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

TL;DR

This study uses high-resolution cosmological simulations to analyze how streaming velocities and Lyman-Werner radiation influence the minimum halo mass for the formation of the first stars, revealing additive effects and providing quantitative models.

Contribution

It presents the first detailed simulation-based analysis of combined streaming velocity and LW radiation effects on Population III star formation thresholds.

Findings

Streaming velocities have a larger impact on the minimum halo mass than LW radiation.

Both effects are additive in influencing star formation thresholds.

Provides fitting functions for the dependence of halo mass on streaming velocity and LW background.

Abstract

The first stars in the Universe, the so-called Population III stars, form in small dark matter minihaloes with virial temperatures $T_{\rm vir} < 10^{4}$~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 $M_{\rm min}$ 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 $M_{\rm min}$ and on $M_{\rm ave}$ (the average minihalo mass for star formation), both individually and in combination. We show that streaming has a bigger impact on $M_{\rm min}$ than the LW background, but also that both effects are additive. We also provide fitting functions quantifying the dependence of $M_{\rm ave}$ and $M_{\rm min}$ on the streaming velocity and the strength of the LW background.