The Nature and Abundance of LYα Clouds in Voids

Abstract

I devise a method to separate low-z Lyα absorbers into void, and non-void clouds. The strong difference between void and non-void EW distribution functions enables a phenomenological definition of voids. I find the filling fraction of voids in the universe is f _v = 0.86_-0.11 ^+0.05, greater than voids defined by galaxies (f _v ≲ 0.5). I show that they cannot be explained as diffuse and expanding with the Hubble flow. Instead I model clouds as based on subgalactic halos whose baryons expand following reionization. Results of modeling show that only isothermal massive halos produce absorbers consistent with observations; NFW halos are not massive enough to restrain the evaporation of baryons, and produce no detectable absorption systems. A halo velocity distribution with slope parameter α = -1.95, and normalized to φ*_V = 0.05φ* can explain the observed distribution of void clouds.