July 12, 2012

Hubble Helps Unravel Mysteries Of Dwarf Galaxies

Lee Rannals for redOrbit.com - Your Universe Online

The Hubble Space Telescope has helped scientists unravel the mystery of the extremely faint dwarf galaxies.

These 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.

The Hubble has given scientists views of three of the small-fry galaxies, revealing that their stars share the same birth date.

The galaxies all started forming stars over 13 billion years ago, all in the first billion years after the universe was born.

The galaxies seen by Hubble are evidence for a transitional phase in the early universe that shut down star-making factories in tiny galaxies.

"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," Tom Brown of the Space Telescope Science Institute and the study's leader said in a statement. "The most likely explanation is reionization."

The reionization of the universe began in the first billion years of its existence, during which, radiation from the first stars knocked electrons off primeval hydrogen atoms, ionizing the cool hydrogen gas. This process allowed the gas to become transparent to ultraviolet light.

The same radiation that sparked universal reionization appears to have halted star-making activities in dwarf galaxies. These galaxies were born about 100 million years before reionization began, and had just started to churn out stars.

The galaxies, which are about 2,000 light-years wide, are smaller cousins of the more luminous star-making dwarf galaxies near the Milky Way.

These galaxies were not massive enough to shield themselves from the harsh ultraviolet light, so what little gas they had was stripped away as the flood of ultraviolet light burned through it.

The latest discovery could explain the so-called "missing satellite problem," where only a few dozen dwarf galaxies have been observed around the Milky Way, despite computer simulations predicting thousands.

One explanation, according to the researchers, is that there has been very little, or no star formation in the smallest of these dwarf galaxies.  This could make these ghostly-galaxies harder to detect.

The Sloan survey recently uncovered over a dozen of these star-starved galaxies in our Milky Way neighborhood.  Astronomers believe the rest of the sky should contain dozens more of these objects.

"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," Jason Tumlinson of the Space Telescope Science Institute, a member of the research team, said in a press release.

"These are the fossils of the earliest galaxies in the universe," Brown said. "They haven't changed in billions of years. These galaxies are unlike most nearby galaxies, which have long star-formation histories."

The study was published in the July 1 issue of The Astrophysical Journal Letters.