Resolved Stellar Populations in Nearby Galaxy Halos

摘要

The results of the three different studies of resolved stellar populations in galaxies across the Hubble sequence based on HST ACS and WFPC2 data are summarized in this contribution. (1) The ACS observations of the stellar halo of the gE galaxy NGC 5128 revealed the core helium-burning stars located in the red clump. Combining the metallicity distribution measured from the upper red giant branch (RGB) and the luminosities of the red clump and asymptotic giant branch (AGB) bump stars the constraints on the star formation history are given. (2) The WFPC2 observations supplemented by the deep near-IR observations with ISAAC on the ESO VLT have been used to derive the ratios of the intermediate-age to old stars in two early-type dwarf companions of NGC 5128. Their intermediate-age star fractions are lower than in outlying dSph satellites of the Milky Way. (3) The ACS observations of the color distribution of the RGB stars in the halo of the edge-on spiral galaxy NGC 891 gives the possibility to derive the metallicity distribution. The average halo metallicity at ～10 kpc above the galaxy disk is [Fe/H] = -0.9 dex, significantly higher than that of the Milky Way halo.