Galaxy on the edge of space

Contributed by
Jan 29, 2011
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For your weekend eye candy pleasure, I have an unusual galaxy for you. Actually, it's an unusual picture of an unusual galaxy!

Check out NGC 6503:


This picture is brought to you by the good folks of Hubble Space Telescope. Click it to get the galactinated 4000 x 2200 pixel version.

This picture is wee bit odd because it combines images from two different filters not generally seen together. One is a near infrared filter just outside the range of the human eye (0.814 microns, if you're keeping a log of all this) and shows mostly stars, colored blue in the picture. The pink/red is coming from a filter that isolates the light from hydrogen gas, and shows where stars are actively forming in giant nebulae. These factories are like our own Orion Nebula, cranking out stars. I've seen images of NGC 6503 before -- like the one inset here from NOAO -- so I have a passing familiarity with it. Like most spirals, it has more older stars toward the center and bluer, younger stars forming in the spiral arms, and that's pretty obvious in the more natural color NOAO image. But in the Hubble shot, the IR filter doesn't really distinguish very well between bright blue stars and older, red ones. Both pour out IR light, so we just see stars all over the place. Of course, the red nebulae are really striking too, making the Hubble image look, well, odd.

The galaxy is a bit of a weirdo, too. First, it's small: only about 30,000 light years across, a third the size of the Milky Way. But size hardly matters, clearly, when forming beautiful spiral arms. This galaxy obviously has no trouble maintaining them.

But its location is strange too. It's on the edge of the great local void: a vast region of space where galaxies are few and far between. Galaxies tend to exist in clusters and superclusters. The Milky Way is part of the the Local Group, a small collection of a few dozen galaxies which itself sits on the outskirts of the Virgo Cluster, 60 million light years away. In the opposite (more or less) direction, toward the constellation of Draco, is the Local Void. Our galaxy is near the edge of this void, but NGC 6503 is actually further into it, 17 million light years away from us. Even then, it's only on the void's edge; estimates vary but the empty region extends for something like 30 - 200 million light years in that direction!

So you can picture it: on one side of us is a collection of hundreds of galaxies in the Virgo cluster, which itself is part of a much larger supercluster containing thousands of galaxies. On the other side of us is an empty region of roughly the same size. Somehow, when the Universe itself was young, the matter in this region must have all condensed toward Virgo, leaving the void nearby. We think the entire Universe is this way, with dense regions of matter surrounding bubble-like voids. If you could step back and look, the Universe might appear like a giant sponge!

Think about that the next time you're scrubbing your dishes.

Image credits: ESA/Hubble and NASA; Adam Block/NOAO/AURA/NSF