When astronomers observe galaxies that are billions of light years away from Earth, they see back in time at how star systems appeared billions of years ago. These observations from galaxies' past have resulted in a remarkable insight into how galaxies grow as big as they are. The first galaxies to form are dwarf galaxies and these subsequently merge to form larger star systems, according to the widely accepted 'cold dark matter' theory. The smallest galaxies around our Milky Way are the nearby dwarf spheroidals, which may be leftovers of our galaxy formation. Further out are slightly misshaped dwarf irregular galaxies, which may be newcomers to our galactic neighbourhood. Astronomers working on the DWARFGALAXIES project wondered if the nearby dwarf spheroidal galaxies have all the same star-forming 'stuff' that we find in more distant dwarf galaxies. Astronomers working on the DWARFGALAXIES project wondered why the nearby dwarf spheroidal galaxies do not have all the same star-forming 'stuff' that we find in more distant dwarf galaxies. Are dwarf spheroidal and dwarf irregular galaxies different objects? Or can they be considered as descending from similar progenitors which some mechanism made evolve differently? To find answers to these questions, the DWARFGALAXIES astronomers brought to bear the combined power of giant telescopes around the world on understanding the so-called dwarf galaxies. Dwarf transition galaxies have intermediate properties to dwarf spheroidal and dwarf irregular galaxies, and so are thought to be caught in the act of transforming from one type to the other. Furthermore, they were found beyond the point that dwarf spheroidal galaxies become rare while the dwarf irregular counterparts flourish. This is the point that is thought to coincide with the edge of the Milky Way's dark matter distribution. The researchers found hints of similarities in the way stars of different ages are distributed within dwarf galaxies, independently on the type of dwarf galaxy. Also the general motions of stars in dwarf spheroidals appear more alike to those of dwarf irregulars if one looks at the stars in these latter systems rather than in the gas. Their observations open up further questions that need follow-up, providing targets for future telescope facilities such as the MOONS spectrograph on the Very Large Telescope. In the meantime, the new data acquired by the researchers will strengthen our understanding of similarities and differences in the properties of the various dwarf galaxy types. The Gaia space mission will provide invaluable information on what may have been the role of interaction with the Milky Way in shaping the nearby dwarf spheroidals.
Dwarf galaxy, dark matter, Milky Way, hydrogen gas, tidal interaction, chemical composition, Very Large Telescope, Gaia