Sombrero Galaxy Has a Split Personality

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There are hundreds of billions of galaxies in the universe, handily divided into three basic types: spiral, elliptical and “irregular.” But even when you think you know a galaxy, it just might surprise you. Such is the case with the so-called “Sombrero Galaxy” (a.k.a. NGC 4594).

Discovered in 1767 by Pierre Mechain, its name derives from the fact that, when viewed from Earth, the galaxy looks like a wide-brimmed hat: a thin disk with a bulge in the center.

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NG 4594 is a flat, disk-shaped spiral galaxy in the constellation Virgo. At least astronomers thought it was a spiral galaxy. Thanks to new data from NASA’s Spitzer Space Telescope, it now appears that NGC 4594 is two kinds of galaxy in one, a galactic split personality, making it the Sybil of the cosmos. These new findings appear in the Monthly Notices of the Royal Astronomical Society.

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Spiral galaxies are the most easily recognizable to the amateur eye, since they are disk-shaped, with a bulge at the center and long “arms” spiraling outward. They usually contain a lot of middle-aged stars swathed in dust and gas.

Elliptical galaxies are older, and usually bigger, with round, flat or elongated shapes. Their stars are older, and there is usually very little dust and gas, having been condensed into stars long ago.

Then there are the irregular galaxies, which defy any attempt to categorize them by shape. These are usually populated by many very young stars.

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To the naked eye — or to telescopes that rely on the visible portion of the light spectrum — the Sombrero Galaxy is a classic disk-shaped galaxy, and appears to be wreathed in a pretty glowing halo. Astronomers thought the halo was small and light — indicative of a spiral galaxy.

But Spitzer views the universe through infrared eyes, giving astronomers a new perspective even on familiar objects like the Sombrero Galaxy. Looking in the infrared, from space, means that Spitzer can see through all the dust and gas obscuring our view.

It turns out that the halo around the Sombrero Galaxy is larger and more massive than previously thought, indicative of a giant elliptical galaxy. So which type of galaxy is it? “The only way to understand all we know about this galaxy is to think of it as two galaxies, one inside the other,” according to Dmitri Gadotti of the European Southern Observatory in Chile.

Okay, so maybe this is the result of two galaxies colliding, with the larger elliptical structure swallowing its smaller spiral sibling whole? Nope. A collision would have torn apart the disk structure. Astronomers are frankly stumped as to how such a large disk managed to survive inside a huge elliptical galaxy.

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One hypothesis is that, about nine billion years ago, this was an elliptical galaxy that got pumped full of gas, causing it to bulk up. That extra bulk meant stronger gravity, which pulled the gas into an orbit around the center, eventually spinning out into a flat disk.

The new data does explain one mystery about the Sombrero Galaxy. Back when they thought it was a simple spiral galaxy, astronomers puzzled over why it should have so many globular clusters — around 2,000 — when most spiral galaxies have a few hundred. But if it is, in fact, an elliptical galaxy, then the large number of globular clusters make sense.

But it still doesn’t explain NGC 4594′s split personality. Pick a side already, Sombrero Galaxy! Or at least spill a few secrets about the early evolution of galaxies, for the sake of our befuddled astronomers.

Image credit: NASA/JPL-Caltech

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