT's giant primary mirror and superior optics will produce images with an angular resolution three times better than the 10 metre Keck and 12 times better than the Hubble Space Telescope, at similar wavelengths, allowing astronomers to study the internal properties of small distant galaxies when the Universe was in its infancy.

Likewise, the Atacama Large Millimeter Array (ALMA), a large interferometer being completed in Chile, will provide a huge step forward in mapping the extremely faint emission from cold hydrogen gas, the principal component of young, distant galaxies and a clear marker of cold molecular gas, compared to the capabilities of present facilities.

"For decades, astronomers were content to build bigger telescopes, arguing that light-gathering power was the primary measure of a telescope's ability," explains Richard Ellis, co-author of the paper presented in today’s issue of the journal Nature and a member of the TMT science advisory committee. "However, adaptive optics and interferometry are now providing ground-based astronomers with the additional gain of angular resolution. The combination of a large aperture and exquisite resolution is very effective for studying the internal properties of distant and faint sources seen as they were when the Universe was young. This is the exciting future we can expect with TMT and ALMA and, thanks to the magnification of a gravitational lens, we have an early demonstration here in this study.”