Metals were the primary driver, next to reionization, of the transformation of the Universe during its first billion years. Their presence refashioned the thermodynamic behavior of the gas collecting in the first bound cosmological objects, and catalyzed the transition from the Population III star formation mode into the pervasive, metal-enriched, clustered, Population I & II mode that heralded the epoch of the galaxies. The behavior of metals in the early Universe was stochastic, inhomogeneous, and subject to nonlinear and nonlocal feedback effects. This project will explore the complexity of metal dispersal by studying the transport and mixing of metals and dust in the first structures. It also targets the nucleosynthetic footprint of pre-galactic metal enrichment in the nearby Universe. The researchers will continue their successful use of large-scale numerical simulations to model the multi-scale build-up of structure, focusing on the transport of metals from their sources in the first supernovae into the cosmic web, their mixing with the primordial gas, their fallback into the first galaxies, and finally, their influence on the fragmentation of the proto-galactic medium and star formation. Importantly, the team will mirror the simulations using both smoothed particle hydrodynamics and adaptive mesh refinement methods, to constrain any numerical systematics. They will characterize nucleosynthetic heterogeneities in the first galaxies, caused by fine-grain mixing of proto star-cluster gas driven by turbulence, and consider the observational characteristics of these first galaxies with an eye to future observations. They will also study the transition from gravitational infall and virialization to turbulent fragmentation and local gravitational instability.
This work addresses the observable properties of first objects and will therefore guide astronomical searches with next-generation infrared facilities. Involved graduate and undergraduate students will learn advanced astrophysics and cutting-edge methodologies for large-scale numerical modeling and the visualization of multi-scale systems. Created animations will be freely available through a comprehensive web-based introduction to the formation of first objects, aimed at the general public. The newly developed algorithms will be made public towards the end of the award. The principal investigator will continue his previous writing of popular articles.
This research has investigated how the first stars transformed the early universe from its pristine state into one of ever-increasing complexity. The specific focus was the enrichment of the early cosmos, which intially only consisted of the hydrogen and helium produced in the Big Bang itself, with heavy chemical elements. Employing powerful supercomputers, we have carried out a suite of numerical simulations on three key aspects of the early enrichment problem: (1) We have studied the formation of the first, so-called Population III, stars with extremely high resolution and unprecedented physical realism; the main result is that the first stars typically formed in small groups, with masses of a few tens of solar masses. Crucially, stars of such mass die as core-collapse supernova explosions, similar to the famous supernova that exploded in the Large Magellanic Cloud in 1987. (2) This sets the stage for the second class of simulations. We now followed, again with extremely high numerical resolution, the evolution of individual Population III supernova remnants. The crucial question was how well the newly produced heavy elements mixed with the surrounding pristine gas. We found that the mixing and transport of the first heavy chemical elements sensitively depend on the mass of the exploding star. With our simulations, we were able to ascertain the realistic conditions in the gas that provides the raw material for the second generation of, so-called Population II, stars. (3) In the third part of our integrated research program we were now able to study the formation of one of the first stellar clusters to form in the universe. We found that such a cluster has a distribution of masses which is already similar to that observed locally in the present-day Milky Way. The significance is two-fold: Firstly, some of the cluster stars will have such low masses, roughly the mass of the Sun or less, that they survive for the entire age of the universe and can still be observed in our local neighborhood, as fossils of those early cosmic times. Secondly, these stellar clusters are the target for upcoming very deep imaging with the James Webb Space Telescope. Thus, our research provides key theoretical input to plan, and interpret, some of the key observations to be made with next-generation telescopes. We have popularized our results in a number of ways: we have authored popular journal articles and book chapters. The PI has given popular outreach talks, and we have had frequent press coverage of our work. In addition, we have trained 5 graduate students in cutting-edge theoretical and computational astrophysics.
