LYMAN-ALPHA ABSORPTION-LINES FROM MINIPANCAKES

Recent numerical simulations show that many Ly alpha absorption lines of column densities N-HI less than or similar to 10(15) cm(-2) are pro duced in transient minipancakes. Such pancakes are modeled here, appro ximating the initial perturbation leading to the formation of the panc ake 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 show n to hold for z(c) similar to 3, where z(c) is the collapse redshift. The Ly alpha absorption-line profiles for a line of sight through the pancake are then calculated. The absorption lines in general have wing s signifying bulk motions in the gas. It is shown that the deviation f rom 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 N-HI > 10(13) cm(-2), high temperatures tend to wash out the signatures of bu lk motion. The analytical modeling of minipancakes associated with Ly alpha forest lines-with 10(13) less than or similar to N-HI less than or similar to 10(15) cm(-2)-gives the corresponding mass scales. In st ructure formation models with cold dark matter and with reheated inter galactic matter (IGM), the gas in the IGM is inhibited from falling in to dark matter potential wells with very small velocity dispersions. S tructures with masses larger than this limit (the Jeans mass) can have an infall of gas and form minipancakes. It is shown here that, for ty pical values of cosmological parameters, absorption lines with N-HI - 10(14) cm(-2) correspond to structures with baryonic mass M-b similar to 10(10) M. with an overdensity of similar to 10 at z similar to 3. T he value of N-HI can change by a factor of similar to 3 in the course of evolution of the pancake in time. It is also shown that there is an upper limit to N-HI from a pancake due to the slow recombination rate and the importance of collisional ionization at high temperatures. Mi nipancakes do not give rise to Ly alpha lines with N-HI greater than o r similar to 10(14.5) cm(-2), for a temperature of the IGM of (T) over bar = 10(4) K, J(-21) = 1, and Omega(IGM) similar to 0.03.