Galaxies and the Universe - Intergalactic Material

Galaxies and the Universe - Intergalactic Material -...

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1/15/12 Gala[ieV and Whe UniYeuVe - InWeugalacWic MaWeuial 1/3 ZZZ.aVWu.Xa.edX/keel/gala[ieV/igm.hWml The Intergalactic Medium We have already seen that intracluster matter exists in large quantities, apparently as a result of early galaxy evolution. It further seems unlikely that galaxy formation would have been 100% efficient by now; there might well be a great deal of matter in diffuse form spread throughout the universe. We are dealing here with "normal" baryonic matter, not exotic forms that manifest themselves mostly by gravitation. The relevant observational tests are: searching for neutral h\drogen and helium absorption in the spectra of high-redshift quasars. the diffuse X-ra\ background radiation. (This turns out to be dominated by faint discrete sources rather than a diffuse gas). searches for systematic dust reddening of very distant objects. (Again, colors of high-redshift objects allow very little scope for widespread intergalactic dust). evidence for e[ternal confinement of diffuse plasmas. The premier test for neutral gas as a component of the IGM is the Gunn-Peterson test (1965 ApJ 142, 1633; see also Shklovskii 1964 Astron. Zh. 41, 408). This uses the mapping between observed frequency and distance to associate continuum absorption shortward of Lyman α in quasar spectra with Lyman α absorption from diffuse matter spread over a wide redshift range. Such absorption is expected to have optical depth thus a function of ] even for constant density, since the mapping between distance interval and redshift interval is nonlinear. Long sought as soon as the region below Lyman α became accessible in quasar spectra, Gunn- Peterson absorption as a continuum has been detected only at redshifts ] > 6, where the IGM is almost opaque in the Lyman continuum ( Becker et al. 2001 AJ 122, 2850 , especially Figure 1 ). These observations show us the end of the cosmological reionization era, as the neutral medium after recombination was ionized by the first stars and kept that way by quasars down to today. Limits to G-P absorption at ] < 6 are at the level of a few percent (Giallongo et al. 1994 ApJL 425, L1). There are plenty of cases of line absorption from multiple discrete clouds or single galaxies, and in fact Lyman edge cutoffs from single galaxies have been seen, but this test gives a strong limit to the overall mass density of neutral hydrogen in the recent Universe: ȍ (H 0 ) < 3 x 10 -7 . Similar, less
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Galaxies and the Universe - Intergalactic Material -...

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