It’s official: The 215th meeting of the American Astronomical Society in Washington, D.C., is the largest astronomical meeting in history. The number of attendees has reached 3,400, several hundred more than the previous record crowd.
All of the astronomers here consist entirely of normal matter — the protons, neutrons, and electrons that make up all that we see in the cosmos. But this morning’s press conference shied away from normal stuff and concentrated on the invisible dark matter that makes up a bigger fraction of the universe. (And they used to call Johnny Carson the king of segues.)
Although astronomers can’t see dark matter, they can measure its effects on objects we can see. This is how David Law of the University of California, Los Angeles and his colleagues probed the shape of the Milky Way’s dark matter halo. Almost all galaxies possess such halos, which typically contain more than 70 percent of a galaxy’s total mass.
Law’s team studied the Milky Way’s dark matter halo by its gravitational effect on the stars being pulled out of the Sagittarius dwarf elliptical galaxy by our galaxy’s tidal forces. The stars in this so-called Sagittarius stream now orbit our galaxy totally within the halo. The team’s analysis shows that the halo is a flattened spheroid — what Law calls a “squashed cosmic beachball” — tilted about 90 degrees to our galaxy’s disk. So, it extends farther from the galaxy’s poles than it does from its plane. Astronomers expected that any bulge would go in the opposite direction, leaving theorists with some work to do.
A few decades ago, astronomers often referred to dark matter as “missing mass” because the law of gravity asserted that more stuff existed than their observations revealed. Now, Stacy McGaugh of the University of Maryland and his colleagues have reversed the problem: they don’t see enough ordinary matter. The Wilkinson Microwave Anisotropy Probe has measured the cosmic abundances of dark and ordinary matter and found that the normal stuff accounts for 17 percent of the total.
McGaugh’s team measured individual structures in the universe and found that none has that high a percentage of normal matter. Rich galaxy clusters come closest, with a ratio of about 14 percent. Big galaxies like the Milky Way have significantly less, and the smallest dwarf galaxies have just 0.2 percent, barely 1 percent of what they should have. So, where is the missing normal matter? No one knows and, as McGaugh says, the ideas proposed range from the easily dismissed to the untestable.
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