
science & techESA/Hubble
A dwarf galaxy so sparse that distant galaxies shine straight through it
Leo A is a dwarf galaxy about 2.5 million light-years away, a small member of the Local Group that includes the Milky Way and Andromeda. Its few million stars are spread so thinly that Hubble can see faint background galaxies through the gaps. It is one of the most isolated galaxies in its group and has no obvious spiral or bar, just a roughly spherical cloud of stars. Oddly, about 90 percent of its stars are less than eight billion years old, so it kept forming stars long after most galaxies of its size had finished. Astronomers are still working out why its star formation waited so long.
Star formation needs cold gas, and galaxies usually use up their gas early or lose it through encounters with neighbours. Leo A has had few such encounters, so its gas appears to have lingered and only gradually collapsed into new stars. Its low mass means its gravity holds gas loosely, which can slow the process further. Studying nearby dwarfs like it shows how small galaxies build up over time.
Leo A shows that galaxies do not all follow one timetable: a small, isolated system can hold on to its raw material for billions of years. That makes it a useful test case for models of galaxy growth, which must explain why some dwarfs are ancient and others are surprisingly young.
Dwarf galaxies are the most numerous kind of galaxy, and nearby ones like Leo A act as fossils of early galaxy formation. Comparing their star histories shows that galaxies of similar size can follow very different paths, depending on their gas and their neighbours.
ESA/Hubble · CC BY 4.0
Image: ESA/Hubble & NASA Acknowledgement: Judy Schmidt (Geckzilla)
