Talk:Lyman-alpha forest

Since neutral hydrogen clouds at different positions between Earth and the distant light source see the photons at different wavelengths (due to the redshift), each individual cloud leaves its fingerprint as an absorption line at a different position in the spectrum as observed on Earth.

Shouldn't this read something more along the lines of:

Since neutral hydrogen clouds in the intergalactic medium are at different degrees of redshift (due to their varying distance), their absorption lines are observed at a range of wavelengths.

Photons that are "seen" at different wavelengths won't be absorbed due to the laws of quantum mechanics (detailed in the second paragraph). Or am I missing something? --86.10.67.229 (talk) 21:21, 28 July 2008 (UTC)Reply

Yes, you are quite correct. I've edited the article accordingly. Regards, RJH (talk) 23:34, 29 December 2011 (UTC)Reply

The image recently added (and since deleted by myself) was unfortunately a bit inaccurate. Rightward of the frequency of the Lyman-alpha transition at the rest frequency of the emitting object, the flux rises more gradually as it approaches the Lyman-alpha transition, and then falls off steeply as you move to shorter wavelengths. Then, beyond the galaxy's rest frequency Lyman-alpha absorption, the flux would still be moderately strong, except where absorbed by neutral hydrogen along the line of sight.

See Figure 1 of paper or Figure 2 of paper for a couple of examples of what this might look like. Note that Figure 1 of the first paper has a weaker forest, since it is actually a paper with evidence for reionization having occurred around a redshift of 6, but it still gives a nice sense of what the forest actually looks like. James McBride (talk) 19:00, 4 March 2012 (UTC)Reply

According to Galaxy Formation & Evolution by Mo, van den Bosch, & White (pg.88); and to Galaxies & Cosmology by Blanchard, Boisse, Combes, & Mazure (pg.310); the number of observed Lyman-alpha forest "clouds", per unit redshift, from z~(4.0-1.5), increases far faster — (dN/dz ~ (1+z)^2.5) — than would be expected, for constant mean-free-path ((n(z) σ(z))^-1 = (n₀ σ₀)^-1) between "clouds", for which case (dN/dz ~ (1+z)^0.5). If Lyman-alpha clouds are associated to (proto-)galactic halos; and if the number of (proto-)galaxies is conserved, e.g. ignoring mergers; then all of the evolution in mean-free-path is attributable to the size (cross-sectional area) of those halos. The increasing ionization, into transparency, of those halos, from z~(4.0-1.5), correlates closely, with the increasing observability, of star forming regions, and Quasars, over the same epoch, i.e. the "Madau diagram" for cosmic star-formation, modified to account for obscuration, cp. Keel (pg. 92); and:

http://www.sciencemag.org/content/328/5978/600/F1.large.jpg

Ipso facto, as (proto-)galaxy halos were heated & ionized by star formation & AGN, their decreasing neutral atomic hydrogen column densities (N(HI)), increasingly revealed the intense activities in the galaxies at their centers. Restated, before z~(1-2), star-formation was obscured (as indicated by infrared observations); and Quasars were obscured; precisely when presumably-proto-galactic halos (Lyman-alpha clouds) were larger, colder, and more neutral, than at present epoch.

Note, too, that after (z~1.5), the observed number of Lyman-alpha "clouds" decreased less rapidly — (dN/dz ~ (1+z)^0.15) — than would be expected, for constant mean-free-path between absorbers. And, according to The Road to Galaxy Formation by Keel (2nd. ed., pg. 122), the inter-galactic ionizing radiation flux, plausibly attributable to intense star-formation and AGN, has plummeted by [at least] a factor of 5, since the peak of Quasar activity, circa (z~2); and, that inter-galactic radiation seemingly causes the "truncation" of spiral galaxy HI disks, out in their peripheries, where N(HI) decreases to ~10¹⁹ cm^-2 = 0.1 cm^-3 × 1000 light-years × 10^-1 neutral fraction (pg.231). Inexpertly, a decreasing inter-galactic radiation field since z~2 would, therefore, allow HI disks to recombine and "regrow", increasing in size (cross-section), and so accounting for the observed increase in "cloud" absorbers since z~1.5, relative to expectations for no evolution. Indeed, well-formed spirals are observed out to (z<1.4); whereas at earlier epochs (z>2), galaxies are irregular, and evidence tight "knots" of star-formation (pg.233). Perhaps a professional could confirm these facts; and, if so, therewith improve the Wikipedia article ? 66.235.38.214 (talk) 08:24, 19 November 2012 (UTC)Reply