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Astronomers discover most
distant known galaxy

23 November 2013

A team of astronomers has discovered a galaxy at a distance of 13.1 billion light years and a redshift of 7.5, and it is surprisingly evolved for such a short time after the big bang, posing problems for theorists.

The team from Universities of California and Texas identified a very distant galaxy candidate using deep optical and infrared images taken by the Hubble Space Telescope. Follow-up observations of this galaxy by the Keck Telescope in Hawai'i confirmed its distance.

In searching for distant galaxies, they selected several candidates, based on their colours, from the approximately 100,000 galaxies identified in the Hubble Space Telescope images taken as a part of the CANDELS survey, the largest project ever performed by the Hubble Space Telescope, with a total allocated time of roughly 900 hours. However, using colours to sort galaxies is tricky because some nearby objects can masquerade as distant galaxies.

Therefore, to measure the distance to these galaxies in a definitive way, astronomers use spectroscopy — specifically, how much the wavelength of a galaxy's light has shifted towards the red-end of the spectrum as it travels from the galaxy to Earth, due to the expansion of the universe. This phenomenon is called "redshift." Since the expansion velocity (redshift) and distances of galaxies are proportional, the redshift gives astronomers a measure of the distance to galaxies.

Hubble CANDELS Field Image with Galaxy z8_GND_5296
Hubble CANDELS Field Image with Galaxy z8_GND_5296

An artist's rendition of the newly discovered most 
		distant galaxy z8_GND_5296.
An artist's rendition of the newly discovered most distant galaxy z8_GND_5296.

"What makes this galaxy unique, compared to other such discoveries, is the spectroscopic confirmation of its distance," said Mobasher, a professor of physics and observational astronomy. "By observing a galaxy that far back in time, we can study the earliest formation of galaxies," he said. "By comparing properties of galaxies at different distances, we can explore the evolution of galaxies throughout the age of the universe."

Commenting on the research, Dr Marek Kukula, Public Astronomer at the Royal Observatory Greenwich, said: "This, along with some other evidence, shows that there are already quite surprisingly evolved galaxies in the very early Universe." This poses a serious problem for theorists because the further back we look the more primitive the galaxies are supposed to be. Yet this, the most distant galaxy know, is already mature at so close a time to the big bang.

The discovery was made possible by a new instrument, MOSFIRE, commissioned on the Keck Telescope. Not only is the instrument extremely sensitive, but it is designed to detect infrared light -- a region of the spectrum to where the wavelength of light emitted from distant galaxies is shifted -- and could target multiple objects at a time. It was the latter feature that allowed the researchers to observe 43 galaxy candidates in only two nights at Keck, and obtain higher quality observations than previous studies.

By performing spectroscopy on these objects, researchers are able to accurately gauge the distances of galaxies by measuring a feature from the ubiquitous element hydrogen called the Lyman alpha transition. It is detected in most galaxies that are seen from a time more than one billion years from the Big Bang, but as astronomers probe earlier in time, the hydrogen emission line, for some reason, becomes increasingly difficult to see.

Of the 43 galaxies observed with MOSFIRE, the research team detected this Lyman alpha feature from only one galaxy, z8-GND-5296, shifted to a redshift of 7.5. The researchers suspect they may have zeroed in on the era when the universe made its transition from an opaque state in which most of the hydrogen is neutral to a translucent state in which most of the hydrogen is ionized (called the Era of Re-ionization).

"The difficulty of detecting the hydrogen emission line does not mean that the galaxies are absent," said Reddy, an assistant professor of astronomy. "It could be that they are hidden from detection behind a wall of neutral hydrogen."

The team's observations showed that z8-GND-5296 is forming stars extremely rapidly — producing each year ~300 times the mass of our sun. By comparison, the Milky Way forms only two to three stars per year. The new distance record-holder lies in the same part of the sky as the previous record-holder (redshift 7.2), which also happens to have a very high rate of star-formation.

"So we're learning something about the distant universe," said Steven Finkelstein at the University of Texas at Austin, who led the project. "There are way more regions of very high star formation than we previously thought. There must be a decent number of them if we happen to find two in the same area of the sky."

"With the construction and commissioning of larger ground-based telescopes — the Thirty Meter Telescope in Hawai'i and Giant Magellan Telescope in Chile — and the 6.5 meter James Webb Space Telescope in space, by the end of this decade we should expect to find many more such galaxies at even larger distances, allowing us to witness the process of galaxy formation as it happens," Mobasher said.


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