Dark Matter Halo Environment for Primordial Star Formation

We study the statistical properties (such as shape and spin) of high-z halos likely hosting the first (PopIII) stars with cosmological simulations including detailed gas physics. In the redshift range considered (11 < z < 16) the average sphericity is 〈s〉 = 0.3 ± 0.1, and for more than 90% of halos the triaxiality parameter is T . 0.4, showing a clear preference for oblateness over prolateness. Larger halos in the simulation tend to be both more spherical and prolate: we find s ∝ M h and T ∝ M h , with αs ≈ 0.128 and αT = 0.276 at z = 11. The spin distributions of dark matter and gas are considerably different at z = 16, with the baryons rotating slower than the dark matter. At lower redshift, instead, the spin distributions of dark matter and gas track each other almost perfectly, as a consequence of a longer time interval available for momentum redistribution between the two components. The spin of both the gas and dark matter follows a lognormal distribution, with a mean value at z = 16 of 〈λ〉 = 0.0184, virtually independent of halo mass. This is in good agreement with previous studies. Using the results of two feedback models (MT1 and MT2) by McKee & Tan (2008) and mapping our halo spin distribution into a PopIII IMF, we find that at high-z the IMF closely tracks the spin lognormal distribution. Depending on the feedback model, though, the distribution can be centered at ≈ 65M⊙ (MT1) or ≈ 140M⊙ (MT2). At later times, model MT1 evolves into a bimodal distribution with a second prominent peak located at 35− 40M⊙ as a result of the non-linear relation between rotation and halo mass. We conclude that the dark matter halo properties might be a key factor shaping the IMF of the first stars.