Lying adjacent to the Local Group of galaxies — a group that includes the Milky Way — is a vast region of space known as the Local Void. This region, roughly 150 million light-years across, is mostly empty — at least in comparison to its crowded neighborhood — but it still boasts the occasional wanderers.
The Hubble space telescope has now captured two such wanderers — two tiny dwarf galaxies Pisces A and B. These galaxies, under the gravitational pull of a nearby galactic “big city,” have now entered a crowded region rich in intergalactic gas. The ensuing interaction with either the intergalactic gas or a gaseous filament has now triggered a firestorm of star birth in the formerly quiescent pair.
“These galaxies may have spent most of their history in the void,” Erik Tollerud from the Space Telescope Science Institute in Baltimore, Maryland, and the lead author of a study describing the process, said in a statement. “If this is true, the void environment would have slowed their evolution. ... The galaxies also are quite compact relative to the typical star-forming galaxies in our galactic neighborhood.”
Pisces A is about 19 million light-years from Earth and Pisces B is roughly 30 million light-years away. Each galaxy contains only about 10 million stars, including 20 to 30 bright blue young stars — a clear sign that they are less than 100 million years old.
Although the pair is currently in a rapid star-forming phase, chances are, if they become satellites of much larger galaxies, this star-formation rate would subside.
“The galaxies could even probably stop forming stars altogether, because they will stop getting new gas to make stars,” Tollerud said. “So they will use up their existing gas. But it’s hard to tell right now exactly when that would happen, so it’s a reasonable guess that the star formation will ramp up at least for a while.”
Dwarf galaxies are the building blocks that formed larger galaxies billions of years ago in the early universe. By studying Pisces A and B — both of which have retained their primitive composition and have relatively high levels of hydrogen — scientists hope to experience star formation and galactic evolution first hand.