Mapping Intergalactic Metals at High Redshift

University Of Hawaii, Honolulu

Investigators

Abstract

AST 0098480 Cowie, A. Over the past decade, numerical models of the growth of structure in the Universe have made vast strides in understanding how the first stars and galaxies in the Universe formed from initial density perturbations. One of the great successes of these models has been in revolutionizing our understanding of the intergalactic gas (IGM) as revealed to us in the "forest" of Lyman alpha absorption lines and associated metal lines seen in the spectra of distant quasars. Previously, absorption in the IGM was understood in terms of individual clouds in pressure balance within a hot underlying smooth IGM component, or in terms of absorption in individual dark matter mini-haloes at high redshift; now the models have successfully shown that the Lyman alpha forest traces the lower overdensity portions of the "cosmic web" of filaments and voids that is the early phase of the buildup of structure --- stars, galaxies and clusters --- in the Universe. The study of quasar absorption lines is thus complementary to the study of high redshift galaxies and clusters, and allows us to see the earliest stages of galaxy formation. Stars form out of he IGM, and it in turn is enriched with heavy elements ("metals") from these stars as they return materials to the gas. Ionizing radiation from stars, galaxies and quasars also affects the balance of ions of hydrogen and other elements in the IGM. The distribution of metals in the IGM and the distribution of ions are therefore both diagnostics of the star formation history of the Universe. In addition, the models predict that the IGM is where the bulk of the baryons in the Universe exist at high redshift, thus, since this gas is fairly easily observable in quasar absorption lines, we are in the lucky position of being able to account for most of the material in the Universe at high redshift. This project will be directly concerned with understanding how the IGM comes to be rather uniformly enriched with metals, even at redshifts of 3 and higher. One possibility is that all the metals that have been observed in the Lyman alpha forest come directly from stars that are forming in the first structures that will later become present-day galaxies, but another possibility is that the whole IGM was pre-enriched by a first generation of stars at very early times: the so-called population III. Since the pre-galactic clumps form in the regions of higher overdensity in the IGM, any enrichment from these stars would necessarily be confined also to these higher density regions. In contrast, the population III stars would enrich the IGM more broadly. As numerical models become more sophisticated at incorporating feedback into the IGM from star formation and evolution, their predictions of metallicity as a function of distance form the star-forming site, and therefore of density, will become more refined. There is therefore a clear rationale for investigating the distribution of metals with density, and this will be a main focus of this project. Funding for this project was provided by the NSF program for Extragalactic Astronomy & Cosmology (AST/EXC). ***