Discovery of Latent Star Formation in the Extended H i Gas around the Local Group Dwarf Irregular Galaxy NGC 6822

Abstract

We carried out a wide-field BRC imaging survey in the outer regions of the Local Group dwarf irregular galaxy NGC 6822 using the Subaru Prime Focus Camera. The B, RC color-magnitude diagram of stars down to MB = 0.51 in the outer region shows the same features as those found for stars in the central part of NGC 6822. The distribution of blue stars (B-RC < 0.5 and -3.5 < MB < 0.5) is completely different from the optical main body of NGC 6822. The blue stars spread widely over about 50' from southeast to northwest far beyond the main body of NGC 6822, tracing the H I gas distribution. It is shown that blue stars in the outer region probably formed ~180 Myr ago. Our data supports the interaction scenario proposed by de Blok & Walter that a passage of the northwest H I cloud produced the tidal arm to the southeast of NCG 6822 and triggered the recent star formation activity found in the entire galaxy. We also discuss the possibility that the northwest H I cloud could be a forming dwarf galaxy or that NGC 6822 could be the nearest low surface brightness galaxy in the universe.