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The Chemical Enrichment of the Intergalactic Medium

Final Activity Report Summary - IGM ENRICHMENT (The Chemical Enrichment of the Intergalactic Medium)

The tenuous medium inbetween galaxies contains most of the matter in the Universe. The enrichment of this intergalactic medium (IGM) with elements heavier than helium, which are all formed in stars, changes the efficiency with which gas clouds can collapse and form stars and galaxies. The distribution of heavy elements also provides us with a unique window onto feedback processes that play a crucial role in the formation and evolution of galaxies, because the same processes (galactic winds driven by supernovae and/or black hole activity) are also thought to be responsible for the dispersal of these elements. The goal of the research program funded by the Excellence Grant was to study the enrichment of the IGM using a multidisciplinary approach, making use of both high-quality observations and state-of-the art simulations.

An international team was formed at Leiden, consisting of the team leader (Schaye) recruited from the USA, an early-stage researcher (Wiersma; graduation expected by the end of 2009) and a postdoc (Dalla Vecchia). In addition, an early stage researcher (Duffy; graduation in December 2009) and postdoc (Booth) were recruited for 2- and 17-months, respectively. Finally, the team included external collaborators and visits to and from their host institutes were funded.

A major highlight was the workshop titled, The chemical enrichment of the intergalactic medium, in May 2009 at the Lorentz Center in Leiden. Sixty five participants from 11 countries came to Leiden to discuss recent results, controversies, and future projects. The Lorentz Center was filled to capacity and there was a healthy mix of senior scientists and students, and of talks and time for discussion.

On the observational side, three studies have been completed. Pieri et al. (2006) found that while the heavy elements are concentrated in regions close to galaxies, the enrichment is likely much more widespread than their immediate surroundings. Schaye et al. (2007) discovered a large population of compact, metal-rich gas clouds. They found that several lines of evidence suggest that most intergalactic metals reside in such high-metallicity patches. The implication is that the metals are poorly mixed on small scales and that most of the Universe may therefore be of primordial composition. Aguirre et al. (2008) studied the abundance of oxygen in the IGM and found it to be similar to that of silicon, but substantially higher than that of carbon, as expected if the metals were produced by massive stars.

A large observational program to study the interactions of galaxies with their environments is well under way. This program is the result of a new collaboration with the Caltech group led by Steidel and will form the core of the PhD thesis of Rakic.

On the simulation side, the project has been expanded beyond the original goals. We were able to take advantage of a unique opportunity: the availability of one the world's fastest computers. Schaye was awarded time on the Bluegene/L computer in Groningen to carry out the ambitious OverWhelming Large Simulations (OWLS) project.

To take full advantage of this opportunity, modules were developed to incorporate relevant physical processes. This resulted in papers on star formation laws (Schaye & Dalla Vecchia 2008), galactic winds (Dalla Vecchia & Schaye 2008, 2009), radiative cooling by heavy elements (Wiersma, Schaye, & Smith 2009a), stellar evolution and metal enrichment (Wiersma et al. 2009b), and the growth of supermassive black holes and feedback from galactic nuclei (Booth & Schaye 2009). Clearly, a lot of science enabled by the Excellence Grant is still to come.