ALMA measures millimeter waves from the sky. These millimeter waves are emitted by cold dust clouds in the Universe. The clouds are so cold that they don’t produce visible light. They’re completely dark. With a normal telescope, you can’t see them. But ALMA can detect their invisible radiation.
But if millimeter waves are invisible for humans, how can astronomers see what ALMA is observing? How can ALMA produce nice pictures, like the Hubble Space Telescope? The trick is simple. Astronomers use ‘false colors’. So the colors in the ALMA pictures aren’t real. Still, they contain a lot of information.
If you have an atlas at home, you may have seen false colors before. Look in the atlas for a so-called ‘physical map’ of the country where you live. A physical map also uses false colors. Low-lying terrain is colored green. Higher terrain is yellow. Very high mountains are brown or red. These colors are used to tell you about the altitude of the terrain. They aren’t the real colors of your country. They are false colors, but they contain a lot of information.
With the ALMA images, it’s more or less the same. Suppose ALMA has observed a distant galaxy in the Universe. For each part of the galaxy, astronomers now know what kind of millimeter waves are emitted, and how much. Using this information, they then create a ‘map’ of the galaxy in false colors.
For the longest millimeter waves, red colors are used. Slightly shorter millimeter waves are colored orange, yellow and green. The very shortest millimeter waves are represented by blue colors. If the millimeter waves are very faint, the corresponding color is dark. If a certain region produces a lot of millimeter waves, the corresponding color is bright. The result is a beautiful, colorful image of the galaxy. But the colors aren’t real!
Of course, astronomers want to compare the ALMA image to a real photo of the galaxy. They want to know which parts of the galaxy produce a lot of millimeter waves, and which parts are dark, even for ALMA. To make this comparison, the false-color ALMA image is often combined with a photo in visible light, made with a large telescope on Earth, or with the Hubble Space Telescope.
Not all ALMA measurements can be turned into images. Sometimes ALMA measures the motions of cold dust in a distant solar system, or the millimeter waves from certain molecules in space. These measurements can’t be turned into an image. Instead, astronomers just use graphs and numbers. They also contain a lot of information.