Looking for galaxies in all the wrong places

Johnny Lee sang about looking for love in all the wrong places. But unlike the travails of poor Johnny, whose lifetime spent in singles bars yielded no traces of what he was dreaming of, in astronomy, searching for cosmic objects in unexpected locations is no fool’s game. After all, space is big, and a galaxy being in the “wrong place” simply means that an observer might not expect it to be there. It’s really a matter of perspective. For instance, if our Sun were located somewhere else in the Milky Way, even our galactic neighbor, the expansive Andromeda Galaxy, could be entirely hidden from our view, its light blotted out by our own galaxy’s plane of dust, gas, and stars.

Galaxies are the building blocks of the cosmos, and they are distributed relatively uniformly across the sky. Yes, there are concentrations known as galaxy clusters and there are places that don’t hold many bright examples. But no telescopic fields away from the dense hub of the Milky Way are entirely devoid of galaxies. There are plenty of galaxies “through” or near the Milky Way, too. The problem with viewing the latter group is that it’s not always easy to get a good look at them.

There are three factors that make observing galaxies in and around the band of the Milky Way challenging. First is something called extinction — the dimming of distant objects that occurs when some of their photons are intercepted and absorbed by molecules of interstellar dust. Our galaxy’s central disk is crowded with stars and gas, but it’s the Milky Way’s prolific dust that really makes observing distant galaxies so difficult.

The amount of extinction an observer experiences is directly proportional to how much of the Milky Way they are looking through. Typically, for every kiloparsec (3,262 light-years) of Milky Way your visual path cuts through, the distant object you’re targeting will appear about 1.8 magnitudes fainter. So, in general, the closer a galaxy is positioned to the galactic plane, the dimmer it looks. A distant galaxy near the galactic plane will also appear redder — a phenomenon called reddening — due to blue light being preferentially absorbed and scattered by the Milky Way’s dust. The best examples of this are the extinction-plagued galaxies Maffei 1 and Maffei 2 in Cassiopeia. They are only a few degrees from the galactic plane and were only discovered in 1967 using specialized, hypersensitive photographic plates with the 36-inch Schmidt telescope at Asiago Observatory in Italy.

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