Deconstructing the Aftermath of a Galactic Fight to the Death

Remember a few months ago when astronomers told us that we’re on the path for a head on collision with the Andromeda galaxy? Well, now we have a preview of what that collision might look like. Astronomers a the Gemini Observatory in Hawaii have captured a picture of the gas and dust wreckage of a colossal galactic fight to the death with wreckage strewn around the polar-ring galaxy NGC 660. But it’s not just a really pretty bloodbath. Astronomers are treating it like a crime scene, forensically analyzing the splatter for figure out just what happened.

It’s an interesting case to study because NGC 660 is a peculiar galaxy, a rare type we don’t know much about. Most of the galaxies we’ve observed are lenticular, an intermediate shape between spiral and elliptical galaxies. But NGC 660, which is about 40,000 light years across and lies about 40 million light-years from Earth in the constellation Pisces, is an edge-on polar-ring galaxy and the only known one of its type with a lenticular galaxy as its host.

Astronomers think polar-ring galaxies are formed through a violent galactic merger. Either one galaxy pierces the other, or one galaxy strips material from the other through tidal forces as it passes by. In both cases, a ring is left over.

Astronomers think this new image from the Gemini observatory is evidence of a newly formed polar-ring galaxy. Specifically, the aftermath of a furious tidal struggle between two galaxies that left one intact and the other a 40,000-light-year-long ring of debris. Though astronomers need to make more observations, there’s enough evidence already to support the tidal stripping theory.

NGC 660 contains more gas than its host galaxy, suggesting that it has pulled material from another. It also doesn’t have the pair of supermassive central black holes astronomers would expect in a merger. Moreover, NGC 660 is inclined about 45 degrees from vertical, an angle too extreme to be caused by any interaction other than tidal forces. The only tell-tale feature that’s missing is the tail of debris astronomers would expect to see extending from NGC 660, a typical splatter pattern after one galaxy has stripped away gas from another. But, again, models support the lack of tail. Besides, if the ring is really old, on the order of 1 billion years or so, it might have moved too far to see in the single shot.

Radio observations have found shock-waves echoing through the galaxy form the merger. The gravitational interaction between the two galaxies created the waves, causing giant clouds of gas to collapse into blue stars, each 100 times more massive than our Sun. These stars exploded as supernovae, which generated more shock waves. This created a domino effect leading to constant star formation in NGC 660’s core. So not only is this a rare polar-ring galaxy, it’s also a starburst galaxy. Not even the Hubble space telescope has captured star formation like this. Radio observations have also suggested NGC 660 is host to a substantial amount of dark matter, the material that influences the dynamics of all galaxies.

Whether or not the Milky Way’s collision with Andromeda will yield similar results is a mystery. But with the collision 4 billion years off, we’re not going to be around to find out.