Delaying the formation of the first stars The impact of streaming velocities and Lyman-Werner radiation in cosmological hydrodynamical simulations

In this thesis, we study which effects can delay the formation of the first stars in the Universe. These Population III stars were the first luminous objects emerging after recombination, starting to end the dark ages. They form in minihaloes or first galaxies, and the minimum mass of these objects is strongly related to the number of Population III stars that formed. In some regions of the Universe, there is an offset velocity between baryons and dark matter, the so-called streaming velocity. In this thesis, we show that in regions with large streaming velocities, the halo masses necessary to form the first stars increase. These regions are the perfect environment for the formation of direct collapse black holes. Feedback in the form of Lyman-Werner radiation emitted by the first stars can destroy molecular hydrogen and prevent the primordial gas cloud from cooling and collapsing. This also leads to an offset in time and halo mass for Population III star formation. To account for the strength of the Lyman-Werner background, we performed simulations that study the escape fraction from minihaloes and the first galaxies. With these results, we provide a piece of the jigsaw of the star formation history of the Universe

The Influence of Streaming Velocities on the Formation of the First Stars

How, when and where the first stars formed are fundamental questions regarding the epoch of Cosmic Dawn. A second order effect in the fluid equations was recently found to make a significant contribution: an offset velocity between gas and dark matter, the so-called streaming velocity. Previous simulations of a limited number of low-mass dark matter haloes suggest that this streaming velocity can delay the formation of the first stars and decrease halo gas fractions and the halo mass function in the low mass regime. However, a systematic exploration of its effects in a large sample of haloes has been lacking until now. In this paper, we present results from a set of cosmological simulations of regions of the Universe with different streaming velocities performed with the moving mesh code AREPO. Our simulations have very high mass resolution, enabling us to accurately resolve minihaloes as small as $10^5 \: {\rm M_{\odot}}$. We show that in the absence of streaming, the least massive halo that contains cold gas has a mass $M_{\rm halo, min} = 5 \times 10^{5} \: {\rm M_{\odot}}$, but that cooling only becomes efficient in a majority of haloes for halo masses greater than $M_{\rm halo,50\%} = 1.6 \times 10^6 \: {\rm M_{\odot}}$. In regions with non-zero streaming velocities, $M_{\rm halo, min}$ and $M_{\rm halo,50\%}$ both increase significantly, by around a factor of a few for each one sigma increase in the value of the local streaming velocity. As a result, in regions with streaming velocities $v_\mathrm{stream} \ge 3\,\sigma_\mathrm{rms}$, cooling of gas in minihaloes is completely suppressed, implying that the first stars in these regions form within atomic cooling haloes.Comment: 13 pages, 16 figures, resubmitted to MNRA

Micronutrient Sprinkles to Control Childhood Anaemia

Over 750 million children have iron-deficiency anemia. A simple powdered sachet may be the key to addressing this global proble