Introduction

Since the discovery of the high-redshift Ly forest over 25 years ago, these abundant absorption features in the spectra of QSOs have been used as evolutionary probes of the intergalactic medium (IGM), galactic halos, and now large-scale structure and chemical evolution. The rapid evolution in the distribution of lines per unit redshift,

(

for ), was consistent with a picture of these features as highly ionized ``clouds'' whose numbers and sizes were controlled by the evolution of the IGM pressure, the metagalactic ionizing radiation field, and galaxy formation. Early observations also suggested that Ly clouds had characteristic sizes kpc, were much more abundant than (L_*) galaxies and showed little clustering in velocity space. They were interpreted as pristine, zero-metallicity gas left over from the recombination era. One therefore expected low-redshift (z < 1) absorption clouds to show only traces of H I, due to photoionization and evaporation in a lower pressure IGM. All these ideas have now changed with new data and new theoretical modeling.

Absorption in the Ly forest of H I (and He II) has long been considered an important tool for studying the high-redshift universe (Miralde-Escudé & Ostriker 1990; Shapiro, Giroux, & Babul 1994; Fardal, Giroux, & Shull 1998). A comparison of the H I and He II absorption lines provides constraints on the photoionizing background radiation, on the history of structure formation, and on internal conditions in the Ly clouds. In the past few years, these discrete Ly lines have been interpreted theoretically by N-body hydrodynamical models (Cen et al. 1994; Hernquist et al. 1996; Zhang et al. 1997) as arising from baryon density fluctuations associated with gravitational instability during the epoch of structure formation. The effects of hydrodynamic shocks, Hubble expansion, photoelectric heating by AGN, and galactic outflows and metal enrichment from early star formation must all be considered in understanding the IGM (Shull 1998).

One of the delightful spectroscopic surprises from the Hubble Space Telescope (HST) was the discovery of Ly absorption lines toward the quasar 3C 273 at

by both the Faint Object Spectrograph (FOS, Bahcall et al. 1991) and the Goddard High Resolution Spectrograph (GHRS, Morris et al. 1991, 1995). In this review, we describe (§2) the current status of our group's long-term program with the HST and VLA to define the parameters and nature of the low-redshift Ly forest. In §3, we discuss related theoretical work on the metagalactic ionizing background, , and the contribution of low-z Ly clouds to the baryon density, .

© Copyright Astronomical Society of Australia 1997