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In the News this month... Mystery object spotted at the dawn of galaxy formation

The Lyman alpha blob known as Himiko
The Lyman alpha blob known as Himiko CREDIT: M. Ouchi et al.
A team of astronomers have discovered a mysterious object in the early universe which could be one of the earliest ancestors of a forming galaxy ever detected. The object is almost 13 billion light years away when the Universe was only 6% of its current age, and is a type of object known as a +>Lyman alpha emitter.

As the Universe expanded after the Big Bang, it slowly cooled, eventually allowing gas to form as the protons and electrons in the hot primordial plasma combined to form hydrogen atoms. Galaxies started to form when this neutral, opaque gas began to collapse under gravity, forming stars which ionised the surrounding gas making the Universe transparent in what is known as the Epoch of Reionisation. This occurred between about 150 million and one billion years after the Big Bang.

This mysterious object, dubbed "Himiko" by its discoverers, is a giant blob of hydrogen gas 55 thousand light years across, roughly the radius of the Milky Way, and formed when the Universe was a mere 800 million years old. Finding such a large object so far back in time was unexpected since it is thought that small objects formed first and then merged to create larger objects over time. Named after particular spectral signature of hydrogen, many smaller examples of these Lyman alpha emitters are known, although most exist at a time when the Universe was between two and three billion years old.

The spectrum of Himiko
This image shows the spectrum of the Himiko object. The top panel shows the two dimensional view of the Keck/DEIMOS data, while the bottom shows the same data in one dimension. CREDIT: M. Ouchi et al.

Himiko is unique in other ways too. It is both the brightest and largest Lyman alpha object yet discovered. Its character is something of a mystery however. In a paper accepted for publication in the Astrophysical Journal, the authors make several suggestions based on their observations so far, including gas ionised by a hidden supermassive black hole, clouds of ionised hydrogen in very early galaxy, gas falling onto a massive dark halo object generating a massive starburst, a merger of giant gas clouds, or outflowing gas from a starburst or merger. With the data available, it is not clear which of these suggestions  is correct, but the researchers point out that observations with the Atacama Large Millimetre Array when it is complete should narrow down the possibilities by characterising the dust and molecular gas properties of the object.

Posted by Megan on Saturday 02nd May 2009 (03:19 UTC) | Add a comment | Permalink


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