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SYFY WIRE Bad Astronomy

Sometimes, galaxies collide

By Phil Plait
 IC 1623 Hubble Image

Sometimes, galaxies collide.

A statement simple in its structure, but breathtaking in its profundity.

This was something not understood until later in the 20th century. It wasn't until the 1920s or so that we even understood that galaxies were entities separate from the Milky Way, colossal structures of their own with billons of stars in them. When this was finally grasped, the size of the Universe jumped by orders of magnitude in human understanding.

They're immense, tens or hundreds of thousands of light years across, with the very largest over a million light years side to side. These are objects far beyond our puny minds to truly grasp.

And sometimes, galaxies collide.

If that statement is amazing in its depth, it is profoundly moving in its beauty.

 IC 1623 Hubble Image

That image from the Hubble Space Telescope shows the object IC 1623, a valentine-shaped glowing chaos that at first brings confusion, until your eye can separate it into two individual galaxies, one on the left and the other on the right, both showing spiral arms but twisted and distorted.

These are galaxies in collision, what happens when the thrall of gravity overcomes the velocities of the galaxies in question. They approach, collide, and while sometimes their speed is enough that they begin to pull apart again, in may cases it's not enough, and they fall back once more to collide and merge into a single, larger galaxy.

The gravity of one pulls on the other, drawing away stars, gas, and dust into long streamers called tidal tails. Two of these are visible in this Hubble image; by stretching the brightness and contrast, two elongated curving arms can be seen above and below the pair.

 IC 1623 Hubble Image Brightened

In general, the three-dimensional structure of objects in space is difficult to ascertain, but physics has provided us with a cheat sheet here. The galaxy on the left, called IC 1623E (for east), has two reddish stripes going directly across it. Those are in part streamers of dust pulled off the galaxy on the right (called IC 1623W). Dust — tiny grains of rocky and/or sooty material created when stars die — blocks visible light, but it blocks blue better than red light. Those streamers are in front of IC 1623E, reddening the starlight behind, giving us a hint of 3D structure. The bulk of IC 1623W might also be in front of the other galaxy, but that's actually harder to tell. Those streamers may have been pulled toward us, while the galaxy itself is on the other side of IC 1623E.

IC 1623 as a whole is glowing furiously in infrared light. This is typically due to stars being born at significantly accelerated rates; massive stars born in this periods of fecundity lives their lives out quickly and explode as supernovae, which churn out dust. The dust is warmed by the stars around it, glowing brightly in infrared.

A study of the light from IC 1623 indicates they are indeed starburst galaxies. Clusters of stars have formed very recently in the pair, many just a few million years old, just the right age to have been whipped into life by the collision. Stars are very small compared to the space between them, so collisions between stars in these events are rare. But gas clouds — nebulae — are huge, so collisions between them are common. This can trigger starbursts, and funnel gas into the galactic center as well. If there's a supermassive black hole there (and there always is, to our knowledge) this can also kickstart the galaxy to become active, with intense high-energy radiation blasting away from the black hole as matter piles up in a disk around it, heats up, and generates vast amounts of energy through friction and magnetic forces.

Eventually, these two barely individual galaxies will indeed become one. They settle down into a roughly spherical or elongated shape, becoming an elliptical galaxy. Perhaps the material will fall back into a disk, and the new galaxy will once again take on the spiral form of its parents.

Time will tell; this event plays out on timescales of tens or even hundreds of millions of years. This collision started before anything resembling humans walked the Earth, and will continue on for much longer than we've already been here. Over that time we evolved bigger, more complex brains; the ability to analyze our surroundings, predict future events; base our understanding on evidence and determine some of the laws of nature, including how matter behaves, the way gravity works, how to collect and magnify light from objects in the sky, and, eventually, to be able to make statements simple but profound in their implications, their scale, and their beauty:

Sometimes, galaxies collide.