New radio observations from a collection of large radio telescopes, including the National Science Foundation’s Green Bank Telescope (GBT), have shown that galaxies nearest the Milky Way are completely devoid of star-forming hydrogen gas.

The Milky Way is the largest in a cluster of galaxies and it occupies the center of this grouping. The closest of these orbiting galaxies, the dwarf spheroidal galaxies, have no hydrogen gas to form stars, and it is thought that the Milky Way’s gravity is stripping the gas away from the smaller galaxies.

With the help of the world’s largest fully steerable radio telescope, the GBT, and other giant telescopes from around the world, the research team was able to identify a distinct zone, reaching about 1,000 light-years outside the Milky Way, in which the dwarf stars were completely lacking hydrogen. Beyond that lay many hydrogen-rich dwarf galaxies, orbiting just out of reach of the Milky Way’s hot, dense plasma halo.

In previous studies astronomers had shown that the more distant dwarf galaxies, dwarf irregulars, did contain the hydrogen necessary to form stars, but they were not sensitive enough to confirm or deny the gas’s presence in the closer dwarf spheroidal galaxies.

“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,” said astronomer Kristine Spekkens, assistant professor at the Royal Military College of Canada and lead author on a paper published in the Astrophysical Journal Letters.

It would seem that, within a certain distance of the galactic disk, the Milky Way’s halo of hot hydrogen plasma affects the composition of these closer dwarf galaxies. Within this zone, gravity can combine with the orbital velocities of the dwarves and strip them of their hydrogen gas.

“These observations therefore reveal a great deal about size of the hot halo and about how companions orbit the Milky Way,” said Spekkens.