Science News

Galaxy From the Early Universe

Sun, 24th Oct 2010

A team of astronomers have successfully looked back about 13 billion years to pinpoint one of the first galaxies to exist after the Big Bang.

Writing in Nature, University of Paris Diderot-based researcher Matt Lehnert and his colleagues explain how they have used the European Southern Observatory's VLT (Very Large Telescope) in Chile to focus on a patch of the sky called the Hubble Ultra Deep Field that contains a galaxy, initially spotted by the Hubble telescope, called UDFy-38135539.

The team used an instrument on the VLT called SINFONI to study the galaxy in detail. Specifically, they were looking at how "stretched out" the light arriving from the galaxy is; this is a phenomenon called red-shifting and is the cosmic equivalent of the siren pitch-drop produced by a passing ambulance.

The team's measurements revealed a red-shift of z=8.6, indicating that the light they were seeing had been travelling across the Universe for about 13 billion years to reach us, making this one of the earliest galaxies in existence following the Big Bang, which occurred about 13.7 billion years ago.

Further analysis on the spectrum of light arriving from UDFy-38135539 has also enabled the team to estimate that the galaxy was rapidly spawning stars at the rate of up to 4 million per year and, from comparisons with other similar closer galaxies, the new-born stars would have contained very little in the way of metals.

Why this discovery is so important is that it helps to define the time limit around an early event called "the epoch of reionisation". Put simply, shortly after the Big Bang, when things cooled sufficiently for protons and electrons to combine together, the early Universe was shrouded in a dense fog of hydrogen. But as soon as the first stars began to shine, the ultraviolet they were pumping out ripped apart the hydrogen again, rendering the Universe once-more transparent.

The results from the present study not only tell us when this must have happened - because the light could not have reached us unless the hydrogen smog had lifted - but they also inform our understanding of the structure of the early galaxy and its neighbours which would have to have blown away the hydrogen fog so the light can be seen...


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Fascinating story, but it raises a question for which I need a simple answer - if there is one - please.

The light seen by the astronomers left that galaxy 13billion years ago.
They are seeing the galaxy in the position it was in 13 billion years ago.
I feel sure that this cannot mean that when the Universe was only 0.7 billion years old, that galaxy was already 13 billion L Y away from Earth’s present position, but I can’t sort out an explanation.  On the other hand, if it was not 13billion L Y away when the light left it, why has it taken 13billion years to reach us?

Help! Bill S, Sun, 24th Oct 2010

The light takes so long to get here because the universe is expanding. As time passes the light needs to travel further and further to get to where we are. If the Universe was expanding faster than the speed of light it would never arrive. Mirkin, Sun, 31st Oct 2010

Thanks Mirkin, I had forgotten about this thread until I found a reference to it on another thread.  I understand your explanation, but it still leaves me with this thought: If we look from the Earth, in opposite directions, at objects that are 13 billion LY away, they are 26 billion LY apart.  We are seeing them as/where they were 13 billion years ago, so they must have been 26 billion LY apart then.  No? Bill S, Thu, 27th Jan 2011

Yep, the lightsphere we see is double any one singly picked distance from Earth Bill. And I think you have a point in your first observation too.

Assume that there is a certain distance, at any point of 'time', from where we won't see any more light reach us, due to a 'expansion'. What hinders that point from having existed one million years ago, two millions years ago? Thirteen millions years ago? The expansion might have been 'slower' as we see it, but what proves that there isn't that possibility?

I'm sorry, my keyboard is falling apart :)
Have to correct my spelling constantly.


Ah, rereading you, no, you have to remember the 'balloon' in where all points move away from each other equally. The 'time' measured will still only be thirteen millions 'light years' to reach us from any point of observation, even though the accumulated 'size' will be the 26 millions light years. yor_on, Fri, 28th Jan 2011

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