The Sombrero galaxy, which in visible light looks like its namesake wide-brimmed hat, is a round elliptical galaxy but has a thin disk embedded inside, making it one of the first known to exhibit characteristics of the two different types, NASA's Jet Propulsion Laboratory in Pasadena, Calif., reported Tuesday.
Astronomers have used NASA's Spitzer Space Telescope to capture an infrared image of the galaxy that reveals it unusual structure.
"The Sombrero is more complex than previously thought," Dimitri Gadotti of the European Southern Observatory in Chile said. "The only way to understand all we know about this galaxy is to think of it as two galaxies, one inside the other."
While it is tempting to think the giant elliptical swallowed a spiral disk, astronomers say this is highly unlikely because that process would have destroyed the disk structure.
Instead, they say, the giant elliptical galaxy may have been inundated with cosmic gas more than 9 billion years ago, with the gas being pulled into the galaxy by gravity and falling into orbit around the center and spinning out into a flat disk.
"This poses all sorts of questions," ESO astronomer Ruben Sanchez-Janssen said. "How did such a large disk take shape and survive inside such a massive elliptical? How unusual is such a formation process?"
The answers could help piece together how other galaxies evolve, the researchers said.