Our closest galactic neighbors cannot make stars and our Milky Way galaxy is the reason why.
It seems that our galaxy is stripping these nearby spheroidal dwarf galaxies of neutral hydrogen gas, a gas that is necessary for the formation of stars.
However, the dwarf galaxies beyond our neighbors are fine and have plenty of hydrogen, meaning they can make plenty of stars.
The Milky Way Galaxy is part of a larger group of galaxies bound by gravity. Ours is, obviously, the largest, and the others are dwarf galaxies. Of these dwarf galaxies, the smallest are spheroidal dwarf galaxies, possibly leftovers from the formations of other galaxies.
Beyond those spheroidal dwarf galaxies are misshapen dwarf irregular galaxies, which aren't part of the group.
"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 astronomer Kristine Spekkens and lead author of the new study.
We know that those distant irregular dwarf galaxies have lots of neutral hydrogen gas. However, until now, we didn't know about the closer spheroidal galaxies. Using giant telescopes across the world, including the National Science Foundation's Green Bank Telescope, astronomers recently learned that the spheroidal galaxies were lacking in all traces of hydrogen gas.
The first explanation for why hydrogen was missing from these galaxies is that there's a supermassive black hole or a nearby star forming somewhere nearby sucking in the material. However, those dwarf galaxies don't have either. They are, however, very susceptible to the larger Milky Way, which is, literally, right next door.
The Milky Way sits in a large and dense halo of hydrogen plasma that extends for hundreds of thousands of light-years. This massive halo, combined with the effects of the high speeds of the dwarf spheroidal galaxies' orbits around the Milky Way, strips away their hydrogen gas.
Those telescopes also showed that beyond a specific point, the dwarf spheroidal galaxies don't appear, and instead, we see more of those other dwarf galaxies, those with the star formation materials. This suggests that these effects only extend so far and perhaps the dwarf spheroidal galaxies' properties are unique because of their closeness to the Milky Way.
Regardless, the Milky Way is why these small galaxies can't make stars. That doesn't make it a very good neighbor.
"These observations therefore reveal a great deal about size of the hot halo and about how companions orbit the Milky Way," says Spekkens.
