The influence of streaming velocities and Lyman-Werner radiation on the formation of the first stars

The first stars in the Universe, the so-called Population III stars, form in small dark matter minihaloes with virial temperatures T-vir < 10(4) K. Cooling in these minihaloes is dominated by molecular hydrogen (H-2), and so Population III star formation is only possible in those minihaloes that form enough H-2 to cool on a short timescale. As H-2 cooling is more effective in more massive minihaloes, there is therefore a critical halo mass scale M-min above which Population III star formation first becomes possible. Two important processes can alter this minimum mass scale: streaming of baryons relative to the dark matter and the photodissociation of H-2 by a high redshift Lyman-Werner (LW) background. In this paper, we present results from a set of high resolution cosmological simulations that examine the impact of these processes on M-min and on M-ave (the average minihalo mass for star formation), both individually and in combination. We show that streaming has a bigger impact on M-min than the LW background, but also that both effects are additive. We also provide a fitting functions quantifying the dependence of M-ave and M-min on the streaming velocity and the strength of the LW background.

Automated summary

Population III star formation occurs in small dark matter minihaloes with specific conditions for H-2 cooling. The minimum mass scale for this formation is influenced by baryon streaming relative to dark matter and photodissociation of H-2 by a high redshift LW background. Results from high resolution cosmological simulations show that these effects are additive and impact M-min and M-ave.