Lyman Alpha Absorption Lines from Mini Pancakes

Recent numerical simulations show that many Lyα absorption lines of column densities NHI < ∼ 10 15 cm are produced in transient, mini pancakes. Such pancakes are modeled here, approximating the initial perturbation leading to the formation of the pancake as a single sinusoidal wave. The density and temperature profiles of the gas in the pancake are determined in the case where cooling and heating rate of the gas is larger than the Hubble time, which is shown to hold for zc ∼ 3, where zc is the collapse redshift. The Lyα absorption line profiles for a line of sight through the pancake are then calculated. The absorption lines in general have wings signifying bulk motions in the gas. It is shown that the deviation from a single Voigt profile is large for small H I column density lines, in which the effect of bulk motions is large. For lines with NHI > 10 13 cm, high temperature tend to wash out the signatures of bulk motion. The analytical modeling of mini pancakes associated with Lyα forest lines— with 10 < ∼ NHI < ∼ 10 15 cm—gives the corresponding mass scales. In structure formation models with CDM and with reheated IGM, the gas in the IGM is inhibited from falling into dark matter potential wells with very small velocity dispersions. Structures with masses larger than this limit (the Jeans mass) can have infall of gas and form