The Role of Hydrogen Molecules in the Radiative Cooling and Fragmentation of Cosmological Sheets

Details The Role of Hydrogen Molecules in the Radiative Cooling and Fragmentation of Cosmological Sheets The Role of Hydrogen Molecules in the Radiative Cooling and Fragmentation of Cosmological Sheets

Apr 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...460..556a&link_type=abstract

Astrophysical Journal v.460, p.556

Astronomy and Astrophysics

Astronomy

52

Atomic Processes, Cosmology: Theory, Hydrodynamics, Cosmology: Large-Scale Structure Of Universe, Molecular Processes, Shock Waves

Scientific paper

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 nine separate species: H, H+, He, He+, He++, H-, H2+, H2, and e-. Several one-dimensional calculations are performed for different initial data parameterized by the perturbation wavelength λ1 along the collapsing direction. Initial wavelengths in the range 1-10 Mpc corresponding to average shock velocities of 9-110 km s-1 are considered. The higher velocity shocks produce higher concentrations of molecular hydrogen ranging from nH2 = 5.8 × 10-2 cm-3 with a mass fraction nH2/n = 2.8 × 1O-3 for the 10 Mpc case to 2.5 × 10-7 and 1.5 × 10-5 for λ1 = 1 Mpc. The gas for those shocks (namely λ1 > 1 Mpc) that produces large concentrations of H2 then cools further through the vibrational/rotational excitation of the molecules. For the λ1 = 10 Mpc case, the temperature drops to 4.3 × 10-3 eV at redshift z = 4.4, where H2 peaks in concentration. This is compared to 0.19 eV in a six species model for which H-, H2+, and H2+ are neglected. However, as the shock speed is decreased, the H2 formation time increases and for the 1 Mpc case H2 does not form rapidly enough to compensate for the decrease in the density because of the cosmological expansion and hence cannot affect the cooling of the gas. 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 ranging from 103 for λ1 = 10 Mpc (dropping from 9 x 106 Msun when H2 is neglected to 9 x 1O3 Msun) to nearly unity for λ1 = 1 Mpc. 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 λ1 = 4 Mpc. We confirm predictions from the one-dimensional studies regarding the size and mass estimates of fragments.

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