Milky Way gets a new layer

From Atlanta at a meeting of the American Astronomical Society

Talk about a death shroud. The corpses of 150 billion stars may blanket the visible disk of our home galaxy, the Milky Way.

Astronomers made this novel proposal to explain results they obtained when they monitored millions of stars in the nearby Large Magellanic Cloud galaxy. These observations sought the identity of some of the Milky Way’s unseen material, or dark matter, believed to account for at least 90 percent of its mass.

Dark matter betrays its presence through gravity. According to general relativity, any dense body—seen or unseen—acts as a gravitational lens, deflecting the path of light passing by. An image of a distant star will appear distorted and brightened by a dense object that lies between the star and Earth (SN: 1/8/00, p. 30: A Dark View of the Universe).

Long-term studies of the Large Magellanic Cloud turned up 15 or so stars that have undergone sudden brightening, indicating they had briefly passed behind a dense object in the vast halo of material that surrounds our galaxy. This suggests that the objects have masses about half that of the sun.

This mass makes white dwarfs, the collapsed, dying embers of sunlike stars, the most likely culprits, says Evalyn Gates of the University of Chicago and the Adler Planetarium in Chicago.

To explain the findings, she and Geza Gyuk of the University of California, San Diego calculate that the Milky Way may harbor a huge, hidden population of white dwarfs.

Determining where such a population might reside required additional reasoning. If they were mostly at the heart of the galaxy, their gravity would distort the orbits of stars there. If they were evenly distributed throughout the Milky Way’s halo of dark matter, the 15 brightenings would indicate a much larger population and would have produced a much greater abundance of heavy elements in the galaxy than astronomers have detected.

Gates and Gyuk instead propose that the dwarfs are confined to a flattened sphere, about 150,000 light-years across and 90,000 light-years high, that swaddles our galaxy’s visible disk. The disk itself has a diameter of about 100,000 light-years and a height of 2,000 light-years. Because it takes about 10 billion years for stars like the sun to exhaust their fuel and become white dwarfs, the objects would rank among the oldest stellar residents of the galaxy.

If most galaxies have such a population, it could offer a clue to galaxy origins and formation, Gates notes. All this may sound highly speculative, she admits, but another team of astronomers recently reported data that suggest the presence of such a dwarf population in our galaxy. Rodrigo A. Ibata of the European Southern Observatory in Munich, Harvey B. Richer of the University of British Columbia in Vancouver, and their colleagues compared the 1995 images of the Hubble Deep Field North, the region viewed in exquisite detail by the Hubble Space Telescope, with those that the telescope took 2 years later.

Several of the objects had moved, indicating that they were not distant galaxies but residents of the Milky Way. Their colors and brightness suggest they could be white dwarfs, the team reports in the Oct. 20, 1999 Astrophysical Journal Letters. Collaborator Ronald L. Gilliland of the Space Telescope Science Institute in Baltimore cautions that the team can’t be certain until it takes a third set of images.

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