The heat and the light of a dusty galaxy

Bad Astronomy
The entire universe in blog form
May 19 2010 11:23 AM

The heat and the light of a dusty galaxy

The European Southern Observatory just released a new image of the spiral galaxy M83, and it's a pretty cool shot:

eso_m83

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

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[Click to embiggen, or go here to get access to monstrously bigger images, including wallpapers.]

M83 is pretty close to us, if you happen to think 15 million light years is close. I'm an astronomer, so yeah, I consider 150 quintillion kilometers -- that's 150,000,000,000,000,000,000 kilometers, or 90 quintillion miles -- practically next door in galactic terms.

This image is fascinating. It was taken with the ridiculously huge 8 meter Very Large Telescope in Chile, and is in the infrared at a wavelength of 2.2 microns, about three times the wavelength of the reddest light your eye can see. That turns out to be important! Galaxies like M83, and our Milky Way for that matter, are littered with huge clouds of dust. These aren't like the clumpy bunnies you find under your sofa when you accidentally drop some M&Ms under there while gaming and realize you haven't vacuumed in a couple of months. Say.

No, this dust is actually composed of complex organic molecules, and they are opaque to visible light. A cloud of dust a few light years across might as well be a concrete wall if you're trying to look past it with a telescope. Whatever's behind it is hidden.

But infrared light has a longer wavelength than visible light, and it passes through that dust. So an image of a galaxy in IR can look a lot different than one taken in visible light. When I first saw the picture above, I didn't even recognize the galaxy! Mind you, I am a vastly huge astronomy dork, and can recognize dozens of galaxies at a glance (that would sound like bragging if the topic were any different). So to be stumped, even for a moment, when seeing such a picture is disturbing. Here's a side-by-side shot to show you the difference:

eso_m83_ir_vis

[Again, click to enspiralate, and to get much huger pix.]

See why I was confused? They look pretty different, and not just due to the colors. In the visible light image on the right, the spiral arms are lit up by really hot stars (some many times hotter than the Sun), and the glowy pink and red parts are clouds of gas where stars are being born. The hydrogen in the clouds gets heated up (astronomers call that being "excited", a gentle reminder that we're dorks) and emits a lot of light, making them very showy and obvious. Those regions are punctuated by long streamers of darkness, the dust clouds scattered along the arms. They are essentially gone from the IR image on the left! The star forming regions are hard to see as well; they don't emit nearly as much IR as visible light, so they fade into the background.

I'll note that just because these images are in the infrared doesn't mean they're tracking heat; at least, not like what you normally think of when you think "infrared". A lot of folks equate IR with heat, but that's not really the case. When you see images from a thermal camera (like in those awful ghost hunting shows) those are way way farther out than what your eye sees, like at 10 - 20 microns. Objects at human body temperature glow at those wavelengths. The picture of M83 was at 2.2 microns, which is where objects at about 1000°C (1800° F) glow. So in a sense we're still seeing heat, but from objects much hotter than even an oven set to broil -- this IR light is from red stars, like giants and supergiants near the ends of their lives.

Looking in the IR tells us a lot about galaxies, more than we'd know otherwise just looking in the visible part of the spectrum. And while the image from the VLT is pretty cool, I have to admit I like the one in visible light better aesthetically. It's prettier. And it just goes to show you: we need our dark side. It provides contrast against which to see our warmer side.

Image credits: ESO/M. Gieles. Acknowledgment: Mischa Schirmer

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