Extragalactic Gas at High Redshift: A Chronograph of Nonlinear Departures from Hubble Flow

Abstract

Studies of the state and evolutionary history of intergalactic and pregalactic gas provide, important diagnostics of the conditions which led to the formation and to the present-day clustering and peculiar motions of galaxies. Three topics are discussed here: (1) cosmological H II regions and the photoionization of the IGM; (2) molecular hydrogen and the radiative cooling of pregalactic shocks; and (3) the possible detection of a hot, metal-enriched IGM in X-ray absorption. We find that the observed quasar population is not sufficient to explain the absence of the Gunn-Peterson effect at z > 3 by overlapping quasar H II regions. We find that metal-free intergalactic and pregalactic shocks which radiatively cool commonly produce an H₂ fraction ≳ 10–³ and cool to T ≲ 10² K. Such shocks should be detectable in quasar spectra as large-column-density H I Ly α absorption lines as well as H₂ Lyman band absorption, unless the shocks break up by z ≳ 3.5. Lastly, we note that the recent discovery of an O VIII X-ray absorption trough towards a BL Lac object at z =.12 is consistent with earlier predictions of the X-ray opacity of a hot IGM with substantial density and oxygen-enrichment.

Alfred P. Sloan Research Fellow