Star Formation History of Dwarfs in Nearby Galaxy Groups

Abstract

1. The data. We study detailed star formation histories (SFH) of 14 dwarf galaxies in the central parts ( 300 Kpc) of the two nearby galaxy groups: M 81 (D=3.6 Mpc) and Cen A (D = 3.8 Mpc). The images of 7 galaxies were selected from the sample of about 50 nearby dwarf galaxies observed with the Advanced Camera for Surveys (ACS) at the Hubble Space Telescope within our prog. 9771 & 10235, (PI I.Karachentsev). The rest of the images (ACS and WFPC2) were taken from the Hubble Archive (prog. 9884, 5898 and 6964). 2. The methods and photometry. Photometry of the resolved stars in the galaxies was made with the DOLPHOT package (Dolphin 2002, MNRAS, 332, 91) for crowded field photometry. Photometric distances for all galaxies in the sample were obtained using tip of the red giant branch (TRGB) distance indicator (Lee et al. 1993, ApJ, 417, 553; Sakai et al. 1996, ApJ, 461, 713). We have developed the StarProbe package for quantitative star formation history determination (Makarov & Makarova 2004, Ap, 47, 229). We create a library of synthetic color-magnitude diagrams from theoretical stellar isochrones, taking the initial mass function, distance to the galaxy, internal and external absorption, and photometric errors into account. The resulting synthetic diagrams are combined linearly and compared quantitatively with photometric data for stars in a galaxy in order to determine star formation rates as a function of age and metallicity.