Lyman Limit opacity (Xavier Prochaska)

Lyman limit systems (LLS) are QSO absorbers that have significant Lyman continuum opacity at 912 A but do not show damping wings (approximately 16.5 < log N(HI) < 19). Our dataset will provide the first survey at z > 2 of partial LLS with 16.5 < log N(HI) < 17.5, as well as the largest path length for all LLS at z < 3.4. The importance of this survey is that LLS likely provide the majority of opacity to ionizing photons in the Universe,and are therefore critical to establishing the intensity of the extreme UV background (e.g. Haardt & Madau 1996). Prochaska et al. (2009) have used the SDSS to constrain the opacity from log N(HI) > 17 (Fig. 4). Although we expect that lower column density absorbers contribute even more signicantly, uncertainties in their number density translate to poor constraints on their total opacity (black curves in Fig. 4). A measurement from the partial LLS surveyed here will fill the gap in previous measurements and close-in on a complete census of the cosmic HI opacity.