The star formation histories of the Carina and sculptor dwarf spheroidal galaxies.

Using simulated color-magnitude diagrams to model a range of star formation scenarios, we have extracted a detailed, global star formation history for two of the Galaxy's dwarf spheroidals. Carina experienced three significant episodes of star formation at ∼15 Gyr, 7 Gyr, and 3 Gyr. Contrary to the generic picture of galaxy evolution, however, the bulk of star formation, at least 50%, occurred during the episode 7 Gyr ago, which may have lasted as long as 2 Gyr. For unknown reasons, Carina formed only 10--20% of its stars at an ancient epoch and then remained quiescent for more than 4 Gyr. The remainder (∼30%) formed relatively recently, only 3 Gyr ago. Interest in the local population of dwarf galaxies has increased lately due to their potential importance in the understanding of faint galaxy counts. We surmise that objects like Carina, which exhibits the most extreme episodic behavior of any of the dwarf spheroidal companions to the Galaxy, are capable of contributing to the observed excess of blue galaxies at B ∼ 24 only if the star formation occurred over a very short timescale (∼10 Myr). We have applied the same methods to the CMD of the Sculptor dSph and infer that the dominant old population is ∼16 Gyr old and formed over a period of 2--3 Gyr, that as many as 30--60% of all apparently single stars in the inner region of Sculptor may be binaries, and that star formation continued at a low level characterized by bursts until ∼2 Gyr ago. Since we find no radial gradients in the age or metallicity distribution within Sculptor, neither age nor metallicity can individually account for the internal 'second parameter' problem observed within the galaxy. We speculate that the possible radial gradient in the binary-star population may be related to the variation of the HB morphology in Sculptor.Ph.D.AstronomyPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/132382/2/9963812.pd