Most galaxies, including our own Milky Way, present only part of themselves to our eyes. They hide much of their mass in huge halos that envelop the more conspicuous spirals. Astronomers know the halo matter is there because it affects the motions of stars in galaxies. But they don’t know what it is. It’s hard to infer the properties of stuff you don’t see, says Rick Rudy, an astronomer with the Aerospace Corporation in Los Angeles.
Rudy believes he may have partly solved the mystery. In 1994 astronomers found a faint gauzy glow around ngc 5907, a galaxy about 36 million light-years away. The glow seemed consistent with the size and shape of the matter needed to make ngc 5907 spin the way it does, so astronomers hoped that this might be the first sign that the dark halos were made of ordinary stars and planets--albeit faint ones--rather than exotic, yet-to-be discovered particles.
Rudy and Chick Woodward of the University of Wyoming recently studied this glow to see what sort of stars caused it. Based on the infrared signature of the faint light, they found to their surprise that the glow seems to be created by second- and third-generation stars--stars created out of gas and dust that has been cooked in the hearts of very large, short-lived stars. The colors you expect from a star depend on its composition, Rudy says. Stars made of primordial material have peculiar colors. But what Rudy and Woodward saw around ngc 5907 resembled the light emitted by stars the size of our sun and smaller.
Rudy and Woodward estimate that most of the glow comes from a population of small, dim red stars with about a tenth of the mass of the sun. The mysterious mass of the halo of at least one galaxy thus comes from relatively dim bulbs that were simply too faint for earlier generations of instruments to detect.
Before astronomers can put the question of the halo mass to rest, however, they must still explain our Milky Way: the Hubble Space Telescope has not found large numbers of small red stars swarming in our halo. If both of these studies hold up, astronomers may have another mystery on their hands--why two seemingly ordinary galaxies have halos made of different kinds of stuff.