Hubble reveals the mysteries behind ancient 'ghost' galaxies

New images show the suprising secrets behind some of the faintest galaxies in the Universe


NASA, ESA, T. Brown (STScI)

The area around the Milky Way should be home to many thousands of dwarf galaxies

They’re very old, very small and incredibly hard to spot. But now, for the first time, the Hubble Space Telescope has been able to study some of the faintest galaxies in our cosmic neighbourhood, helping astronomers to explain why they contain so few stars.

These ancient fossils, barely changed for 13 billion years, could help explain what astronomers describe at the 'missing satellite' problem, where very few satellite galaxies have been found around the Milky Way, despite the thousands predicted.

Over the past decade astronomers have been trawling through images from the Sloan Digital Sky Survey and have discovered more and more of these ancient galaxies, but it's only now, with images provided by the Hubble Space Telescope, that scientists can begin to decipher the mysteries behind them.

The three rare galaxies viewed by Hubble - Hercules, Leo IV and Ursa Major - began forming stars over 13 billion years ago, and then abruptly stopped, all within the first billion years following the Big Bang.

Hubble's images provide evidence for a transitional phase in the early Universe when the capability of tiny galaxies to produce stars was stopped. This phase seems to coincide with the time when radiation from the first stars burned off a fog of cold hydrogen, a process called reionisation. The same radiation that sparked universal reionisation also appears to have stopped star-making activities in dwarf galaxies. Unlike their larger counterparts, these small galaxies were not massive enough to shield themselves from the harsh ultraviolet light and what little gas they had was stripped away. Their gas supply depleted, the galaxies could not make new stars.

This new data could help explain why only a few dozen dwarf galaxies have been observed around the Milky Way even though computer simulations predict that thousands should exist. Many of these smaller dwarf galaxies contain very few stars but a great deal of dark matter, making them almost invisible. Many thousands more of these galaxies may actually contain no stars at all. 


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