Towards a phylogenetic analysis of galaxy evolution: A case study with the dwarf galaxies of the Local Group

Abstract

Context. The Hubble tuning-fork diagram has always been the preferred scheme for classifying galaxies. It is based only on morphology. In contrast, biologists have long taken the genealogical relatedness of living entities into account for classification purposes. Aims. Assuming branching evolution of galaxies as a "descent with modification", we show here that the concepts and tools of phylogenetic systematics that are widely used in biology can be heuristically transposed to the case of galaxies. Methods. This approach, which we call "astrocladistics", is applied to dwarf galaxies of the Local Group and provides the first evolutionary tree for real galaxies. Results. The trees that we present here are solid enough to support the existence of a hierarchical organisation in the diversity of dwarf galaxies of the Local Group. They also show that these galaxies all stem from a common ancestral kind of object. We find that some kinds of dlrrs are progenitors of both dSphs and other kinds of dlrrs. We also identify three evolutionary groups, each one with its own characteristics and own evolution. Conclusions. The present work opens a new way to analysing galaxy evolution and a path towards a new systematics of galaxies. Work on other galaxies in the Universe is in progress. © ESO 2006.