Lyman alpha question

In article .com,
"sean" writes:
This is the quote from gcn 5237..."A preliminary analysis
shows the Lya forest up to a rather weak Lya line, "
Isnt that the same as a weak L break?

Nope. The Lyman alpha line is at rest 122 nm; the "break" is at rest
91 nm. The "forest" is between these wavelengths, corresponding to
gas along the line of sight but less redshifted than the background
object.

For instance Im still unsure of what exactly the `Lyman limit` is.

It's another word for the break. Physically, the Lyman alpha
absorption line is a transition from the n=1 to n=2 level of
hydrogen. The "break" is a transition from n=1 to the electron being
separated from the nucleus. Only photons near 122 nm in the rest
frame can easily be absorbed and make a line, but _any_ photon with
wavelength 91 nm can be absorbed. The absorption cross section
decreases, though, as the wavelength gets shorter.

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Steve Willner Phone 617-495-7123
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