Mathematics – Logic
Scientific paper
Jul 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999hst..prop.8272a&link_type=abstract
HST Proposal ID #8272
Mathematics
Logic
Hst Proposal Id #8272 Field Stellar Populations
Scientific paper
The Galaxy's dwarf spheroidal {dSph} companions show a surprising diversity in the epochs of their major star formation episode{s}, extending from 15 Gyr to 3 Gyr. To aid in understanding the origin of this diversity, we need similar results for M31's dSph companions, the nearest set of morphologically similar galaxies. Observations of old main sequence turnoffs in the M31 dSphs are impractical, but with WFPC2 we can establish their horizontal branch {HB} morphologies, which observations of Galactic dSphs have shown to be a reliable mean age indicator. Cycle 4 observations indicate that the bulk of the star formation in And I took place 10 Gyr ago, though the presence of blue HB and RR Lyr stars testify to a minor older component. Analysis of Cycle 6 observations of And II are nearing completion, and observations of And III are scheduled for execution in Cycle 7. Here we seek HST time to complete our study of the M31 dSphs by observing And V & VI, newly discovered M3 1 dSph companions. The primary aim is to determine the HB morphology and thus fix the epoch of the major star formation episode{s} in these galaxies. Other results that will follow include an estimate of the true {unprojected} distance from M31 and limits on the fraction of younger stars, since the data will reveal any main sequence stars younger than 2-3 Gyr.
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