Most galaxies are either roughly spherical or take the shape of a flat disk - but new observations from NASA's Spitzer Space Telescope show that the Sombrero galaxy is both.
Shaped a little like the hat for which it's named, the Sombrero galaxy is roughly elliptical, but has a thin disk embedded inside. Also known as NGC 4594, it's located 28 million light-years away in the constellation Virgo.
"The Sombrero is more complex than previously thought," says Dimitri Gadotti of the European Southern Observatory in Chile. "The only way to understand all we know about this galaxy is to think of it as two galaxies, one inside the other."
It's unlikely that the giant elliptical galaxy swallowed a spiral disk, as this would have destroyed the disk structure. Instead, the team suggests that a giant elliptical galaxy was inundated with gas more than nine billion years ago.
The gas would have been pulled into the galaxy by gravity, falling into orbit around the center and spinning out into a flat disk, and stars would have formed from the gas in the disk.
"This poses all sorts of questions," said Rubén Sánchez-Janssen from the European Southern Observatory. "How did such a large disk take shape and survive inside such a massive elliptical? How unusual is such a formation process?"
There may be another example of such a merged galaxy. Centaurus A, appears also to be an elliptical galaxy with a disk inside it, although its disk doesn't contain many stars.
The astronomers speculate that Centaurus A could be at an earlier stage of evolution than the Sombrero and might eventually look similar.