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2.4.2. Plasma in Cool Low Surface Density Clouds

In addition to the hot plasma detected in X-rays, there is a considerable amount of plasma in cool, thermally stable, photoionized clouds with temperatures less than about 20,000 K. These cool low column density clouds are detected by the Lyman-alpha resonance absorption line in quasar spectra. They may be found as far as 5 Mpc from a galaxy, and generally seem to avoid both the voids and strong concentrations of galaxies (Shull, Stocke, & Penton 1996). This is consistent with the view that such clouds were left behind in the gravitational assembly of groups of galaxies; those that would have belonged to cluster members were incorporated and shock heated by the assembly of the clusters, joining the hot phase. In our budget for plasma in groups we thus use the sum of the mean baryon densities in these clouds and in cluster baryons detected by X-ray emission (eq. [29]).

The estimate of the baryon abundance in such clouds is very sensitive to model details such as ionization and geometry. Let us assume as an illustrative model that the clouds are uniform density, isothermal spheres in photoionization equilibrium, and the size is of the order of 100 kpc, determined empirically by common absorption between two nearby lines of sight. Then the mass of the clouds that yield neutral hydrogen column density NHI are estimated, following Shull, Stocke & Penton (1996), to be

Equation 30 (30)

where J-23 is the ionizing UV flux at the Lyman limit in units of 10-23 erg s-1 cm-2 sr-1 Hz-1, with a power index alpha, T is the gas temperature, N14 is the neutral hydrogen column density in units of 1014 cm-2, and R is a typical cloud size. The neutral hydrogen fraction x = nHI / nH appeq 6 x 10-5 J-1/2-23 [T/(2 x 104)]-0.375 N1/214 (R / 100 kpc)-1/2 [(3 + alpha)/4.5]1/2. With the detection frequency of the absorbers at z appeq 0, dNc / dz = phi0 (pi R2) c2 / H0 approx 86, the mass density of the clouds is

Equation 31 (31)

This assumes spherical clouds. If the clouds are oblate the density decreases by the aspect ratio, which we assume to be between 1/5 and 1.

Direct simulations of cloud formation and absorption spectra permit a much more detailed though model-dependent picture of the clouds. It has been demonstrated that these simulations give good fits to the observations at z approx 3 (see below) but they do not not yet yield useful approximations to the situation at z = 0. Thus the direct estimates of the density parameter in this component remain quite uncertain.

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