Astronomers have discovered that our nearest galactic neighbors, a menagerie of small dwarf galaxies, no longer have any hydrogen gas. The Milky Way has stripped them of all their star-making material. The work was published in the Astrophysical Journal Letters this week.
Our galaxy is the largest member of a small clutch of galaxies bound together by gravity. Swarming around the Milky Way are dwarf spheroidals, likely the leftover building blocks of galaxy formation. Further out are similarly small “irregular” galaxies, which are the newcomers to our galactic ‘hood.
“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,” says Kristine Spekkens from the Royal Military College of Canada. This includes large reservoirs of neutral hydrogen gas, the fuel for star formation. Using observations from large radio telescopes such as the Green Bank Telescope in West Virginia, Spekkens and colleagues probed dwarf galaxies that have surrounded the Milky Way for billions of years, looking for tiny amounts 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,” Spekkens explains in a news release. Beyond this point -- which extends 1,000 light-years from the edge of the Milky Way’s star-filled disk to the edge of its dark matter distribution -- dwarf spheroidals become increasing rare. Their gas-rich, irregular counterparts, on the other hand, flourish.
Larger, mature galaxies typically lose their star-forming material during furious star formation or due to jets of material driven by the supermassive black holes at their centers -- energetic processes that dwarf galaxies orbiting the Milky Way don’t have. Instead, the team believe that, up to a certain distance from our galactic disk, the diffuse halo of hot hydrogen plasma is dense enough to affect dwarf galaxy composition. In this zone, the pressure created by the orbital velocities of the dwarf spheroidals -- which can reach over a million kilometers an hour -- strip away any detectable traces of neutral hydrogen, shutting down star formation.