Exploring Baby Galaxies with Charles Steidel
 Example of ultraviolet dropout method
by Fred Heeren
When I visited Caltech astronomer Chuck Steidel in 1996, he had recently discovered a method to fill in the enormous knowledge gap between our observation of modern galaxies and the universe’s first light (the cosmic microwave background radiation). For 20 years, astronomers had searched diligently—but unsuccessfully—for a way to single out a population of the earliest galaxies. Steidel had developed a method that proved itself capable of doing just that, so that today it continues to give astronomers the world over the data they need to learn how galaxies evolved. As is the case in many fields, astronomers are trying to settle questions that the general public thinks little about, often because laypeople are still coming to grips with much more basic questions such as Did the universe appear—poof—all at once or did it evolve into its present state? These are questions where science crosses into—and sometimes crosses swords with—religion. For many who take an anti-evolutionary stand as a matter of spiritual principle, the word evolution should not be applied to anything having to do with creation, cosmic or otherwise. Yet, if an evolving universe implies a beginning (and it does, for relativistic reasons), science has taken a tremendous leap toward rapprochement with Christian faith on the matter of creation. Traveling backward in time with their shrinking subject, cosmologists can only watch the cosmos disappear at the beginning, pointing to a universe that came out of nothing—a universe that wasn’t there. No one need ask: “Were you there?” Chuck Steidel has tapped into nature’s own motion picture of past events, now showing in the present. Anyone who cares to view it can now see for himself what was and wasn’t there, at various stages of the deep past. While other astronomers at first assumed that larger telescopes would be necessary before finding truly primeval galaxies, Steidel began finding dozens of them—and today, thousands of them. His method, called ultraviolet dropout, is based on the fact that intergalactic hydrogen gas absorbs the ultraviolet light of the most distant galaxies, causing them to disappear when seen through an ultraviolet filter. Steidel identified early galaxies that are present in pictures of the cosmos when viewed through red and green filters, but that aren’t there when viewed through an ultraviolet filter. Visual evidence for a universe that isn’t there starts with the observation of galaxies that aren’t there. In the years ahead, telescopes beyond our obfuscating atmosphere, like NASA’s James Webb Space Telescope (collecting six times as much light as the Hubble), may begin to give us glimpses of the “Dark Ages,” when the first galaxies began to form. As our improving technologies bring us closer to the beginning, they will lead people to ponder, once again, what happened before that. Interviewing Charles Steidel
“The way that people have looked for these in the past tended to be looking for particular, spectacular fireworks of stars going off all at once,” Steidel told me. He was only 32—a young-looking 32—and could have passed more easily as a student than as a professor as he talked with me in his Caltech office, surrounded by Hubble Deep Sky images. “So they were looking for relatively rare events, using narrow-band filters tuned to find an emission line that comes from hydrogen atoms. And you have to have the filter exactly tuned to that wavelength to see it.”
“And I’ve heard it’s like trying to find a needle in a haystack,” I offered. “It’s much more difficult.” “So rather than try to find something that stands out you’re trying to find something that drops out?” “That’s correct. It’s a very simple technique, where we take pictures through different filters, very deep images of the sky with CCD detectors, and we take three filters, and we look for objects that are present through two of those filters, and they completely disappear in the third. And the reason they disappear is because they’re at a high redshift.” The high redshift denotes greater distances—and earlier periods, because of the time required for light to reach us from those greater distances. These young galaxies contain young, hot stars, emitting strongly in the ultraviolet. However, ultraviolet radiation from the most distant galaxies is absorbed by a greater amount of intervening hydrogen gas along the way. Today, Steidel uses the 200-inch Hale Telescope at California’s Palomar Observatory to find these primeval galaxies with his ultraviolet dropout technique, then flies to the 10-meter Keck telescope in Hawaii to measure their redshift, which corresponds to their distance and time period. And what do these galaxies look like? “We actually think we’re seeing the central bulge regions of galaxies forming,” said Steidel, “that is, the round part in the middle of a spiral or an elliptical galaxy, where you expect all of the star formation to be happening in a relatively small region. And those parts of galaxies we see today are also the parts that we think are the oldest stars in those galaxies.” “And you’re saying that modern galaxies have the oldest stars in the bulges, is that right?” “That’s right …. It’s still somewhat controversial. But there isn’t any doubt that we’re finding a number of things that match fairly closely to the number that you would expect to find if you were looking at the progenitors of the present-day, bright galaxies.” Steidel’s galaxy surveys have shown that galaxies were already arranged in clusters at that early time. But the individual, primeval galaxies lacked the characteristics of today’s spirals and ellipticals. More recently, Steidel has focused on a slightly later period, from about 10 to 12 billion years ago, when star formation appears to peak. If seeing is believing, then, as Steidel says, the universe “has absolutely changed with time.” His methods have helped astronomers identify populations of galaxies at various stages, where their differences from one to another are unmistakable. In the years ahead, telescopes beyond our obfuscating atmosphere, like NASA’s James Webb Space Telescope (collecting six times as much light as the Hubble), may begin to give us glimpses of the “Dark Ages,” when the first galaxies began to form. As our improving technologies bring us closer to the beginning, they will lead people to ponder, once again, what happened before that. In my interviews with researchers, I usually bring up such crossover questions when the scientists or their studies naturally suggest them. But I worried that I’d crossed over too clumsily into this territory with Steidel when I asked him what he thought about a universe that appeared to come into being out of nothing. He hesitated and said, “What happened before, you know, it’s …” and his voice trailed off. Finally I suggested: “Something must have happened before.” “I think about that extremely rarely.” Shoot, I’d gone too far, I thought. But then he added: “On the other hand, I do have a very wide appreciation for whatever put things there—because it’s just the greatest thing to go out on the catwalk around the dome, in the middle of the night, and just look up there, or look at a picture of the Hubble Deep Field, and see all the things that are out there, and—you know— it’s a beautiful universe out there.” Indeed, come to think of it, the way it all came together may be an even more impressive fact to ponder than the fact that at one time, that is, before time, the universe wasn’t there.
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