The case of the Milky Way robbery involves a greedy cosmic bully and a whole lot of missing hydrogen. The bully in this case is our home galaxy, the Milky Way, and the missing hydrogen once belonged to nearby dwarf galaxies. But guess who has it now? That's right, the bully.
"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," Kristine Spekkens, an astronomer at the Royal Military College of Canada, explained in a recent press release.
The answer is no, they don't.
Spekkens and her colleagues say the small galaxies that cluster along the edges of Milky Way's spiral are entirely devoid of star-forming hydrogen gas. The gravity of the Milky Way robs these galaxies, known as dwarf spheroidals, of their gas -- the same gravity that holds them close, part of a bundle of galaxies of which the Milky Way is the largest.
Beyond the grip of Milky Way's galactic pull are dwarf irregular galaxies, which have only recently joined the cosmic block party and aren't made to fork over precious hydrogen to join the fun.
"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," explained Spekkens, lead author of a paper published this week in the Astrophysical Journal Letters. And beyond that break, which lies roughly 1,000 light-years from the edge of the Milky Way, are galaxies once again rich in hydrogen -- the irregulars.
Researchers came to these conclusions with the help of new and incredibly detailed radio telescope readings provided by the National Science Foundation's Green Bank Telescope in West Virginia.
Milky Way's gravity doesn't simply suck the hydrogen straight from of its neighbors. Its theft is more like a Ponzi scheme than a mugging; it pulls them in slowly and then robs them blind. Researchers say the pressures of the Milky Way's dense gravitational halo -- a hot disk of plasma -- combined with the orbital spin of these small galaxies, causes the neighbors to be slowly stripped of all their hydrogen, thus shutting down their ability to create new stars.
"These observations therefore reveal a great deal about size of the hot halo and about how companions orbit the Milky Way," Spekkens concluded.