Galaxy Evolution Studies with Metal-deficient Blue Compact Dwarf Galaxies

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AST 0205785ThuanGalaxy formation is one of the most fundamental problems in astrophysics. Most high-z objectsobserved thus far are found to be rich in heavy elements, thus not qualifying as bona fide primevalgalaxies. An alternative approach is to search among nearby galaxies that may be actively formingstars for the first time at the present epoch. The best candidates for such a search are extremely metalpoor blue compact dwarfs (BCDs). They constitute excellent laboratories for pursuing the following cosmological problems. The standard hot big bang model of nucleosynthesis (SBBN) is one of the key quantitative tests of big bang cosmology. The primordial abundance Yp of 4He, together with that of Deuterium, is crucial for checking the consistency of SBBN. The relative insensitivity of 4He production to the baryonic matter density means that Yp needs to be determined to a precision of better than 1%. Dr. Trinh Thuan, at the University of Virginia, will evaluate systematic effects and correct Yp for them to attain that precision. He will search for more extremely metal-deficient BCDs in the newly released Sloan Digital Sky Survey spectral data base to improve the determination of Yp. The study of the BCD stellar populations will allow them to understand the properties of massive stellar populations in very low metallicity environments, like those prevailing in the era of galaxy formation. Moreover, Cold Dark Matter models predict that low-mass dwarf galaxies could still be forming at the present epoch because they originate from density fluctuations considerably smaller than those giving rise to the giant ones. Thus establishing the existence of young dwarf galaxies in the local universe will put strong constraints on the primordial density fluctuation spectrum. Dr. Thuan will apply age-dating techniques which he has developed and refined to a sample of very metal-deficient BCDs. There is a dichotomy in the way star formation proceeds in BCDs. In some, star formation occurs in a "passive" low-level diffuse fashion, with a low extinction. In others, "active" star formation occurs at a high level in Super Star Clusters, mixed with dense gas and a large amount of dust. Dr. Thuan will study the physical parameters whichgovern that dichotomy. Dr. Thuan will also study how the metallicity of the neutral gas compare with that of the ionized gas. Is the metallicity of the H ii region higher because of self-enrichment? Which constraints can be put on the time scale for the mixing of newly formed heavy elements in the H ii region with the HI gas? If the metallicity of the HI gas is very low, that would put constraints on the amount of metal pre-enrichment by the hypothetical Population III stars during the early pregalactic epoch. Such studies will shed new light on the study of abundances in high-redshift damped Lyman-alpha systems. Finally, Dr. Thuan will use these models to study correlations of the efficiency of mass exchange between the galaxy and its environment with global properties of BCDs such as their sizes, luminosities, metallicities, star formation rates, etc. ***