NASA’s Hubble Space Telescope has sent images of two tiny dwarf galaxies that have wandered from a vast cosmic wilderness into a nearby “big city” packed with galaxies, ahead of starting a firestorm of star birth.
After being quiescent for billions of years, the galaxies, called Pisces A and B, are late bloomers because they have spent most of their existence in the Local Void, a region of the universe sparsely populated with galaxies. The Local Void is roughly 150 million light-years across.
“These Hubble images may be snapshots of what present-day dwarf galaxies may have been like at earlier epochs,” said lead researcher Erik Tollerud of the Space Telescope Science Institute in Baltimore, Maryland. “Studying these and other similar galaxies can provide further clues to dwarf galaxy formation and evolution.”
The Hubble observations suggest that:
Under the steady pull of gravity from the galactic big city, the loner dwarf galaxies have at last entered a crowded region that is denser in intergalactic gas. In this gas-rich environment, star birth may have been triggered by gas raining down on the galaxies as they plow through the denser region.
Another idea is that the duo may have encountered a gaseous filament, which compresses gas in the galaxies and stokes star birth. Tollerud’s team determined that the objects are at the edge of a nearby filament of dense gas. Each galaxy contains only about 10 million stars.
Dwarf galaxies are the building blocks from which larger galaxies were formed billions of years ago in the early universe. Inhabiting a sparse desert of largely empty space for most of the universe’s history, these two galaxies avoided that busy construction period.
Pisces A is about 19 million light-years from Earth and Pisces B roughly 30 million light-years away. Tollerud’s team estimates that less than 100 million years ago, the galaxies doubled their star-formation rate. Eventually, the star formation may slow down again if the galaxies become satellites of a much larger galaxy.
“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.”
Tollerud’s team hopes to observe other similar galaxies with Hubble. He also plans to scour the Panoramic Survey Telescope and Rapid Response System survey (PanSTARRS) for potential dwarf galaxies. Future wide-survey telescopes, such as the Large Synoptic Survey Telescope (LSST) in Chile and the large radio telescope in China, should be able to find many of these puny galactic neighbors.
The team’s results will appear in the Aug. 11 issue of The Astrophysical Journal.