Sheets and Filaments as the Origin of the High-Velocity Tail of the Lyman-Alpha Forest

Abstract

Simulations of large-scale structure formation predict the formation of sheet- and filamentary structures, which are often invoked as the origin of the Lyman-alpha forest. In their simplest description, these sheets and filaments require a differential distribution of observed line-of-sight velocity widths ($b$) which will decrease as power-laws at velocities well above the observed peak in this distribution: for filaments, the differential distribution is $dN/db \propto b^{-3}$, while for sheets it is $dN/db \propto b^{-2}$. These functional dependences on $b$ arise a priori due to the geometry of these absorbing structures -- assuming random orientations relative to the line-of-sight -- and are otherwise unrelated to the physical state in the absorbing structure. We find the the distribution at $b>35$ in three previously published data sets to be steeper than $dN/dB \propto b^{-3}$ (99.99% confidence). This implies that evidence of the finite length of these kinds of absorbing structures is present in the $b$-distribution data.

12 pages, 2 figures, Latex, requires aaspp4.sty,12pt.sty,flushrt.sty; Submitted, ApJ Letters