Resolving the Ly alpha forest

In this paper we critically examine predictions of the Ly alpha forest with in the standard cold dark matter (SCDM) model, paying particular attention to the low end of the column-density distribution. We show in particular th at the width of these lines, typically measured by the b-parameter of a Voi gt profile, is sensitive to spatial resolution in numerical simulations and has previously been overestimated. The new result, which predicts a distri bution with a median b of around 20-22 km s(-1) at z = 3, is substantially below that observed. We examine a number of possible causes of this discrep ancy and argue that it is unlikely to be rectified by an increase in the th ermal broadening of the absorbing gas but is instead telling us something a bout the distribution of matter on these scales. Although the median differ s, the shape of the b-parameter distribution agrees quite well with that ob served, and the high-end tail is naturally produced by the filamentary natu re of gravitational collapse in these models. In particular, we demonstrate that lines of sight that obliquely intersect filaments or sheets tend to p roduce absorption lines with larger b parameters. We also examine the physi cal nature of the gas that is responsible for the forest, showing that for lines with neutral column densities below N-H1 similar to 10(14) cm(-2) (fo r this model at z = 3), the peculiar infall velocity is actually slower tha n the Hubble flow, while larger lines have, on average, turned around and a re collapsing.