According to predictive models, there should be thousands of ancient and tiny ‘dwarf galaxies' in our neighborhood — but to date, astronomers have found but a few. And those discovered contain puzzlingly few stars, giving rise to the name ‘ghost galaxies.' This problem has led to the theory that dwarf galaxies must have abruptly stopped making stars about 13 billion years ago. But why?

Ghost galaxies are among the oldest, least chemically evolved galaxies found in the universe. Because they've remained largely unchanged since the early days of the universe, they're considered astronomical fossils. But their surprisingly low numbers has given rise to what is called the ‘missing satellite' problem. Compounding this puzzle has been scientists' inability to properly observe these "ultra-faint" galaxies.

But now, astronomers working with the Hubble Space Telescope believe they may be onto something — a cosmological process known as reionization.


Normally, reionization is credited for giving galaxies the property to form stars. The process first started happening in the first billion years following the Big Bang when radiation from the first stars knocked electrons off early hydrogen atoms, ionizing the cool hydrogen gas. If it wasn't for this, hydrogen gas wouldn't be transparent to ultraviolet light; reionization changed the nature of the cosmos such that it possessed ionized hydrogen that had been split into its component electrons and protons.

But according to a study conducted by Tom Brown of the Space Telescope Science Institute in Baltimore, this same process may have killed the star-making abilities of dwarf galaxies. By studying three irregular galaxies, Hercules, Leo IV and Ursa Major, Brown and his team concluded that the reionization process stripped these objects of the crucial gasses required to build more stars and grow into larger galaxies.


Essentially, reionization stopped the evolution of these galaxies dead in its tracks, removing the crucial ingredients required to become fully robust galaxies. And it all happened 13 billion years ago, at a time when the universe was still very young.

Just for scale, dwarf galaxies are only 2,000 light-years wide (compared to the Milky Way's 100,000 light-year diameter), and are between 330,000 and 490,000 light-years from Earth. But they also coalesced about 100 million years before reionization began — a premature and ultimately fatal time to be born.

Brown's results appeared in the July 1 issue of the Astrophysical Journal Letters.

Source: NASA. Image source: NASA.