Lyman forest

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. 

The total baryonic contribution in the universe, including the hypothesised WHIM, sums to flb = 0.028 0.005. The corresponding baryon fraction flb/flm is 0.10 0.02. This is to be compared with primordial nucleosynthesis at z 109 fli, = 0.04 0.004. In addition the CMB peak heights at z 1000 yield a similar value fib = 0.044 0.003. Finally, Lyman alpha forest modelling at 2 3 suggests that fli, ss 0.04. There is also the indirect measurement of baryon fraction from the intracluster gas fraction of 15%. From this, combined with flm, we also find that fib 0.04. 

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.)... 

The question of loss of metals from galaxies is profound because of the existence of metals in low column density Lyman-a forest systems (Ellison et al. 1999), which are probably gas clouds residing outside of galaxies. Where the heavy elements came from in these systems is still a mystery it is possible that they were seeded with elements... 

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). 

Miralda-Escude Rees 1997), where Z is the metallicity (by mass) and mp the mass of the proton. Since Zq = 0.02 (Grevesse Sauval 1998), if the Lya forest at 2 5 had already been enriched to a metallicity Z ya 10 -3 o, eq. (3.6) implies that by that epoch stars had emitted approximately three Lyman continuum (LyC) photons per baryon in the universe. Whether this photon production is sufficient to have reionised the IGM by these redshifts depends critically on the unknown escape fraction of LyC photons from the sites of star formation. 

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