http://arxiv.org/abs/1907.02958
The first galaxies forming a few hundred million years after the Big Bang are the key drivers of cosmic evolution and ideal laboratories to study theories of galaxies formation. Their radiative feedback regulates star formation not only in nearby but also in distant halos. In this study, we aim to systematically study the role of UV radiation in suppressing star formation in primordial galaxies by means of destroying molecular hydrogen, the main coolant in primordial gas and to provide estimates of cold dense gas at the onset of star formation. To accomplish this goal, we perform three dimensional cosmological simulations of minihalos in different environments forming at $ z\sim 25$ immersed in a background radiation field with varying strength of UV flux below the Lyman limit between 0.01-1000 in units of $\rm J_{21}=10^{-21}~erg/cm^2/s/Hz/sr$. Particularly, we include photo-detachment of $\rm H^-$, the self-shielding of $\rm H_2$ which both were neglected in previous studies and use updated reaction rates. Our results show that $\rm H_2$ formation is suppressed, delaying gravitational collapse until halos reach the atomic cooling limit. We find that the formation of cold dense molecular gas and subsequently star formation gets delayed by 100 to 230 Myr depending on the level of the background radiation and the growth history of the dark matter halos in our simulations. The fraction of dense self-shielded gas available to cool and form stars is a strong function of the background flux and exponentially declines with the strength of incident UV flux above $\rm J_{21} \geq 1$. We also find that taking into account $\rm H_2$ self-shielding is crucial for estimating the amount of cold dense gas available for star formation. Our results suggest that a significant fraction of first stars could be observed between $z=10-15$ with the James Webb Space Telescope.
M. Latif and S. Khochfar
Mon, 8 Jul 19
22/43
Comments: Submitted for publication, comments are welcome
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