The Physics of Extragalactic Gas: One Argument for a Next Generation Space Telescope

Observations of the Galactic ISM have had tremendous impact on our understanding of the physics of galactic gas and the processes of galaxy formation. Similar observations at z > 2 reveal the neutral baryonic content of the universe, trace the evolution of metal enrichment, shed light on process of nucleosynthesis and dust formation, and yield precise measurements of galactic velocity fields. Owing to the limitations of UV spectroscopy, however, researchers are unable to examine galactic gas at 0 < z < 2, an epoch spanning ≈ 80% of the current universe. To complement the multitude of ongoing programs to identify and research z < 2 galaxies, a next generation space telescope is essential to investigate the gas which feeds and records the history of galaxy formation. 1. What is the Problem? In the next decade, numerous observational programs will identify and investigate overwhelming numbers of z < 2 galaxies. These surveys will characterize the history of star formation, the evolution of galaxy morphology, and the assembly of large-scale structure. Altogether, these efforts are likely to revolutionize our view of stars and galaxies at z < 2. In contrast to these achievements, these observations will have minimal impact on our understanding of the gas which feeds star formation. At all redshift, gas is the major baryonic component of the universe and, at z > 1, probably the dominant baryonic component of most galaxies. The principal challenge associated with examining the physics (e.g. metallicity, density, ionization state, temperature) of this gas is that the majority of diagnostics lie within the ultraviolet (UV) pass-band. To cover this epoch, one requires a next generation space telescope. With the advent of echelle spectrographs on 10m-class optical telescopes, researchers have pursued quasar absorption line (QAL) studies at unprecedented levels in the early universe. These observations have revolutionized our understanding of the Lyα forest (e.g. Miralda-Escudé et al. 1996; Rauch 1998), measured the baryonic density of the universe (Burles & Tytler 1998; Rauch et al. 1997), revealed metals in among the least overdense structures observed (Tytler et al. 1995; Ellison et al. 2000), provided new insight into the chemical enrichment history of the universe (Pettini et al. 1997; Prochaska & Wolfe 2000), and traced the velocity fields of protogalaxies (Prochaska & Wolfe 1997). These studies have tremendous impact on our theoretical description of gas in the early universe. For example, aside from the CMB, comparisons of CDM predictions