Spiral galaxies are inherently interesting. Something about their beauty is so enticing... but when you look at them more carefully, the science and physics behind them is terrifically compelling. And when you use different eyes -- say, radio telescopes -- then you see something different entirely:
This shows two views of the lovely face-on spiral galaxy NGC 6946. On the left is a visible light image, and on the right is the radio view, taken by the Westerbork Synthesis Radio Telescope (taken over the course of 192 hours). Amazingly, these two images are to the same scale!
Spiral galaxies emit light across the entire electromagnetic spectrum, including visible and radio light, but what emits that light is different. Stars and warm gas emit visible light, but cold hydrogen glows at radio wavelengths. At a wavelength of 21 centimeters (about 8.5 inches, much, much longer wavelength than visible light, by a factor of tens of millions!) cold hydrogen can actually be quite bright, making it a perfect target for big radio telescopes.
In this image on the right I superposed both shots so you can see just how much more there is to NGC 6946 than the eye sees. What this image immediately tells us is that cold hydrogen extends well beyond the region where hydrogen is warmer, toward the center of the galaxy. It also shows the gas still takes on a spiral shape well past the visible boundaries of the galaxy.
A more detailed analysis indicates there are over 100 holes in the cold hydrogen gas as well, and these correspond to areas where stars are actively forming. That's hardly a surprise! Stars use up hydrogen gas when forming, and then heat up what remains around them in the neighborhood. Once warmed up, the gas doesn't emit as much 21cm radio waves.
The astronomers also found a lot of this gas is moving at high speeds, up to 100 km/sec (60 miles/second, fast enough to go from the Earth to the Moon in a little over an hour!). This is probably gas that's been blown up and out of the galaxy by stars and supernovae, only to fall back down due to the gravity of the galaxy. That's not known for sure, but we do see such fountains in other galaxies, including our own.
I'll be honest: I'm more of a medium-to-high energy guy than radio guy. That's why I tend to talk more about X-rays and gamma rays from astronomical objects, but every part of the spectrum tells a story. Radio astronomy has been around for almost a century now, and it is still and always will provide insight into the mechanisms behind the Universe.
Image credit: Rense Boomsma/Digitized Sky Survey/WSRT; ASTRON/JIVE Archive