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Session 54 - Large Scale Structure.
Display session, Tuesday, January 16
North Banquet Hall, Convention Center
We extend previous studies of nonlinear hydrodynamical effects on the fragmentation of cosmological sheets in a dark matter dominated universe by allowing for the formation of hydrogen molecules. This is accomplished by solving a reaction flow system in nonequilibrium for the baryonic fluid that includes 27 chemical reactions and 9 separate species: H, H^+, He, He^+, He^++, H^-, H_2^+, H_2, and e^-. Several one-dimensional calculations are performed for different initial data parameterized by the perturbation wavelength \lambda_1 along the collapsing direction. Initial wavelengths in the range 1 to 10 Mpc corresponding to average shock velocities of 9 to 110 km/s are considered. The higher velocity shocks produce higher concentrations of molecular hydrogen ranging from n_H2=5.8\times 10^-2 cm^-3 with a mass fraction n_H2/n=2.8\times 10^-3 for the 10 Mpc case to 2.5\times 10^-7 cm^-3 and 1.5\times 10^-5 for \lambda_1=1 Mpc. The gas for those shocks (namely \lambda_1 > 1 Mpc) that produces large concentrations of H_2 then cools further through the vibrational/rotational excitation of the molecules. Because the cooling is isobaric, the accompanying increase in density together with the drop in temperature combine to collapse the gas to smaller volumes and to reduce the Jeans mass by factors up to 10^3 for \lambda_1=10 Mpc (dropping from 9\times 10^6 M_ødot when H_2 is neglected to 9\times10^3 M_ødot). Hence, the faster moving shocks are likely to fragment into smaller units that may be associated with massive stars. The fragmentation process is investigated with two-dimensional simulations for the case \lambda_1=4 Mpc. We confirm predictions from the 1D studies regarding the size and mass estimates of fragments.
