The Nature and Origin of the Nonvoid Lyα Cloud Population

Abstract

I continue my study of the low-redshift Lyα cloud population. Previous work showed how galaxy catalogs could be used to attribute relative degrees of isolation to low-redshift Lyα clouds found in Hubble Space Telescope-Goddard High Resolution Spectrograph spectra. This enabled the separation of clouds into two distinct populations corresponding to two distinct environments, variously characterized as void/unshocked and nonvoid/shocked. Void clouds have a steep equivalent width distribution (i.e., many smaller absorbers), while nonvoid clouds have a flat distribution. I show that N-body/hydro simulations of Lyα clouds are inconsistent with observations of the clouds as a function of their environments. Simulations fail to predict the existence of significant numbers of detectable void clouds and incorrectly predict the characteristics of nonvoid clouds. Implicated in this failure is the so-called fluctuating Gunn-Peterson approximation (FGPA), which envisions that Lyα absorbers are formed in the large-scale structures of coalescing matter. A recent paper (of mine) has modeled the void cloud population as subgalactic perturbations that have expanded in response to reionization. It is notable that success in this modeling was contingent on using the more massive isothermal halo in place of the standard Navarro, Frenk, & White halo, for it was found that gravitational restraint on evaporation of baryons is key to producing detectable void absorbers. In this paper I extend my modeling of Lyα clouds to nonvoid clouds, using the same basic cloud model. In the case of voids, clouds are in a quiescent environment, while nonvoid clouds are thought of as void clouds that have accreted to the denser, turbulent intergalactic medium surrounding galaxies and are thus subjected to bow shock stripping. Model void clouds are analytically shock-stripped, and a column density spectrum (CDS) is derived, based on the same halo velocity distribution function as that used to explain the void CDS. The nonvoid CDS produced by shocked subgalactic clouds are found to be capable of producing an excellent fit to the observed nonvoid CDS without recourse to the FGPA mechanism.