Hubble's half-dozen cosmic train wrecks

One of the most fantastic statements that can be made about the Universe is: Sometimes, galaxies collide.

A galaxy is an immense thing, billions or even trillions of stars, uncountable clouds of gas and dust, (usually) a huge halo of dark matter, a central supermassive black hole, all in one place and held together by their mutual gravity. A typical galaxy is tens of thousands of light years across, and some, like our home galaxy the Milky Way, well over 100,000. Mind you, a single light year is about ten trillion kilometers.

Galaxies are huge.

So big, in fact, that they sometimes crowd together a bit, and when that happens they can pass closely, scrape each other, or even suffer a full-on collision.

And when such a cosmic train wreck occurs, the results are staggeringly beautiful.

Galaxy collisions observed using Hubble Space Telescope. Clockwise from upper left: NGC 3256, NGC 1614, NGC 4194, NGC34, NGC6052, NGC 3690. Credit: NASA & ESA

That is a collage of six different galactic collisions involving a dozen galaxies, as seen by a Hubble survey called HiPEEC, for Hubble imaging Probe of Extreme Environments and Clusters. The idea is to look at interacting or colliding galaxies in the nearby Universe, find which ones are undergoing bursts of star formation, and then look at the clusters of stars in them to see what they look like. We think these kinds of star clusters formed early in the Universe as well, but it's hard to examine ones in galaxies billions of light years away. By looking at nearby ones we can get an idea of what they're like.

The astronomers used Hubble to get the colors of the stars in clusters from the near ultraviolet to the near infrared, which allows them to figure to not only what kinds of stars are in them but how many of them there are as well. Different conditions make different kinds of stars, so in some you might see a higher ratio of high-mass stars versus low-mass ones, or vice versa. That sort of info is very useful to understand how stars are born.

Some of the individual images are stunning. Like this one, of NGC 4194:

NGC 4194, aka the Medusa Merger, is a collision of two galaxies that look like a Portuguese Man-of-War. Credit: ESA/Hubble & NASA, A. Adamo

This pair of colliding galaxies is about 125 million light years away or so, and is nicknamed the Medusa Merger, for a resemblance to the Gorgon due to tendrils of stars and gas at the top. Personally I think it looks more like a Portuguese Man-of-War, but maybe that's just me.

This collision has been going on a long time, several hundred million years. When galaxies collide it's very rare for stars to directly, physically impact each other; stars are small compared to the distances between them (roughly a million versus tens of trillions of kilometers). Giant gas clouds, however, can be several light years across, so collisions between them are common. The impact causes them to collapse and form stars at elevated rates, and in NGC 4914 stars are forming a dozen or so times faster than they are in the Milky Way. This starburst has been going on for a hundred million years, too.

In the image you can see that at least one of the initial galaxies was a spiral, with pinwheel streamers of red gas and darker dust (including that one thick smear on the right) around the center. The long streams to the lower right are called tidal tails, where gas and stars are pulled like taffy by the gravity of one galaxy on the other. It's a dead giveaway of a collision.

You can see that even better in NGC 34:

NGC 34 is a galaxy collision showing a classic tidal tail of stars and gas pulled away from one of the galaxies by the gravity of the other. Credit: ESA/Hubble & NASA, A. Adamo et al.

This pair is much farther at 260 million light years away. Like NGC 4194 it's in the late stages of a merger, the two colliding galaxies becoming one. The astronomers found 569 star clusters in this object, a nice amount to study. The rate of star birth is elevated, but not as much as the other galaxies studied, roughly 5 times our own galaxy's.

The final stage of the collision happened 300–600 million years ago, and they've been coalescing ever since. They found one cluster in there with a mass of 10 million times the Sun's, so it has a lot of stars in it, and is roughly 100 million years old.

My favorite one of those studied, though, is NGC 6052, because it looks like someone threw one circular saw blade into another at a 90° angle, and they put some arm into it:

NGC 6052 is a pair of colliding spiral galaxies, part of a survey to look at star clusters that form in the aftermath. Credit: ESA/Hubble & NASA, A. Adamo et al.

One of the galaxies is a face-on spiral, the blue color due to lots of recently born massive stars shining far more luminously than the Sun. The other is closer to edge-on. Besides the cool shape here, I also like that, on the left, the dust lines in the face-on galaxy are superposed on the other, a silhouette of sorts made of huge clouds of dark grains of silicaceous (rocky) and long-chain carbon molecules that are opaque to light.

This collision is obviously in the early stages; both galaxies seem to have so far retained their overall shapes, but that likely won't last, and in fact there's plenty of evidence for distortion in the spiral arms. The astronomers found nearly 2,000 star clusters in this one, made even more amazing when you consider this pair is 210 million light years from us! As expected since this merger is just starting, many of the clusters are young, some less than 10 million years old.

It's weird to think that violence on a scale like this leads to creation, but the Universe is funny that way. Stars are born in the aftermath of collisions in incredible numbers, and in fact any big galaxy out there (again, including ours) grew to that size in part by eating other galaxies.

Imparting our morals on the Universe won't get you far, but turning that around to appreciate the way we tend to perceive things can be illuminating, even as illuminating as the tens of millions of stars born in such events as these.