The small ghost-like galaxies are thought to be some of the oldest and most pristine galaxies in the universe, but scientists were puzzled by how few stars they contained.
Hubble has determined stars in three of the small-fry galaxies share the same birth date, and that the galaxies all started forming stars more than 13 billion years ago, then abruptly stopped, NASA reported Wednesday.
This all happened in just the first billion years after the universe was born in the big bang.
This suggests the relic galaxies are evidence for a transitional phase in the early universe that shut down star-making factories in tiny galaxies as the first stars burned off a fog of cold hydrogen in a process called reionization.
"These galaxies are all ancient and they're all the same age, so you know something came down like a guillotine and turned off the star formation at the same time in these galaxies," study leader Tom Brown of the Space Telescope Science Institute in Baltimore said. "The most likely explanation is reionization."
The small irregular galaxies were born about 100 million years before reionization began and had just started to churn out stars, but unlike their larger relatives the puny galaxies were not massive enough to shield themselves from harsh ultraviolet light, previously blocked by the cold hydrogen fog, that rushed through them stripping way what little gas they had.