HST-ACS photometry of the isolated dwarf galaxy VV124=UGC 4879: Detection of the blue horizontal branch and identification of two young star clusters

摘要

We present deep V and I photometry of the isolated dwarf galaxy VV124=UGC 4879, obtained from archival images taken with the Hubble Space Telescope-Advanced Camera for Surveys. In the color-magnitude diagrams of stars at distances greater than 40′′ from the center of the galaxy, we clearly identify a well-populated, old horizontal branch (HB) for the first time. We show that the distribution of these stars is more extended than that of red clump stars. This implies that very old and metal-poor populations dominate in the outskirts of VV124. We also identify a massive (M = 1.2 ± 0.2 × 10~4 M⊙) young (age = 250 ± 50 Myr) star cluster (C1), as well as another of younger age (C2, ≈ 30 ± 10 Myr) with a mass similar to classical open clusters (M ≤ 3.3 ± 0.5 × 10 ~3 M⊙). Both clusters lie at projected distances less than 100 pc from the center of the galaxy.