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SYFY WIRE Bad Astronomy

A galaxy torn apart blasts out a six-million-degree scream

By Phil Plait
The face-on spiral galaxy is lovely in optical light (blue) but has a very big secret in X-rays (purple). Credit: X-ray: NASA/CXC/Huntingdon Inst. for X-ray Astronomy/G.Garmire, Optical: ESO/VLT

Serendipity has a role in astronomy.

An astronomer, Gordon Garmire, noticed that the face-on spiral galaxy NGC 1232 had never been observed in X-rays before. At a distance of just 60 million light years (I know, "just," but that's not too far away as galaxies go) the Chandra X-ray Observatory should be able to see about 100 point sources; that is, unresolved sources of X-rays like black holes gobbling down matter, fiercely hot magnetic neutron stars, gas from exploding stars, and the like.

So he pitched his observing proposal and got 41 hours of total observing time, long enough to see plenty of interesting things. But what he also got was a big surprise. Sure, there were lots of bright X-rays dots scattered across the galaxy… but there was also this:

The face-on spiral galaxy is lovely in optical light (blue) but has a very big secret in X-rays (purple). Credit: X-ray: NASA/CXC/Huntingdon Inst. for X-ray Astronomy/G.Garmire, Optical: ESO/VLT

Yeah. The blue shows the galaxy in visible light using the Very Large Telescope in Chile, and the purple shows the X-rays from Chandra. That huge glowing purple cloud is, well, just that: a huge cloud of extremely hot gas, probably around 6 million Kelvins*.

Huge clouds of searingly hot gas are actually fairly common around galaxies. What's weird about this one is that there's no sign of it in visible light. Nothing. Usually you see some indications of gas at lower temperatures, but there's just nothing there.

So what's going on? The most reasonable assumption is that a dwarf galaxy is colliding with the much larger NGC 1232. The gas from the smaller galaxy is being pulled out by the gravity of the far larger galaxy, stretching it out into this elongated comet-like shape. NGC 1232, like most spirals, should have a halo of gas around, and as the dwarf galaxy slams into it the event creates a vast shock wave heating the gas to million of degrees. Although it's tens of thousands of light years long, the gas is actually pretty thin; it may have a density of only one atom per liter of space — in that same volume at sea level Earth's atmosphere there are more like 1022 atoms!

The total mass of the cloud is hard to determine. It depends on how thick it is, and since we're seeing it in only two dimensions (the thickness along our line of sight is impossible to know) the total mass can't be found. However it can be estimated if you assume a shape for it, though. If it's flat, like a sheet or a pancake, it has a total mass of about 43,000 times the mass of the Sun. If it's more spherical, the total mass is far higher, about 3 million times the Sun's mass.

Given that we don't see anything in optical light I'd wager it's closer to the lower end of that estimate, otherwise I'd expect to see something if that much gas were slamming into the disk of the spiral. But that's just a guess.

Incidentally, the fuzzy blob to the upper right may be a background galaxy (it's not mentioned in the journal paper), but the blob just to the lower right may be affiliated with this collision. It's not clear, but the author suggests it could be a huge region of star formation in NGC 1232, perhaps triggered by the impact. We know how much X-ray emission to expect from such features, and this one is so big, hot, and bright that if it is a nebula making stars it's huge and extremely fecund, among the largest such regions known. I'm a little skeptical of that, but it does sit right at the front of that gas cloud, where you'd expect such a thing to happen. Maybe there's a lot of gas streaming down from the collision into NGC 1232, and it's triggered star formation in gas clouds there. If so, wow.

If this huge cloud really is gas stripped from a dwarf galaxy colliding with the spiral, this is the first time such a thing has ever been discovered only by X-rays. That in itself is helpful; it's hard to see dwarf galaxy collisions from a distance in optical light, whereas gigantic clouds of X-ray emitting gas might be easier. If more of these are seen it may help determine how often such collisions happen. Galaxies grow by eating smaller galaxiesthe Milky Way is eating several right now — so getting a collision rate can help us understand how galaxies like ours grow so big.

Oh, and what about the original proposal, where he was looking for point sources? Well, that did in fact work:

Optical and X-ray observation of NGC 1232 showing the extended gas cloud as well as dozens of point sources, high-energy objects likely located in the spiral galaxy. Credit: NASA/CXC/Huntingdon Inst. for X-ray Astronomy/G.Garmire

That's the same image but this time the point sources are shown (in the first image above a processing technique was used to remove them so the fainter extended gas cloud could be more easily seen). There are likely more there than you can see in this image, but as the author states, "The [point] source studies are still ongoing". In other words, he was way more excited by the gas cloud, and wrote that journal paper first.

I don't blame him. He was expecting a routine survey of a lovely but otherwise typical large galaxy, and got evidence of a superheated eructation from a cosmic collision instead. I think I'd drop everything and write that paper first, too.

*At this high a temperature, Kelvin and Celsius are the same, if that helps.