Bad Astronomy

Flying saucer galaxy!

Hubble spies invading saucer from space!

Hubble image of spiral galaxy NGC 7049
Click to Brobdignagify.

OK, it’s not an alien spaceship. But Holy Haleakala, what a gorgeous galaxy! It’s NGC 7049, a kind of hybrid galaxy, one that is part disk (like our spiral Milky Way) and part elliptical. These types of galaxies are fairly rare. They are shaped like disk galaxies – a flattened disk surrounding a central bulge – but have almost no gas compared to a normal spiral galaxy. It’s the gas that makes spiral galaxies obvious to the eye. As gas orbits the galactic center in a disk, odd gravitational instabilities build up, creating spiral-shaped traffic jams in the disk. Gas piles up there, collapses, and forms stars. The stars light up the gas, and from a few dozen million light years away, you see a gorgeous spiral galaxy.

But NGC 7049 is not gifted with gas, so the disk is fuzzy and featureless… almost. This galaxy does have a ring of beautiful dust lanes circling the center, which is unusual even for these types of galaxies! We see the dust in silhouette because it absorbs light from stars, blocking the starlight behind it. It takes a considerable quantity of dust to absorb that much light, which is weird– dust is usually formed when there’s lots of star formation. But without gas, NGC 7049 can’t make new stars, and so there shouldn’t be much dust. What gives?

My first thought when I saw this picture was that the galaxy must have recently eaten a smaller galaxy and taken on all its dust. And it turns out that may very well be the case. Studies of these types of galaxies shows that some do have gas, but in some cases the gas is orbiting the galaxy in the wrong direction, the opposite direction as the stars do. That’s a sure sign that a smaller galaxy got too close, collided with the larger galaxy, and finally merged with it. In some of those events the geometry of the collision with smaller galaxy will yield a counter-rotating block of stars, gas, and dust.

So that’s probably what happened to NGC 7049. And now, a few hundred million years later, we reap the benefit of being able to see this spectacular and lovely galaxy.

Image credit: NASA, ESA, and W. Harris (McMaster University, Ontario, Canada)