Nasa Scientists Solve Mystery of Ghost Galaxies / Images

These computer simulations show a swarm of dark matter clumps around our Milky Way galaxy. Some of the dark-matter concentrations are massive enough to spark star formation.

Credit: Nasa

The green blobs are those dark-matter chunks massive enough to obtain gas from the intergalactic medium and trigger ongoing star formation, eventually creating dwarf galaxies.

The red blobs are ultra-faint dwarf galaxies that stopped forming stars long ago.

These Hubble images show the dim, star-starved dwarf galaxy Leo IV. The image shows only few stars in Leo IV galaxy.

The small and faint star-starved dwarf galaxy, Leo IV, is one of more than a dozen ultra-faint dwarf galaxies found lurking around the Milky Way. These galaxies are dominated by dark matter.

 

For several years, astronomers were puzzled as to why ghost galaxies contained only a few stars. But now scientists have solved the mystery, according to a Nasa report.

Nasa scientists have found that more than 13 billion years ago these galaxies started forming stars but the star formation abruptly stopped because of the big bang.

The discovery was made while analysing data retrieved from the Hubble space telescope. The Hubble telescope had studied three galaxies, Hercules, Leo IV, and Ursa Major. The galaxies' distance from earth ranges from 330,000 light-years to 490,000 light-years.

Ghost galaxies are the tiniest, the oldest and the most pristine galaxies in the universe. They have been discovered over the past decade by scientists using automated computer techniques to search through images of the Sloan Digital Sky Survey but scientists did not know as to why these galaxies had very few stars. Now they know that it was because of the big bang.

Reionisation of the universe began in the first billion years after the big bang. During this period, radiation from the first stars knocked electrons off primeval hydrogen atoms, ionising the cool hydrogen gas. This process allowed hydrogen gas to become transparent to ultraviolet light.

Ironically, the same radiation that sparked universal reionisation appears to have squelched star-making activities in dwarf galaxies.

The small irregular galaxies were born about 100 million years before reionisation began and they 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 tiny 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 and the galaxies could not make new stars.

The ghost galaxies have very little stellar pollution and have only a few thousand stars. Even though they have very few stars, scientists claim that they have an abundance of dark matter around them.

Normal dwarf galaxies near the Milky Way contain 10 times more dark matter than the ordinary matter that makes up gas and stars. In ultra-faint dwarf galaxies, dark matter outweighs ordinary matter by at least a factor of 100.

"The small galaxies in our study are made up mostly of dark matter because their hydrogen gas was ionised and the stars got turned off," Brown explained.

Scientists claim that this discovery will help them solve another mystery about the missing satellite galaxies.

Computer simulation has shown thousands of satellite galaxies existing in the Milky Way but till now scientists have found only a few dozen galaxies.

Scientists are baffled as to how several satellite galaxies have gone missing.