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SYFY WIRE Bad Astronomy

Warp into a galaxy

By Phil Plait
NGC 5084, a lovely but odd disk galaxy with an obviously warped disk. Credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona

Stop me if this shocks you, but I’m something of an astronomy nerd.

One of my favorite perqs of this job is that people will send me images of gorgeous astronomical objects, and then I get to dive into the professional journals reading up on them so I can find stuff to write about. It’s fun! I wind up learning a lot nearly every time, even if I can sometimes figure out a lot simply by inspecting the image.

I have a wonderful example of this for you today. My friend Adam Block is an astronomer who observes using the 0.8-meter Schulman telescope on Mt. Lemmon in Arizona, where skies are general dark and steady*. He is a terrific astrophotographer, taking magnificent images of a wide range of cosmic beauties.

In a recent email he notified me of a new image of a galaxy called NGC 5084, and I was excited to see it. Even so, I was blown away by his work. See for yourself.

NGC 5084, a lovely but odd disk galaxy with an obviously warped disk. Credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona

Oh, yeah.

NGC 5084 is a disk galaxy, a flattened circle much like our Milky Way. We see it almost precisely edge-on, so it looks like a thick line. Like most such beasts, it has a central bulge, a flattened ellipsoid of old, red stars marking the middle. You can also see long dark tendrils: These are dust lanes, vast clouds of material made up of silicates (rock) and complex carbon chains called polycyclic aromatic hydrocarbons (or PAHs) which are essentially soot. They’re opaque to visible light so they appear in silhouette against the glow of billions of stars behind them.

But the most obvious feature of NGC 5084 is that the disk isn’t flat. It’s warped! It bends up on the left and down on the right. What’s that about?

Many galaxies have warped disks like that! While the cause isn’t 100% certain, a plausible scenario is that the galaxy recently underwent a near-miss or even a collision with a smaller galaxy. The gravity of the smaller galaxy can distort the disk, creating a ripple in it.

In general a ripple like that would die down rather rapidly on galactic timescales, but we see something like half of all disk galaxies have warps! That includes our own Milky Way and the Andromeda galaxy. This means something must be happening to maintain the warp for much longer times than you’d naively expect, or else we’d see far fewer galaxies with them.

This can happen if the galaxy has a huge halo of dark matter around it — invisible because it doesn’t emit light, but still has a significant mass and therefore gravity — and this halo isn’t quite symmetric. If it’s a flattened ellipsoid (like a beach ball someone is sitting on), and tilted a bit with respect to the plane of the disk, then small disturbances in the disk can be maintained for a long time by the gravitational tugging of the halo.

Interestingly, I found a paper that indicates that NGC 5084 does have a very large and massive dark mater halo, and there’s evidence it’s eating its smaller satellite galaxies! So that fits.

Cool. But it’s not the coolest thing about this galaxy.

NGC 5084 is huge. I mean huge; it’s twice the diameter of the Milky Way, and may have a mass equal to ten trillion times that of the Sun.

Holy heavy heavenly objects! That’s far more massive than our galaxy. In fact, NGC 5084 is one of if not the most massive disk galaxy known. It must have eaten a lot of other galaxies to get so big.

I also found out that it’s a LINER — that means it has a low-ionization nuclear emission-line region. That’s a galaxy where spectra show atoms like oxygen are mildly ionized (they have one or two electrons stripped from them) in the middle of the galaxy. Most likely this is due to a supermassive black hole in the core gobbling down matter. As the matter falls in it piles up into a huge disk that gets unfathomably hot. This material then glows so fiercely it can zap material much farther out, causing it to glow as well. In general a galaxy like this is called an active galaxy, and LINERs are a subset of them.

I was also curious about the two smaller galaxies near NGC 5084 in the image. The bigger one on the right is called ESO 576-31, and is likely a satellite galaxy of NGC 5084. The distance is about the same.

But the other one is odd, isn’t it? It looks like a hamburger! That’s probably another disk galaxy where the dust going through the middle is far thicker, blocking all the light behind it, making the galaxy look like it’s sliced in two. This galaxy is called 2MASX J13204225-2149079 and it’s something like 600 million light years away from us, seven or so times farther away than NGC 5084, so it’s way in the background.

Although it’s hard to tell in this image, there are also a dozen or so galaxies orbiting NGC 5084; they make up a compact group called, oddly enough, the NGC 5084 Group (which includes ESO 576-31). This is also common; the Milky Way is part of the Local Group of a few dozen galaxies including the Andromeda and Triangulum spiral galaxies.

Of course, not all this can be gleaned from Adam’s gorgeous image — this galaxy has been studied for decades. But a lot of it is there if you know what to look for.

And if all you want to look for is artistic beauty, that’s there too. But there’s beauty — and art — in science, and that’s a powerful motivator for me to continue to post such spectacular images.

*The one time I visited Adam at Mt. Lemmon was part of a trip with Science Getaways, a company my wife and I run where we take people on science vacations, which includes lots of astronomy. We took 20+ people up to the observatory… and it snowed. Figures. However, Adam is prepared for such things, and gave us a great talk, showed us the telescope, and we had a lot of fun watching cosmic rays zap through a cloud chamber. In fact, that last part was one of the most memorable parts of the evening according to our guests!