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Twinkle, twinkle, microlens: in trying to probe the dark matter surrounding, the Milky Way, astronomers have confirmed the identity of a nearby gravitational lens
Natural History, July-August, 2004 by Charles Liu
In the early 1990s a team of astronomers led by Charles Alcock at the Lawrence Livermore National Laboratory began a pioneering, multiyear study of unseen matter in our Milky Way. They scanned millions of stars in the sky night after night, watching and waiting for signs of "microlensing"--gravity-induced magnification of background stars by small, massive objects passing in front of them. No one knew for sure how many such events the team would see. Some predicted that the search might not yield even a single microlensing event (including one young graduate student who now writes this column).
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Now, older and hopefully wiser, I and my fellow naysayers cheerfully admit we were wrong; the experiment was a grand success. After eight years of painstaking observations, Alcock and his collaborators recorded several dozen microlensing events, all caused by planet- or star-size bodies within the "dark halo" of unseen matter that envelops our galaxy. A few dozen microlensing events may not sound like a lot, but it was a large enough sample to help open a new frontier of astronomical research, and in the process coin a spiffy new acronym: MACHO, which stands for "massive compact halo object."
Since those early days, more than a thousand microlensing events have been recorded by research groups around the world. Recently, astronomers reached another milestone in the study of microlensing. One of the earliest MACHOs detected, which caused an event dubbed LMC-5, has now been positively identified--lifting an anonymous, faint red star into the limelight of astronomical fame.
The Milky Way has an ellipsoidal "bulge" of stars at its center, about 15,000 light-years long and 5,000 fight-years high, which is bisected by a flat disk of stars and gas 100,000 light-years across and about 1,000 fight-years thick. (You'll get the idea if you imagine a pizza with a plum stuck through the center of the crust.) The Sun, with Earth orbiting around it, resides in the disk, about midway between the bulge and the disk's edge.
The bright, familiar shape of the disk and bulge is enveloped by a spherical cloud of sparsely scattered stars called the stellar halo, which extends some 50,000 to 100,000 light-years from the center of the galaxy. The disk, bulge, and stellar halo in turn are all embedded in the center of a structure called the dark-matter halo (or dark halo), whose extent is still not definitively known; what is known, though, is that the dark halo dwarfs the galaxy's other components in both size and mass.
As its name suggests, the dark halo emits no light. Astronomers know of its existence from its gravity alone: without the dark halo, our galaxy's disk would not be stable. The structure and composition of the dark halo is one of the outstanding puzzles in astronomy--though astronomers suspect that about a fifth of its mass is comprised of small, massive bodies. But if no one can see it, how can astronomers figure out what it's made of?
Since gravity is the one measurable feature of the dark halo, Alcock and his colleagues decided to probe it with gravitational lensing [see "The Quest for the Golden Lens," by Charles Liu, September 2003]. The random orbital motions of its small, massive bodies, the investigators reasoned, would occasionally put those dark bodies directly in front of a distant star. As a body moved into, then out of, the line of sight between the star and Earth, its gravity would temporarily act like a telescope lens. As seen from Earth, the background star would brighten, then fade away, over a period of days or weeks, in a pattern predicted by Einstein's general theory of relativity.
In principle, gravitational lensing is simple: a massive object bends space much the way a heavy bowling ball would bend a trampoline if it were rolled around on the springy surface. Light beams passing through bent space bend too, almost as if they were focused by a magnifying glass. The devil, of course, is in the details. Solving the equations of general relativity can lead to a morass of difficult mathematics--and the natural "magnifying glass" created by an intervening dark object may act more like the thick glass at the base of a bottle of soda pop than like a carefully polished lens. To understand exactly what happens in a microlensing event, theoretical models are compared with the observed data to see which initial mathematical conditions lead to the best match with the magnification pattern. Rarely, alas, is enough information available--either theoretically or observationally--to afford an unambiguous interpretation.
Fortunately, the observational data on LMC-5, a microlensing event recorded in the direction of the Large Magellanic Cloud, were exquisitely detailed. In January 1993, an unremarkable bluish star in the cloud, nearly 200,000 light-years from Earth, began to grow noticeably brighter, and reached its peak on February 5, at nearly fifty times its usual brightness. Then it slowly faded, returning to its normal, unremarkable brightness by mid-March. Unmistakably, the event was caused by microlensing.
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