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A Nearby Unseen Giant

Week of August 2, 1999

I've always been fascinated by things that we miss that are almost literally right in front of us. In astronomy, this can happen for many reasons. One is that the object (or objects) are very faint, like the huge reservoir of incredibly faint comets orbiting beyond Pluto. These Edgeworth-Kuiper Belt objects had to wait until very large telescopes with sensitive detectors were invented to be discovered.

Another good example are the new types of lightning just discovered. Though observed for years by pilots and others, these red sprites and blue fans are so faint that again only very sensitive detectors can record their signature. Now they have spawned a cottage industry in observations and theoretical work.

Some things, though, can be a bit bigger than 100 kilometer snowballs and more substantial than millisecond long ethereal plasma displays. Sometimes whole galaxies can be hidden away from us, yet so close we could practically stub our toes on them.

How can a whole galaxy hide from our prying eyes? Imagine you stand in a large room, one hundred meters on a side. The ceiling is low, say, one meter above your head, and transparent (a true glass ceiling). Let's further imagine the room is filled with smoke. Now look around: you can clearly see people a few meters away, fuzzily see them 10 meters away, and only barely see them 50 meters away. However, when you look up, you can easily see people if they are standing on the ceiling. Although people fade into the murk if they are inside the room with you, if you look up you can see much farther; you could even see airplanes flying overhead, or the stars at night. That's because there is less smoke above you: the ceiling is low.

This is how it is for us living inside the Milky Way Galaxy. The Galaxy is very flat, and filled with dust and gas. When we try to look out the long way, our view is blocked. When we look up (or even down) we can see vast distances. At least, this is true when we use our eyes.

But we have more than our eyes. We have detectors that can see colors of light our eyes cannot, like ultraviolet, infrared (also known as IR) and radio waves. Radio and IR light have a very useful property: they is relatively unaffected by dust. While visible light is absorbed by dust, IR and radio are unimpeded. So we use them to pierce the veil of dust, and we sometimes find treasure.

image of Dwingeloo galaxy A team of observers (including people from the U.S., UK and the Netherlands) found such a treasure recently. Using a radio telescope at the Dwingeloo observatory in the Netherlands, they were surveying the plane of the Milky Way when they stumbled on the object in the picture (the picture itself was made later and is a combination of visible and infrared light). It's a whole galaxy, and the size of it amazed them: it's roughly 3 arcminutes across, or about 1/10 the size of the Full Moon! This implies it is very close, and indeed it lies only about 10 million light years away, making it one of the nearest big galaxies in the sky. It is about 1/4 the mass of our own Milky Way, making it a pretty big galaxy, yet it wasn't discovered until 1994! (A full size image can be seen if you click on the image; it will take you to the Astronomy Picture of the Day website, a favorite of mine and many others).

Other galaxies have also been discovered nearby as well. Wanna know more about them? Wait until next week's Snack!

My thanks to Andrew Loan for correcting a couple of errors I made on this page; I originally said the observing team was Australian, and that the image was wholly infrared. He should know; he was part of the team that discovered the galaxy!

©2008 Phil Plait. All Rights Reserved.

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