Scientists who were surveying a catalog of gas clouds spotted something strange: five "blue blobs" composed of young blue stars in the Virgo galaxy cluster.
Unusually, these stars were completely isolated from their parent galaxies, and their stars were arranged in an irregular pattern. Based on those details, the researchers think they've discovered a new type of stellar system — a collection of gravitationally bound stars that's not quite a galaxy but not a known type of star cluster, either.
Even more mysteriously, the blobs were determined to have little atomic hydrogen gas, which is a crucial ingredient in star formation. So how did the young stars form, especially given their distance from the closest potential parent galaxies?
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The research team — led by Michael Jones, a postdoctoral fellow at the University of Arizona Steward Observatory — noted the presence of heavy metals in the blobs. "This tells us that these stellar systems formed from gas that was stripped from a big galaxy, because how metals are built up is by many repeated episodes of star formation, and you only really get that in a big galaxy," Jones said in a statement.
There are only two main ways gas is stripped from a galaxy: tidal stripping, in which the gravitational attraction between passing galaxies pulls gas away from them, and ram pressure stripping, which is "[w]hen a galaxy belly flops into a cluster that is full of hot gas, then its gas gets forced out behind it," Jones said. "That's the mechanism that we think we're seeing here to create these objects."
The astronomers suspect that, over time, the stars in the blobs will split into smaller stellar clusters and spread out.
The team's findings were presented June 15 at the 240th American Astronomical Society meeting in Pasadena, California.
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