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SYFY WIRE Bad Astronomy

Overlapping galaxies

By Phil Plait
Two overlapping galaxies, some 800 million light years away, were captured by Hubble accidentally while viewing a much closer galaxy (NGC 253, off screen to the right).

As I (and many before me) have said many times, space is big. That's why we call it space.

The sky is so big that by eye it's relatively rare that two objects are really close together. That's why we get excited when two planets pass near each other, or the Moon gets near one (also: it's very pretty when that happens). The handful of stars that are really close to each other by eye get a lot of attention too, like Albireo in Cygnus, and Mizar and Alcor in the handle of the Big Dipper.

The odds of two stars actually overlapping are incredibly small; stars appear so small in our sky that only a handful can be seen as anything other than unresolved dots.

But some objects are bigger. A lot bigger: Like, galaxies. These are enormous collections of billions or trillions of stars, and even though many are very far away indeed, their spatial extent is so big they can appear to overlap each other. This is actually rather common in nearby galaxies — Andromeda, for example, the nearest big spiral, has a satellite companion called M32 that is superposed against its disk.

Those are physically associated, though (M32 orbits Andromeda). Do we ever see two randomly placed galaxies that overlap?

Yes! NGC 3314 is one of the coolest examples, where a face-on spiral is almost perfectly centered over a more inclined one. Others can be found.

The edge-on spiral galaxy NGC 253 is about 11 million light years from Earth, seen here using the Wide Field Imager camera on the MPG/ESO 2.2-metre telescope. The location of two overlapping background galaxies is arrowed. Credit: ESO

… sometimes by accident. NGC 253 is a magnificent spiral galaxy seen very nearly edge-on. It's close enough to us that its individual stars can easily be seen using Hubble Space Telescope. A survey of NGC 253 was done to look at those stars, creating a large (and very narrow; see below) image of it. When the astronomers looked near the edge, though, they saw something pretty amazing: Two galaxies, far in the background, and they're overlapping!

In ground-based images (like the one above) they appear as faint fuzzy blobs. But with Hubble's keen eye, they look like this:

Two overlapping galaxies, some 800 million light years away, were captured by Hubble accidentally while viewing a much closer galaxy (NGC 253, off screen to the right).

Oh wow. The little spiral is in front of the big one, between us and it. The tell-tale for that is dust. Spiral galaxies have lots of dust in them, made up of tiny silicate grains or long-chain carbon molecules (really, soot). These are both opaque, blocking light from stars behind, and generally following the spiral arm structure. You can see wispy dark tendrils between the center of the big and little galaxies, and it fits the pattern of the smaller one. Clearly, the smaller one is in front.

The big galaxy — called by its catalog name, 2MASXJ00482185-2507365 — is about 800 million light years away (NGC 253 is only about 11 million, so this pair is way way in the background). It's unknown how far away the smaller one is but it's clearly a little bit closer at least.

The dust is so well-defined in the smaller one, and both galaxies resolved so well, that astronomers took advantage of this lucky happenstance to measure the amount of dust in the little galaxy and map it out. They found that the dust extends well beyond the disk of visible stars in the smaller galaxy, the first time it's been seen so far out in a galaxy pair like this, and in many ways is similar to the distribution of dust in our home galaxy, the Milky Way. They were able to test various methods of analyzing the dust distribution, too, which is useful for future studies if more galaxies like this are found.

I like this! Almost every time an observation is made, especially using Hubble, everything is carefully planned out as to how it will be done, how it will be analyzed, and what is hoped to be understood. Accidents like this aren't very common, and I'm happy to see it being taken advantage of. When I worked on Hubble I found lots of little coincidences — interestingly shaped galaxies, weird stars, asteroids in the foreground, and the like — and I sent out many emails to other astronomers letting them know about them if they wanted to take a stab at analysis. I once found a nebula in a neighboring galaxy to us that hadn't ever been resolved before, and published a short paper on it!

There's a lot of science lurking in observations made for different science. It's fun to see it dug up.

And I have to mention one other thing. In many image of galaxies like this, you can see lots of foreground stars. That's because we live inside the Milky Way galaxy and have to peer past our own stars to see galaxies beyond.

NGC 253 (well, half of it), an edge-on spiral surveyed by Hubble Space Telescope to investigate its population of stars. Credit: NASA, ESA, J. Dalcanton and B. Williams (University of Washington)

But in this case things are different. All those foreground stars you see in the Hubble image are not Milky Way stars: They're in NGC 253! That's why most of them are faint and look to be of uniform brightness; we're seeing the brightest, most luminous stars in NGC 253.

And now consider this: In this image we are looking out of the Milky Way, through NGC 253, then through that smaller galaxy, to see 2MASXJ00482185-2507365 beyond!

It's a galaxy overlapping a galaxy overlapping a galaxy overlapping a galaxy. And I have to wonder if the faint red fuzzy thing on the left of center of 2MASXJ00482185-2507365 and near the edge of the spiral arms is an even more distant galaxy, billions of light years away. It sure looks like it. If so, add another "galaxy" to that series.

Quite the pileup. Maybe "space" isn't all that big after all*.

*Yes, it is.