Mass - forests (Tripp, Savage, and Jenkins 2000 ApJL 534,...

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Mass: the line widths are (just barely) resolvable. Together with a typical size, this was long used to give a virial mass (i.e. assuming gravitationally bound clouds). This comes out extremely large, suggesting nongravitationally bound systems (that is, they are externally confined). The column densities from equivalent widths, compared to these mases, imply that the clouds are almost completely ionized. The H I we see is only a trace constituent of ionized material. However, with what has come out about the spatial and velocity structure of the IGM, it's not at all clear that we can derive useful masses for individual features in this way. Highly ionized stuctures thus seem to exist in the IGM, which were at one time much more common than bright galaxies. Typical parameters are thought to be T = 3 x 10 4 K, M ~ 10 7 solar masses, R ~ 10 kpc, and metal abundances about 10 -3 solar (that is, "pregalactic" material). The heavy-element abundance is clearly not zero. There is an O VI forest to match the H and He II
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Unformatted text preview: forests (Tripp, Savage, and Jenkins 2000 ApJL 534, L1), a major target of this year's FUSE observations. The ionization balance is so poorly constrained that estimates of the metallicity are extremely crude. X-ray absorption measurements of O VI-O VIII work on a few lines of sight, and suggest that the hot and hotter phases of the IGM are major reservoirs of baryons. In fact, more properly, the major reservoirs of baryons. The following table lists some recent consensus values (courtesy of G. Bothun) on the fractions of critical density contributed by various kinds of (normal) matter. Phase Density fraction All baryons 0.041 Galaxies 0.003 0.001 H I 0.00039-0.00063 Lyman forest 0.008-0.012 O VI medium 0.003-0.008 The enrichment of the IGM may go beyond supernova ejecta leaking from galaxies and clusters, back to the first generation of massive stars before galaxy formation proper occurred....
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