Modeling the Void HiColumn Density Spectrum with Subgalactic Halos

Abstract

The equivalent width distribution function (EWDF) of H I absorbers specific to the void environment has been recently derived (Manning). The findings revealed void line densities d/dz 500 at equivalent widths ≥ 15.8 mA (N 2.6 × 1012 cm-2). I show that the void absorbers cannot be diffuse (or so-called filamentary) clouds, expanding with the Hubble flow, as suggested by N-body/hydro simulations. Absorbers are here modeled as the baryonic remnants of subgalactic perturbations that have expanded away from their dark halos in response to reionization at z ≈ 6.5. A one-dimensional Lagrangian hydro/gravity code is used to follow the dynamic evolution and ionization structure of the baryonic clouds for a range of halo circular velocities. The simulation products at z = 0 can be combined according to various models of the halo velocity distribution function to form a column density spectrum that can be compared with that which is observed (Manning). To explain the observations with these models requires a search of parameter space somewhat beyond the envelope of convention. For a given circular velocity, a halo model more massive than the Navarro, Frenk, & White halo is required to reproduce the observed line density of absorbers. A more massive, nonsingular isothermal halo is used with a more favorable outcome. I find that such clouds may explain the observed EWDF if the halo velocity distribution function is as steep as that advanced by Klypin et al. Observations are best explained when individual halos have subhalos that occupy the flanks of the parent. A picture emerges in which growth by accretion of subhalos is possible. Further analysis suggests that the mass distribution around a cloud may extend significantly farther than the virial radius. Accounting for the total void mass density remains an outstanding problem.