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Session 62 - Dark Matter in Large Scale Structures.
Oral session, Thursday, January 08
International Ballroom East,
Observations of the intergalactic medium (IGM) have been accumulating for the past 3 decades, but their interpretation has changed and advanced remarkably in the past 3 years. Also during the past 3 years the observational data have expanded to include not only the neutral hydrogen component of the IGM observed as the Lyman alpha forest in the spectra of high-redshift quasars, but also the singly-ionized helium component of this gas. The new He II absorption measurements provide interesting constraints on the lowest density regions of the IGM, which occupy most of the volume of the universe, whereas the much better-studied HI absorption arises primarily in moderately overdense regions. Together they provide a consistent picture of the high-redshift IGM as a tracer of cosmic structure formation via gravitational instability. The intergalactic gas appears to be photoionized by quasar radiation at z = 2-3 and has a mean density comparable to the baryon density inferred from big bang nucleosynthesis calculations and observations of the light element abundances. I will review briefly the basis for these claims.
