Some autumn night, hold a dime at arm’s length and block a tiny piece of the constellation Pegasus the Winged Horse. Hidden behind that coin may be the most famous galaxy cluster in all the heavens.
First seen by French astronomer Édouard Jean-Marie Stephan in 1877, the five galaxies famously known as Stephan’s Quintet constitute the first compact group ever found. Back in the 19th century, however, no one knew that these smudgy patches were far more than mere clouds of gas in our Milky Way Galaxy. With the advent of better telescopes and spectroscopy, astronomers eventually saw each as a city of billions of stars. But there was mystery here, stupendous mystery, and it arose not slowly but all at once.
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The puzzle involved redshift, the most important evidence of a galaxy’s distance. Redshift — a change in the wavelength of light an object emits as it moves away from us — indicates how fast a galaxy recedes in our expanding universe. Imagine a balloon covered with polka dots. As the entire cosmos (the balloon) expands, the galaxies (dots) logically get farther apart. A dot close to the one you live on will move slowly away from you; more distant dots will move away faster.
The galaxies in Stephan’s Quintet all seem physically connected, so they should display similar redshifts as the space between our Milky Way and them grows continuously larger. Indeed, more than merely appearing physically near each other, tendrils of dusty gas appear to connect them loosely. It seems as if each galaxy’s gravity pulls off strings of material from the others, like pizza cheese. They appear interlocked.
There was just one problem. Four of the galaxies show a large redshift, corresponding to a recessional speed of 4,000 miles (6,400 kilometers) a second. This means they lie about 300 million light-years away. But galaxy NGC 7320, at lower left in the photograph, shows a redshift indicating a slower speed of just 491 miles (790 km) per second. This would mean it’s only 30 million light-years away.
This discrepancy was not some mere technicality; it was huge. If one galaxy at the same distance as others has a very different redshift, then redshift cannot be a reliable indicator of distance. Some astronomers, led by the iconoclast Halton Arp (see object 38 on our list), concluded that redshift can’t be trusted. The size of the universe was suddenly in doubt, as were many of the tenets of cosmology, such as how quickly everything is expanding. Thus, Stephan’s Quintet became the paradigm for the mysterious, and for cosmological insecurity.
Many astronomers insisted the streams of material between NGC 7320 and the other galaxies proved they were physically bound together in space. Moreover, all five galaxies have approximately the same apparent size, further evidence they sit at the same distance. After all, if the low-redshift galaxy was really much closer to us, shouldn’t it appear far larger than the others?
The Hubble Space Telescope finally solved this issue in 2000. Its razor-sharp imaging resolved individual stars in the low-redshift galaxy, while the other four remained starless. This proved NGC 7320 is much closer to us than the others.
At last, mystery laid to rest. It’s a line-of-sight situation. One galaxy sits in front of the other four — 10 times closer, in fact. The seeming strings of matter do not really extend from NGC 7320 to the others. And NGC 7320 appears the same size as the other four because it happens to be a smaller galaxy.
Cosmologists let out a collective sigh of relief. Redshift is reliable after all. The universe is safe. Astronomers now prefer to label the four galaxies that are actually a physically bound unit as Hickson Compact Group 92. This actually includes a separate galaxy that was not a part of the original quintet, so that even though NGC 7320 has been expelled from the club, Compact Group 92 remains a fivesome.
But relief did not last long. Amazingly enough, a different puzzle suddenly arose. Near the nucleus of the quintet’s uppermost galaxy is a benign-looking star that spectroscopy now shows is a quasar with an enormous redshift. Meaning, it must lie billions of light-years beyond the galaxy. Yet there it is, clearly in front of it, and seemingly interacting with the galaxy’s gas.
This resurrects the same cosmic puzzle — with a vengeance. How could a galaxy envelop a quasar whose redshift indicates it’s several billion light-years farther away?
It’s déjà vu all over again. It’s another doozy of a discordant distance issue in the very same rabbit patch. Indeed, this time the visual evidence is even more ironclad. Might redshift be unreliable after all? Or does the distant quasar just happen to align with a weird empty hole that tunnels all the way through this galaxy?
Stephan’s Quintet just won’t quit the Strangeness Society.
