Stars and Gas Crash in Galaxy in Celestial Tsunami

colliding galaxies
Annotated image showing dazzling eyelid-like features bursting with stars in galaxy IC 2163 formed from a tsunami of stars and gas triggered by a glancing collision with galaxy NGC 2207 (a portion of its spiral arm is shown on right side of image). ALMA image of carbon monoxide (orange), which revealed motion of the gas in these features, is shown on top of Hubble image (blue) of the galaxy. M. Kaufman; B. Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO); NASA/ESA Hubble Space Telescope

When galaxies in the universe crash together, the results can be pretty spectacular. In some cases, the intertwined galaxies warp each other into twisted shapes. The resulting shock waves that reverberate through the interacting galaxies spur huge bursts of star formation.

All those things happened in the galaxy IC 2163, a spiral that lies some 114 million light-years away from Earth. Just by looking at it, you can tell that something colossal happened to it as it careened past the galaxy NGC 2207. The resulting galactic tangle looks like a huge pair of eyelids in the galaxy. (In this picture, IC 2163 is the galaxy on the left.)

Making a Galactic Eyelid

Galaxy collisions aren't unusual. They are, in fact, how galaxies grow and change. The Milky Way itself was built up by the merger of many smaller ones. In fact, it is still cannabilizing dwarf galaxies. The process is common, and astronomers see evidence for it happening in nearly every galaxy and galaxy clusters they can observe. However, the creation of galactic "eyelid" features in a collision is a rare occurrence. They're short-lived, and that tells astronomers something about the process that made them.

First of all, they seem to get made when galaxies graze pass close by one another in the collision process. During that "sideswipe", the outer arms of the participating galaxies brush up against each other. That's usually the first encounter during the collisions. Think of it like a huge ocean wave rushing up to shore. It gathers speed until it gets close to the shoreline, and then it ends up dumping its water and sand onto the beach. The action sculpts the beach and shoves dunes of sand around the shoreline.

Ultimately, in the case of the galaxies, they end up merging and dumping clouds of gas and dust all over each other. In this case, the gases in the galaxy arms decelerates (slows down) very quickly. It cools down and condenses just as quickly. The gases pile up and cool during the sideswipe and eventually they start combining to form massive new stars. This process is something that our own Milky Galaxy may suffer through when it goes through a merger with the Andromeda Galaxy in a few billion years. 

In the big picture, the "pile-up" regions form the eyelids seen in the annotated image. What's happening here is really quite fascinating. These are huge clumps of gas called "molecular gas clouds". They're moving very quickly — upwards of 100 kilometers (about 60 miles) per second. When they smash together, that's when the star formation regions begin their work. Generally, the thick clouds create very hot stars that are many times more massive than our Sun. They live relatively short lives as they consume their fuel. In about ten million years, the same "eyelid" regions will be bristling with massive stars blowing up as supernovae.

How Do Astronomers Know What's Happening?

The raging storms of star formation give off tremendous amounts of light and heat. While they're visible in optical light (the light we see with our eyes), they also emit ultraviolet, radio waves, and infrared light. The Atacama Large-Millimeter Array in Chile can detect specific regions of the spectrum in radio and close to infrared, which makes it a perfect tool to track the tsunami of star-forming action in the "eyelid" regions. In particular, it can trace carbon monoxide gas, which tells them how much other molecular gas exists. Since those gases are the fuel for star formation, tracking the actions of gas gives astronomers a great snapshot into the lead-up to starburst activity in a galaxy merger. Their observations are a great look into a short-lived phenomenon of a few million years during a galaxy collision that can take tens of millions of years to complete.

Why short-lived? In a few million years, those eyelids will be gone; all their gases will be "eaten up" by hot young newborn stars. That's just one effect of a galaxy collision, and it changes the way the resulting galaxies will look for many millions of years to come. Observations by ALMA and other observatories give astronomers a multi-wavelength look at a process that has occurred many, many times in the 13.7 billion years since the universe formed.