Milky Way Secretly Steals from Nearby Dwarf Galaxies

The starry Milky Way Galaxy, seemingly innocent, is secretly stealing from nearby dwarf galaxies, making them devoid of star-forming gas, according to new research.

Swarming around our home Galaxy is an army of smaller dwarf galaxies, the smallest of which are nearby dwarf spheroidals - supposedly the leftover building blocks of galaxy formation. And even farther out, escaping the clutches of the Milky Way, are a number of similarly sized and slightly misshaped dwarf irregular galaxies - newcomers to the neighborhood.

"Astronomers wondered if, after billions of years of interaction, the nearby dwarf spheroidal galaxies have all the same star-forming 'stuff' that we find in more distant dwarf galaxies," astronomer Kristine Spekkens, the study's lead author, said in a news release.

The star-forming "stuff" Spekkens is referring to is hydrogen gas. Previous studies have shown that the more distant dwarf irregular galaxies have large reservoirs of hydrogen gas, but the same couldn't be said for sure of the smaller dwarf spheroidals.

So astronomers using the National Science Foundation's Green Bank Telescope (GBT) in West Virginia, along with data from other large radio telescopes, probed the Milky Way's neighbors for traces of atomic hydrogen.

"What we found is that there is a clear break, a point near our home Galaxy where dwarf galaxies are completely devoid of any traces of neutral atomic hydrogen," noted Spekkens. So any nearby galaxy found within this "danger zone," which extends approximately 1,000 light-years from the edge of the Milky Way, lies within our galaxy's greedy grasp.

This explains why dwarf spheroidals situated within the boundary become vanishingly rare while their farther out, gas-rich, dwarf irregular counterparts flourish.

There are many ways that larger, mature galaxies can lose their star-forming material, but the dwarf galaxies that orbit the Milky Way lack the necessary energetic processes to be subject to those factors. So instead, they are at the mercy of the broader influences of the Milky Way.

The study is described in further detail in The Astrophysical Journal.