Three-Dimensional Numerical Simulations of Thermal-Gravitational Instability in Protogalactic Halo Environment

We study thermal-gravitational instability in simplified models for protogalactic halos using three-dimensional hydrodynamic simulations. The simulations started with isothermal density perturbations of various power spectra, and followed the evolution of gas with radiative cooling down to T = 10 K, background heating, and self-gravity for up to ∼ 20 cooling times. Then cooled and condensed clouds were identified and their physical properties were examined in detail. In our models, the cooling time scale is several times shorter than the gravitational time scale. Hence, during early stage clouds start to form around initial density peaks by thermal instability. Small clouds appear first and they are pressure-bound. Subsequently, the clouds grow through compression by the background pressure as well as gravitational infall. During late stage cloudcloud collisions become important, and clouds grow mostly through gravitational merging. Gravitationally bound clouds with mass Mc & 6 × 10 6 M⊙ are found in the late stage. They are approximately in virial equilibrium and have radius Rc ≃ 150− 200 pc. Those clouds have gained angular momentum through tidal torque as well as merging, so they have large angular momentum with the spin parameter 〈λs〉 ∼ 0.3. The clouds formed in a denser background tend to have smaller spin parameters, since the self-gravity, compared to the radiative cooling, is relatively less important at higher density. The H2 cooling below T = 10 4 K does not drastically change the evolution and properties of clouds, since it is much less efficient than the H Lyα cooling. The slope of initial density power spectrum affects the morphology of cloud distribution, but the properties of individual Department of Earth Sciences, Pusan National University, Pusan 609-735, Korea: [email protected], [email protected] Korea Astronomy & Space Science Institute, Whaam-Dong, Yuseong-Gu, Taejeon 305-348, Korea: [email protected] Department of Astronomy & Space Science, Chungnam National University, Daejeon 305-764, Korea: [email protected]