The Leo II Dwarf Spheroidal - an Old Galaxy With a Large Intermediate-age Population

Abstract

Deep F555W (broadband V) and F814W (broadband I) observations have been obtained of the Leo II dwarf spheroidal galaxy with the Hubble Space Telescope Wide Field Planetary Camera 2 instrument. The resulting color- magnitude diagrams reach V = 27.4 mag and I = 26.6 mag, and the fiducial sequence has a main-sequence turnoff at V = 25.1 mag. A metallicity of [Fe/H] = -1.60+/-0.25 dex and reddening E (V - I) = 0.035+/-0.019 were determined simultaneously using the technique of Sarajedini [AJ, 107,618 (1994)]. A well-defined horizontal branch is seen at V_HB_ = 22.18+/-0.08 mag and the true distance modulus is (m - M)_0_ = 21.55+/-0.18 mag. The apparent distance moduli in V and I are (m - M)_V_=21.63+/-0.18 mag and (m - M)_I__ = 21.60+/-0.18 mag, respectively. Using a variety of age indicators, we have determined that the main stellar population of Leo II has an age of 9+/-1 Gyr. By analyzing the distribution of stars near the base of the red giant branch, we have determined that half of the stars in Leo II probably formed during a period of star formation lasting about 4 Gyr. The Leo II dwarf spheroidal galaxy started forming stars about 14+/-1 Gyr ago and formed most of its stellar population during the succeeding 7 +/- 1 Gyr with the typical star having been formed about 9+/-1 Gyr ago. The rate of star formation has been negligibly small for the last ~7 Gyr.