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Session 35 - Old Stellar Populations Beyond the Milky Way - I.
Oral session, Wednesday, June 11
North Main Hall A,

[35.07] Old Stellar Populations in Local Group Dwarf Galaxies

E. K. Grebel (AIW)

The properties of the resolvable old stellar populations in the Local Group present a unique opportunity for studying the detailed early evolutionary history of different types of galaxies and of our immediate extragalactic neighborhood. This knowledge together with proper motions and space motions will help us to evaluate the impact of interactions on galaxy evolution including the Milky Way.

Star formation histories, metallicities, and fractions and ages of subpopulations of Local Group dwarf galaxies vary substantially even within the same morphological galaxy type. However, it seems that all Local Group dwarf members contain old stellar populations. The fraction of old stars appears to depend to a certain extent on the environment and the distance to the large spirals.

While old populations can be studied in great detail in the closest dwarf galaxies, crowding and faintness limit our knowledge in distant Local Group members. Tracers of old stellar populations and different methods to study old populations are reviewed. Accurate age information can only be obtained through photometry reaching well below the main-sequence turnoff(s). While recent HST studies provide data of unprecedented depth and quality, large-area coverage and good number statistics are paramount for the detection of subpopulations distinct in age, metallicity, and/or spatial distribution. Surprisingly even in dwarf spheroidals, the lowest-mass galaxies known, stellar populations may exhibit a considerable spread in age, metallicity, and position indicating a very complex evolutionary history.

Several programs are currently underway for in-depth studies of the properties of old stellar populations in Local Group dwarf galaxies. After reviewing the present knowledge on these populations they will be put in context of the overall star formation history of these galaxies, and a unified evolutionary scenario will be suggested.

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Program listing for Wednesday