Washington: For many years, astronomers have been trying to find out why some puny, extremely faint dwarf galaxies spotted in our Milky Way galaxy’s back yard contain so few stars.
These ghost-like galaxies are thought to be some of the tiniest, oldest, and most pristine galaxies in the universe. They have been discovered over the past decade by astronomers using automated computer techniques to search through the images of the Sloan Digital Sky Survey.
Now, NASA’s Hubble Space Telescope has finally help solve the mystery of these star-starved galaxies.
Hubble views of three of the small-fry galaxies reveal that their stars share the same birth date. The galaxies all started forming stars more than 13 billion years ago -- and then abruptly stopped -- all in the first billion years after the universe was born in the big bang.
The relic galaxies are evidence for a transitional phase in the early universe that shut down star-making factories in tiny galaxies. During this time, 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. The most likely explanation is reionization,” said Tom Brown of the Space Telescope Science Institute in Baltimore, Md., the study’s leader.
The reionization of the universe began in the first billion years after the big bang. During this epoch, radiation from the first stars knocked electrons off primeval hydrogen atoms, ionizing the cool hydrogen gas. This process allowed the hydrogen gas to become transparent to ultraviolet light.
Ironically, the same radiation that sparked universal reionization appears to have squelched star-making activities in dwarf galaxies, such as those in Brown’s study.
The small irregular galaxies were born about 100 million years before reionization began and had just started to churn out stars. Roughly 2,000 light-years wide, the galaxies are the smaller cousins of the more luminous star-making dwarf galaxies near our Milky Way.
Unlike their larger relatives, the puny galaxies were not massive enough to shield themselves from the harsh ultraviolet light. What little gas they had was stripped away as the flood of ultraviolet light rushed through them. Their gas supply depleted, the galaxies could not make new stars.
The discovery could help explain the so-called “missing satellite problem,” where only a few dozen dwarf galaxies have been observed around the Milky Way while the simpler computer simulations predict that thousands should exist. One possible explanation is that there has been very little, or even no star formation in the smallest of these dwarf galaxies, making them difficult to detect.
The Sloan survey recently uncovered more than a dozen of these star-starved galaxies in our Milky Way’s neighbourhood while scanning just a quarter of the sky. Astronomers think the rest of the sky should contain dozens more of these objects, dubbed ultra-faint dwarf galaxies. The evidence for squelched star formation in some of the smallest of these dwarfs suggests that there may be thousands more where essentially no stars formed at all.
“By measuring the star formation histories of the observed dwarfs, Hubble has confirmed earlier theoretical predictions that star formation in the smallest clumps would be shut down by reionization,” said Jason Tumlinson of the Space Telescope Science Institute, a member of the research team.
“These are the fossils of the earliest galaxies in the universe. They haven’t changed in billions of years. These galaxies are unlike most nearby galaxies, which have long star-formation histories,” he added.
Brown’s results appeared in the July 1 issue of The Astrophysical Journal Letters.