View Full Version : Exploring back to z=200

2004-Oct-07, 01:44 PM
Here&#39;s a pointer to a paper about using the highly red-shifted 21cm neutral hydrogen line to explore the formation of the first galaxies at about 30<z<20.
DETECTING THE EARLIEST GALAXIES THROUGH TWO NEW SOURCES OF 21CM FLUCTUATIONS (http://www.arxiv.org/PS_cache/astro-ph/pdf/0410/0410129.pdf)

Barkana & Loeb discuss using the redshifted 21 cm line from as far back as z=200 and its interactions with new galaxies, presumed to be at about z=25 +/-5, and map the early timeline during the period otherwise obscured by re-ionization.

One thing that this paper points out is that because there is little that intercepts the redshifted 21cm line, we can theoretically use it to get a picture of how Hydrogen flowed and moved in the early universe leading up to the formation of galaxies.

Another thing that the paper points out is that this method can only work going back to about z=200 [perhaps 6 million years after the big bang], since before that time, the neutral Hydrogen was too hot to give off the very low-energy 21cm line.

The paper cites LOFAR and SKA as tools that may make these measurements possible. Measuring galaxies at z=25 will use five-meter radio waves.

Exploring back to z=200 will use a wavelength a little over 40 meters. Getting reasonable positional accuracy at THAT wavelength will require a radio interferometer array in solar orbit. Fortunately keeping a dish parabolic to within a 1/8 of a one of those waves does not require precision measurements, or a lot of material. Exploring Hydrogen before z=30 may be one of the big astronomy projects by the middle of this century, or a little before.

2004-Oct-07, 03:37 PM
Stupid question: What is "z" ?

2004-Oct-07, 06:41 PM
Originally posted by Wouter@Oct 7 2004, 03:37 PM
What is "z" ?
&#39;z&#39; is a way of describing redshift. We actually measure &#39;z&#39;, and from that infer age, or time since the big bang, so it is a more direct thing to quote than saying &#39;when the universe was 6 million years old&#39;.

If something [such as Hydrogen] has a spectral line at a particular wavelength, say 122 nanometers for example, we will see its wavelengh shifted by a factor of 1+z, so at z=4 that line would appear at 610 nanometers.

In cosmology, &#39;z&#39; is often translated to distance or age.

2004-Oct-07, 08:09 PM
So the point is, that z=200 means the redshift is really high, which means that we can look back to the VERY young universe? :)