“Stealth Merger” of Dwarf Galaxies

The Subaru Telescope captured a “stealth merger” of dwarf galaxies, where an in-falling satellite galaxy is nearly undetectable by conventional means yet has a substantial influence on its host galaxy. Subaru’s high-resolution images show individual stars in a dense stream of stars in the outer regions of a nearby dwarf galaxy (NGC 4449). This galaxy is famous for a burst of recent star formation, depicted as blue hues in the center of the large galaxy. The red color in its periphery and in its satellite indicates the presence of older, red-giant stars. The ongoing merger in NGC 4449 gives us clue to the formation of the halo structure around dwarf galaxies as well as the intense burst of star formation.