In what has become an annual tradition here at BA Central, literally the day I post my gallery of best pictures of the year, something comes along that really would’ve made it in had I seen it even a few hours earlier. In this case, it’s a combined Chandra X-Ray Observatory and optical Very Large Telescope image of galaxy clusters colliding that’s so weird that at first I thought for sure it was Photoshopped! But it’s real, so check this out:
What you’re looking at is a collision on a massive scale: not just two galaxies, but two clusters of galaxies slamming into each other, forming this object, called Abell 2052. The total mass of this combined cluster is almost beyond imagining: something like a quadrillion times the mass of the Sun — 1,000,000,000,000,000 Suns! Note that our galaxy has about a hundred billion stars in it, so Abell 2052 is about 10,000 more massive. Yikes.
No, don’t fret: I’m not betraying everything I know to be true and suddenly supporting astrology! I’m just having a little joke at the expense of NGC 4435 and 4438, two galaxies in the Virgo Cluster known as "The Eyes", and seen in lovely detail by the Very Large Telescope:
[Click for orbus giganticus, and you really should; the details are beautiful.]
Clearly, these guys know each other. NGC 4438 (upper left) is distorted and drawn out, which is a sure bet that it’s undergone a collision with another galaxy in the recent past. Given how close NGC 4435 (lower right) is to it, that seems like the culprit (though M86, not seen in this shot, is also close by and may be to blame). They may have actually passed right through each other as recently as 100 million years ago! Direct hits between galaxies aren’t like car accidents where the vehicles stop dead; galaxies are mostly empty space, and stars are so small compared to the galaxies themselves that a direct impact between two stars is incredibly unlikely.
But the gravitational pulls from the opposing galaxies can affect each other, teasing out long tails of material just like the one streaming from NGC 4438 . The scattering of dust is also another clue. Although stars don’t collide, gas clouds are much larger, some dozens of light years across. Those do in fact slam into each other, causing them to collapse and form stars (though there’s some evidence that’s not always the case). Vigorous star formation can cause lots of dust to be created, and that’s what we’re seeing in NGC 4438. And it’s all weird and distorted too, clinching the case.
You may notice NGC 4435 is a bit featureless. That’s actually common in disk galaxies that live in clusters. As they move through the cluster at high speed, the intergalactic medium — thin gas expelled from the galaxies — can strip away the gas and dust in a galaxy, like opening a car window can blow out stale air inside.
Galaxy collisions are pretty cool, and a rich field for study. And if you’re patient, you’ll get a great view of one: our galaxy is headed for a close encounter with the Andromeda Galaxy. Given that it and the Milky Way are among the biggest and most massive spiral galaxies in the local Universe, it’ll be a spectacular show. Better reserve your seats now, though. You only have a billion or two years to wait!
Astronomers are like forensic investigators. We have all this data taken from the scene of some sort of event, and have to piece together what happened. But those folks on CSI have it easy: they get to actually walk around the scene, poke and prod it, examine various stains, and even take physical evidence back to the lab. Astronomers are stuck standing a quintillion kilometers away, and we only get to see things at one angle.
But oh, what an angle. If it pleases the court, I’d like to enter this evidence for your consideration:
That’s my kind of evidence (click to embiggen). It’s an image of the lovely grand-design spiral galaxy M81, one of the nearest major galaxies to our own. At about 12 million light years away it’s bright enough to be seen in binoculars (and in fact some extremely keen-eyed observers have been able to see it with their unaided eyes). That means it’s close enough to study in detail… and what detail!
Every now and again I think I’ve pretty much seen it all when it comes to astronomical images, and I’m getting jaded.
And then I see a picture like this:
Yeah, I still get a thrill from seeing things like this! Click to massively embiggen.
The image shows what’s called the Hickson Compact Group 31, a small collection of galaxies. It’s a combination of images from Hubble (visible light, shown in red, green, and blue), Spitzer (infrared, shown as orange), and the Galaxy Explorer or GALEX (ultraviolet, seen here as purple).
If I saw this picture with no caption, I’d know I was seeing dwarf galaxies colliding; the shape and the glow from newly-forming stars is a dead giveaway. But I’d also guess that the galaxies were young; old galaxies tend not to have much gas in them, and there’s clearly plenty of that in those galaxies! But in fact the galaxies here are very old; there are globular clusters (spherical collections of perhaps a million stars each that tend to orbit outside of galaxies) in the group that can be dated to being 10 or so billion years old. That means these are old objects, reinvigorated by their collision.
In fact, star clusters inside the galaxies can be dated as well, and appear to be only a few million years old. Oddly, the gas content of the galaxies is very high, with about five times as much as the Milky Way has. That’s pretty weird; it should’ve been used up a long time ago. Apparently, these galaxies have lived very sedate lives until very recently. I’ll note that they are relatively close to us, about 166 million light years away. Usually, colliding dwarf galaxies like this are seen billions of light years away, so we really are seeing them as they appeared recently.
Apparently, the lower-case g-shaped object on the left is the result of two galaxies smashing into each other, and the longer galaxy above them is separate. The spiral to the right is part of this as well and may be involved in the gravitational dance; you can see a splotchy arm of material pointing right at it from the collision on the left. Typically in collisions the gravity of one galaxy draws matter out of the other, and that can collapse to form stars. The red glow is from gas excited by newly born stars, and the blue glow is from these stars themselves. The galaxies are pouring out ultraviolet light (the purple glow) which is another dead giveaway of vigorous star formation.
The background galaxies are gorgeous, too. There’s a phenomenal distant open spiral on the bottom, to the left of center, and what looks like yet another pair of interacting galaxies at the bottom left, obviously much farther away than the Hickson group. Take a minute to look around the high-res version to see what else you might find!
Yup. I guess you can teach old galaxies new tricks… and even sometimes jaded astronomers, too.