Resolving the Lyman-alpha Forest

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Abstract
In this paper we critically examine predictions of the Ly-alpha forest within the standard cold dark matter model. We show in particular that the width of these lines, typically measured by the b-parameter of a Voigt profile, is very sensitive to spatial resolution in numerical simulations and has previously been overestimated. The new result, which predicts a distribution with a median b of around 20 km/s at z=3, is substantially below that observed. We examine a number of possible causes and argue that it is unlikely to be rectified by an increase in the temperature of the absorbing gas, but is instead telling us something about the distribution of matter on these scales. The shape of the b-parameter distribution agrees quite well with that observed, and the high-end tail is naturally produced by the filamentary nature of gravitational collapse in these models. We also examine the physical nature of the gas which is responsible for the forest, showing that for lines with neutral column densities below N_HI ~ 10^14 cm^-2 (for this model), the peculiar infall velocity is actually slower than the Hubble flow, while larger lines have, on average, turned around and collapsed.

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