Yakobchuk T. Stellar populations and star formation history of dwarf galaxies with low metallicity.

The results of the photometric analysis of the stellar populations in the low-metallicity dwarf galaxies are presented with the use of the Hubble Space Telescope data. A deep photometry was obtained for stars in the blue compact dwarf galaxy I Zw 18. The galaxy was found to have no halo of the red giant branch stars which can indicate its young age. The measurements of the tip of the possible red giant branch in I Zw 18 resulted in the ages of these stars of 1.2 Gyr in the main body and 1.5–2 Gyr in the C component. This gave the distance of 11–14 Mpc to I Zw 18, in agreement with the estimates using the other types of stars. In the dwarf galaxy SBS 1415+437 there was a spatial age gradient revealed for the stellar population younger than 50 Myr. The metallicity of SBS 1415+437 determined from the red giant branch stars is found to be [Fe/H] = –2.41 ± 0.22 which is lower by one order of magnitude than the value found from its HII regions. The difference can imply the young age of SBS 1415+437 of ~2 Gyr. However, in the case of very low initial metallicity the age of 10 Gyr is also cannot be ruled out. Stars belonging to the red clump have been detected in the color-magnitude diagram for the galaxy KK 230. From its properties the metallicity of oldest stars in the galaxy was found to be equal to Z = 0.0004 and the age to less than 2–3 Gyr. The results presented in the thesis led to the conclusion about the possible existence of local dwarf galaxies in which star-formation has started quite recently. This conclusion is important for verifying modern cosmological theories of the galaxy formation and evolution.