An international team of scientists led by David Martinez-Delgado (Max Planck Institute for Astronomy, Germany) has conducted research that reveals 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.
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Researchers used the Subaru Telescope to obtain high-resolution images of individual stars in a dense stream of stars in the outer regions of a nearby dwarf galaxy (NGC 4449), these outlying stars are the remains of an even smaller companion galaxy in the process of merging with its host. NGC 4449, the host galaxy, is the smallest primary galaxy in which a stellar stream from an ongoing merger has been identified and studied in detail. NGC 4449 is located 12.5 million light years from Earth and is a member of a group of galaxies in the constellation Canes Venatici. These final observations at Subaru in 2011 clearly showed the stealth merger of two dwarf galaxies. Modern cosmological theory posits that large galaxies were built up from smaller ones through an orderly succession of mergers. Although astronomers have observed many mergers involving massive galaxies, it has been difficult to find mergers of two dwarf galaxies. Theory suggests that similar processes of merging should occur on a smaller scale, with small galaxies eating even smaller ones, this is how galaxies grow. The new observations support the idea that the stellar halos around many dwarf galaxies are the remnants of smaller satellites that were shredded in past merger events. The ongoing merger in NGC 4449 may also be responsible for the intense burst of star formation seen in the galaxy.