23 October 2013

New titleholder for most distant galaxy hints at universe's opaque beginnings

by Will Parker

Newly discovered galaxy z8-GND-5296 is providing astronomers with an intriguing glimpse of what the universe was like when it was only about 700 million years old - 13.1 billion years ago.

Detailing their work in the journal Nature, the astronomers explain how the discovery was made possible by a new instrument, MOSFIRE, commissioned on the Keck Telescope. MOSFIRE is extremely sensitive in the infrared spectrum and can target multiple objects at the same time. It was this 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, the scientists were 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, which exhibited a redshift of 7.5. Only five other galaxies have ever been confirmed to have a redshift greater than 7, with the previous high being 7.215.

"We were thrilled to see this galaxy," said Steven Finkelstein at the University of Texas at Austin. "And then our next thought was, 'Why did we not see anything else? We're using the best instrument on the best telescope with the best galaxy sample. We had the best weather - it was gorgeous. And still, we only saw this emission line from one of our sample of 43 observed galaxies, when we expected to see around six. What's going on?'"

The scientists 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 was neutral to a translucent state in which most of the hydrogen was ionized. Which means it's not necessarily that the distant galaxies aren't there; they may be hidden from detection behind a wall of neutral hydrogen fog, which blocks the hydrogen emission signal.

This, scientists believe, is one of two major changes in the fundamental essence of the universe since its beginning - the other being a transition from a plasma state to a neutral state. "Everything seems to have changed since then," explained team member Vithal Tilvi, from Texas A&M University. "If it was neutral everywhere today, the night sky that we see wouldn't be as beautiful. What I'm working on is studying exactly why and exactly where this happened. Was this transition sudden, or was it gradual?"

Tilvi adds that the new titleholder is unlikely to hang onto its crown for long, 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 that should enable scientists to find many more such galaxies at even larger distances.

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Source: Texas A&M University