Final Report Summary - GALAXIES BIRTH (The birth of the first stars and galaxies)
We have conducted a set of smoothed particle hydrodynamics (SPH) cosmological simulations, the First Billion Years Simulations (FiBY), of which the highest resolution simulation has a SPH particle mass of approximately 1000 solar masses throughout a cubic volume of 8 Mpc side lenght. The latter comprises one of the highest resolution hydrodynamical simulations to date of such a cosmological volume down to redshift z = 6. The simulation includes sub-grid physical models for population-III and population-II star formation, molecular networks and cooling, metal enrichment from stellar populations, metal line cooling from 11 elements, seeding mechanisms for black holes from population-III remnants and feedback from them, supernovae feedback, production of dust from stars and supernovae, and is coupled in post-processing to a radiative transfer routine. These sub-grid models are prone to uncertainties in our understanding of physical processes, and one of the aims of this project is to advance our understanding and modelling of sub-grid physical processes. The statistical properties of the galaxies in the FiBY agree well with those observed for the galaxy population at z = 6 making it an ideal testbed for investigating the formation of the first stars and galaxies.
Results of this project have shown:
Re-ionization of the Universe- Proto-galaxies with stellar masses lees than 100 million solar masses can re-ionize the Universe at approximately z = 10. The escape fraction of UV photons from low-mass galaxies is larger than from massive galaxies. This is mainly due to supernova feedback being very effective in blowing out gas and creating channels of low column density for photons to escape in low mass haloes. Our results are the first to calculate self-consistently the ionization fraction of hydrogen based on the escape fraction for thousands of galaxies, the clumping factor of the IGM, and the multi-frequency spectra of stars with different ages and metallicities from a cosmological hydrodynamical simulation.
The impact of molecular hydrogen abundance on POPII and POPIII star formation- Lyman-Werner (LW) radiation with energies between 11.2 to 13.6 eV from population-III and population-II stars is able to photo-dissociate hydrogen molecules in neighbouring primordial haloes, thereby suppressing the cooling of gas below approximately 10000 K and population-III star formation in these haloes. The lack of associated feedback by pair-instability supernovae results in enhanced POPII star formation at later times. We find a factor two more in total stellar mass forms by z=6 allowing for LW feedback. Our results are the first to show the self-regulating nature of star formation due to a combination of radiative feedback in form of LW dissociation of molecules and mechanical feedback from supernovae feedback.
Formation of massive seed black holes- Gas in primordial haloes close to sites of star formation lack molecules due to Lyman-Werner radiation to cool down and the Jeans mass of the gas reaches values large enough, which under certain physical conditions could lead to the formation of massive seed black holes via the direct collapse of gas. With initial seed masses of 10000 to 100000 solar masses and a predicted number density of 0.04 per cubic Mpc at z greater than 6 they are abundant enough to provide the seeds for super-massive black holes observed at z=6. These studies are again among the first to take into account the spatial distribution of LW sources in a fully self-consistent cosmological setting.
Dark matter response to baryons- Facilitating the high resolution of the FiBY, we show that during the initial collapse of gas the adiabatic contraction model is a good approximation for the response of the halo. Once supernovae feedback kicks in dark matter haloes start expanding again due to the change of potential in the center. This abrupt change leads to the formation of a core in the dark matter halo profile similar to the ones observed in dwarf galaxies. Interestingly, we find haloes which host central galaxies that are off-center from the dark matter potential minimum, thereby heating the halo core via dynamical friction.
The transition from POPIII to POPII star formation- The critical metallicity at which POPIII star formation transitions to POPII star formation is has only weak consequences on the overall topology of star formation in the cosmic. Enrichment of the ISM takes place over time scales so short that in general one generation of stars is enough to elevate the metallicity above a wide range in critical values. Using the large volume of the FiBY we are able to investigate how efficient metal pollution of nearby haloes due to supernovae winds is. Results indicate a pollution by metals in the outer parts of nearby haloes, however, dense star formation sites within them are not significantly affected.
Results of this project have shown:
Re-ionization of the Universe- Proto-galaxies with stellar masses lees than 100 million solar masses can re-ionize the Universe at approximately z = 10. The escape fraction of UV photons from low-mass galaxies is larger than from massive galaxies. This is mainly due to supernova feedback being very effective in blowing out gas and creating channels of low column density for photons to escape in low mass haloes. Our results are the first to calculate self-consistently the ionization fraction of hydrogen based on the escape fraction for thousands of galaxies, the clumping factor of the IGM, and the multi-frequency spectra of stars with different ages and metallicities from a cosmological hydrodynamical simulation.
The impact of molecular hydrogen abundance on POPII and POPIII star formation- Lyman-Werner (LW) radiation with energies between 11.2 to 13.6 eV from population-III and population-II stars is able to photo-dissociate hydrogen molecules in neighbouring primordial haloes, thereby suppressing the cooling of gas below approximately 10000 K and population-III star formation in these haloes. The lack of associated feedback by pair-instability supernovae results in enhanced POPII star formation at later times. We find a factor two more in total stellar mass forms by z=6 allowing for LW feedback. Our results are the first to show the self-regulating nature of star formation due to a combination of radiative feedback in form of LW dissociation of molecules and mechanical feedback from supernovae feedback.
Formation of massive seed black holes- Gas in primordial haloes close to sites of star formation lack molecules due to Lyman-Werner radiation to cool down and the Jeans mass of the gas reaches values large enough, which under certain physical conditions could lead to the formation of massive seed black holes via the direct collapse of gas. With initial seed masses of 10000 to 100000 solar masses and a predicted number density of 0.04 per cubic Mpc at z greater than 6 they are abundant enough to provide the seeds for super-massive black holes observed at z=6. These studies are again among the first to take into account the spatial distribution of LW sources in a fully self-consistent cosmological setting.
Dark matter response to baryons- Facilitating the high resolution of the FiBY, we show that during the initial collapse of gas the adiabatic contraction model is a good approximation for the response of the halo. Once supernovae feedback kicks in dark matter haloes start expanding again due to the change of potential in the center. This abrupt change leads to the formation of a core in the dark matter halo profile similar to the ones observed in dwarf galaxies. Interestingly, we find haloes which host central galaxies that are off-center from the dark matter potential minimum, thereby heating the halo core via dynamical friction.
The transition from POPIII to POPII star formation- The critical metallicity at which POPIII star formation transitions to POPII star formation is has only weak consequences on the overall topology of star formation in the cosmic. Enrichment of the ISM takes place over time scales so short that in general one generation of stars is enough to elevate the metallicity above a wide range in critical values. Using the large volume of the FiBY we are able to investigate how efficient metal pollution of nearby haloes due to supernovae winds is. Results indicate a pollution by metals in the outer parts of nearby haloes, however, dense star formation sites within them are not significantly affected.