DISCOVERY OF RR LYRAE STARS IN THE SCULPTOR GROUP DWARF GALAXIES

摘要

We report on discovery of first unambiguous evidence of the presence of ancient stars in galaxies beyond the Local Group. In two Sculptor Group galaxies, ESO 294-010 and ESO 410-005, we have used deep multi-epoch Hubble Space Telescope ACS observations to image stellar populations fainter than the horizontal branch magnitude. In both galaxies we uncovered a well populated blue horizontal branch, as well as numerous RR Lyrae variable stars. The presence of these low mass star testifies that the star formation in dwarf galaxies started at least 10 Gyr ago.