From the first stars to the first galaxies

We present the results from three-dimensional radiation-hydrodynamics calculations of the formation and evolution of Hii regions around the first stars. We carry out ray-tracing simulations of ionizing radiation transfer by embedding a population III star at the center of the primordial gas cloud formed in a small dark halo. In the case with a very massive (∼200 M⊙) star, ionization front makes a transition from D-type to R-type within the lifetime of the central star, and a large Hii region with diameter ∼1 kpc is formed. The photo-ionized and photo-heated halo gas is evacuated at a velocity of ∼30 km/s, but falls back to the halo when the halo has grown to ∼108 M⊙ in about 100 Myrs. We follow the chemical evolution of the photo-ionized gas in the fossil Hii region. In our new non-equilibrium chemistry solver, we include deuterium chemistry and cooling by HD molecules. The fractionation of HD/H2 occurs when the gas temperature becomes as low as 100 K, and the HD fraction reaches 10−5 in the case with no background UV radiation. The gas then cools by HD line cooling down to a few tens Kelvin. Interestingly, at high redshift (z > 10), the minimum gas temperature is floored by that of the Cosmic Microwave Background Radiation. The characteristic mass of the collapsing gas clump is found to be ∼10 M⊙, in an interesting contrast to a typical clump mass of ∼1000 M⊙ of the first primordial gas clouds.