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: http://www.mso.anu.edu.au/pfrancis/roleplay/html_dir/firstlight/node9.html
Дата изменения: Unknown Дата индексирования: Tue Oct 2 14:53:56 2012 Кодировка: Поисковые слова: m 106 |
Your main research interests concern neutral hydrogen.
In the early universe, most galaxies have presumably not yet formed. That means that there must be enormous quantities of gas lying around, waiting for the chance to collapse under its own weight into lumps and form stars and galaxies. If we could actually observe this gas, as it moves around and assembles itself into lumps, we would be directly observing the process of galaxy formation. If we look at the amount of gas as a function of redshift, it should drop as galaxies form, telling us the era of galaxy formation.
Optical astronomers seem to think that they can observe galaxy formation by taking optical images. What emits visible light? Stars. And where do stars live? In galaxies. So you can only see visible light once a galaxy has formed. So how do they propose to study galaxy formation using optical telescopes? This has always puzzled you. Furthermore, even once a galaxy has formed its first stars, it will probably be dusty for a while, just like local starburst galaxies, and dust is remarkably good at blocking optical radiation.
In the local universe, gas can be studied in enormous detail using 21cm radiation. This has been responsible for a large fraction of our knowledge of galaxies in the local universe. Alas, with current technologies it is impossible to use this technique at even moderate redshifts: telescopes simply aren't sensitive enough.
The only way we can study gas in the early universe at present is through QSO absorption-line spectroscopy. If the gas lies directly in front of a bright background QSO, then high resolution optical spectroscopy of the absorption it causes in the spectrum of the background QSO can tell us many things about the gas. This technique has been brilliantly sucessful: a wide variety of exciting and important things have been learned. But QSO absorption-line work is forever restricted to statistical sampling: all you can ever measure is what happens to lie in front of a background QSO: something even a milli-arcsec away is completely unknown. Thus the shapes, sizes, geometries and velocity structures of whatever gas clouds are causing the absorption are almost completely unknown.