ar X iv : a st ro - p h / 96 10 22 5 v 1 2 8 O ct 1 99 6 GHRS Observations of the Lyman α Forest

I review the results obtained using the GHRS on low redshift Lyman α absorbers. Until the advent of HST and the GHRS, the existence of such absorbers was doubted. The confirmation of their existence, in one of the first GHRS GTO team results to be published, must rank as one of the HSTs most interesting results. The GHRS resolution allows us to probe equivalent widths well below those detectable with the FOS, and has led to a number of interesting new questions. One example is the apparent disagreement between the GHRS result that there are many Lyman α absorbers which are not associated with luminous galaxies, and FOS studies which suggest that all such absorbers have a nearby galaxy causing them. This almost certainly shows that the equivalent width (or column density) range reachable by the GHRS includes gas from a wide range of causes, and not only the halos of luminous galaxies. With these data, we are seeing the debris left over from Galaxy formation, material flung out from galaxy interactions and starbursts, and also diffuse halo material at the outer edges of normal galaxies. 1. The picture prior to HST The Lyman α 'forest' refers to the dense overlapping absorption seen shortward of the Lyman α emission line in high redshift QSO spectra (see figure 1 for an example, taken from Lu et al. 1996). This absorption is caused by a huge number of foreground condensations of warm gas, with a significant component of neutral hydrogen. Nice reviews of the observational situation just prior to the launch of HST are given by Carswell 1988, and Hunstead 1988. A non-specialist summary would be: the detectable neutral hydrogen column densities in the forest vary from less than 10 13 cm −2 up to ∼10 17 cm −2 , above which the systems become optically thick to the Lyman continuum (referred to as Lyman-limit systems), and generally show Mg II absorption. There is a roughly power law fall-off in density of absorbers with column density, such that it is considered reasonable to associate these rarer Lyman-limit systems with the inner regions of the halos of luminous (L*) galaxies. Above 10 15.5 cm −2 , C IV absorption is often detected, and so these systems are called 'metal-line' systems, although recent Keck HIRES observations suggest that the carbon abundances are comparable in the lower column density systems as well (around 0.01 solar, …