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STARLIGHT Report Summary

Project ID: 339177
Funded under: FP7-IDEAS-ERC
Country: Germany

Mid-Term Report Summary - STARLIGHT (Formation of the First Stars)

The appearance of the first stars marked a primary transition in cosmic history. Their light ended the so-called “dark ages”, and they played a key role in the metal enrichment and the reionization of the Universe, thereby shaping the galaxies we see today. Understanding high-redshift star formation is central to many areas of modern astrophysics. However, studying stellar birth in the early Universe is a relatively young field of science, and so still little is known about the origin and observable characteristics of the first stellar populations. Shedding light on the physical processes that govern the formation of stars in the early Universe is the primary research goal of STARLIGHT. Progress in this field requires a concerted, multi-facetted approach that combines a range of complementary expertise and innovative techniques. Using novel multi-scale computer simulations with high predictive power interlaced with high-resolution observations over a wide range of wavelength, researchers in STARLIGHT aim at addressing the following four key scientific questions:

(1) What are the physical processes that governed the birth and evolution of the first and second generations of stars in the Universe?
(2) What are their statistical characteristics and how do these depend on environmental conditions?
(3) What are the observational signatures of the first and second generations of stars and how can we study their properties with current Earth-bound and spaceborne instruments?
(4) How did these stars affect their birth habitat and influence subsequent cosmic evolution?

The STARLIGHT team has made significant progress on all these four topics, either by answering certain aspects of the questions outlined above or by preparing the tools to do so in the near future. For example, on very small scales, researchers in STARLIGHT have studied the fragmentation behavior of the accretion disks around the first stars (so called Population III stars) and were able to constrain the expected mass spectrum of these objects. Clearly, more work is needed, in particular radiative feedback and magnetic fields need to be taken into account self-consistently. On intermediate scales of individual halos, STARLIGHT members have studied the escape probability of photons emitted from the first stars. This is important, because this radiative feedback is able to change the star formation behavior in nearby halos. In particular, strong irradiation can prevent and delay star formation and may lead to the formation of extremely massive stars, which then directly collapse into a supermassive black hole at the end of their lifetime. This process can be the seed for some of the extreme active galactic nuclei currently observed at high redshift. Members of STARLIGHT are investigating all phases of this sequence. On even larger scales, the team is building up a comprehensive database of high-resolution zoom-in simulations of early galaxies at high redshift. As a next step, this unique dataset will be used to make detailed and quantitative observational predictions, e.g. for the James Webb Space Telescope that will be launched soon.

On the methodological side, members of STARLIGHT have designed and implemented new radiative transfer modules for high-resolution radiation-magnetohydrodynamic simulations of reactive flows in astrophysics. The methods are fully coupled to the time-dependent chemical networks developed in the group in the past years. One approach is based on the HEALPIX compartementalization of the unit sphere, and it is used for studying radiative feedback processes in present-day and primordial star formation. The other approach employs a Voronoi tesselation of the computational domain and is specifically geared to studying radiative feedback in the early Universe. Furthermore, the team is constantly improving und updating the chemical networks necessary to correctly describe the thermodynamic state of the star forming gas and its composition.

In a complementary line of research, members of the team have developed a semi-analytic merger tree model of early star formation which can be used to statistically constrain Pop III star formation. This approach is extremely flexible and enabled STARLIGHT researchers to predict the number of low-mass Population III star expected to be observable in the Milky Way today, to calculate the expected number count of Population III supernovae, which can help to distinguish between different cosmological models, and it has been extended to compute gravitational wave emission expected from the merger of primordial black hole binaries.

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