Extremely Metal-Poor Stars in Dwarf Galaxies around the Milky Way

Abstract

Extremely metal‐poor stars are believed to provide a unique opportunity to investigate the detailed nucleosynthesis by first generations of stars as well as the early stage of the Milky Way formation. Such objects have been found by recent surveys of dwarf galaxies around the Milky Way. Chemical abundance studies for stars in dwarf galaxies revealed the diversity of the abundance ratios (e.g. α/Fe ratios) and their metallicity dependence, suggesting that these dwarf galaxies are not simply remnants of the building blocks of the Milky Way halo, but have original enrichment histories. More recent studies for fainter galaxies (so‐called ultra‐faint dwarf galaxies) suggest a higher fraction of extremely metal‐poor stars in these galaxies than in classical dwarf galaxies, and the similarity of metallicity distribution to the field stars. However, chemical abundance ratios show large scatter, suggesting inhomogeneous chemical evolution. Comparisons of chemical abundance ratios between dwarf galaxies and the Milky Way halo are discussed.