The Lyman Alpha Forest

There are two important properties of the Lya forest which we should keep in mind. One is that it is highly ionised, so that the H I we see directly is only a small fraction ( 10-3 to 10-6) of the total amount of hydrogen present. With this large ionisation 

Max Pettini Element Abundances Through The Cosmic Ages 

The question of interest is Where do these metals come from . Obviously from stars (we do not know of any other way to produce carbon ), but are these stars located in the vicinity of the Lya clouds observed—which after all are still at the high column density end of the distribution of values of N(H I) for intergalactic absorption—or are we seeing a more widespread level of metal enrichment, perhaps associated with the formation of the first stars which re-ionised the universe at z 6 (Songaila Cowie 2002)  

A level of metal enrichment of 10-3 to 10 2 of solar in regions of the IGM with N(H I) 1014 cm-2 may still be understood in terms of supernova driven winds from galaxies. The work of Aguirre et al. (2001) shows that such outflows which, as we shall shortly see ( 4.5) are observed directly in Lyman break galaxies at z = 3, may propagate out to radii of several hundred kpc before they stall. However, if O VI is also present in Lya forest clouds of lower column density, as claimed by Schaye et al. (2000), an origin in pregalactic stars at much earlier epochs is probably required (Madau, Ferrara, Rees 2001). 

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There is a significant but subdominant mass in dark baryons. Spheroid stars amount to 10% of the baryons or 0.004 in terms of fDisk stars contribute 5% or 0.002 in Intracluster gas amounts to 5% or fib = 0.002. The Lyman alpha forest (at z 0) contains 29 4% of the baryons or = 0.008. This is all we actually observe in any quantifiable amount. In addition, intermediate temperature intergalactic gas, the so-called warm/hot intergalactic medium (WHIM) has been detected, at a temperature of 105 — 106K. It is estimated from simulations (at z 0) to amount to 30% of the local baryons or f = 0.012, with however a large uncertainty. Indeed the WHIM simulations do not resolve the Jeans mass at the resolution limit, and and the existence of WHIM is purely a theoretical inference, at least in so far as its quantitative fraction is concerned. 

A couple more "warm" candidates probably go about here, including mirror or shadow neutrinos or majorons (ApOl, Sect. 12.5). These must have masses in excess of 0.25 - 0.4 keV or something bad will come down the chimney (probably reionization that smears out the Lyman alpha forest clouds). A 40 eV neutrino popped up in Ap97 (Sect. 12.2), and it was not clear how to avoid having so many of these that the universe would fold up into Pauli s pocket. (See Ap02 for generic objections to warm DM of any sort.)... 

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