Volume 5, Issue 35 February 2008
The Hunt for Dark Matter
Leo Blitz shows an Allen Telescope Array dish feed to Paul Allen, the telescope's namesake, at the facility's October dedication. The feed enables the telescope to convert faint cosmic radio signals into measurable voltages. Photo credit: Colby Kraybill
What you see is not what you get everywhere in the universe. Leo Blitz, a Berkeley professor of astronomy, encountered this disturbing discrepancy early in his scientific career. As a postdoctoral fellow at Berkeley in the 1970s, he found that the outer parts of the Milky Way spin far faster than a galaxy of its size should while still holding together.
Similar discrepancies have cropped up in the motion of other galaxies, galactic clusters, and even light traveling past distant objects. Further studies implied the existence of a substance that neither absorbs nor emits light yet is abundant enough to affect the movements of the universe. Known as dark matter, this stealthy substance makes up nearly a quarter of the cosmos. "It completely dominates the gravity on large scales between galaxies because there's so much of it," Blitz says.
An expert in dark matter and the evolution of galaxies, Blitz also serves as director of the university's Radio Astronomy Laboratory. In this role, he manages the Combined Array for Research in Millimeter-wave Astronomy (CARMA), located high in California's Inyo Mountains, and the brand-new Allen Telescope Array (ATA) near Mount Lassen.
For Blitz and other astronomers seeking insights into dark matter, computer simulations offered some useful hints. These models predict that the universe should contain far more dwarf galaxies than the tiny fraction that astronomers can identify. Could the rest exist as galaxies made up solely of dark matter?
The Allen Telescope Array, located near Mount Lassen, conducts both conventional astronomy studies and scans the skies for intelligent alien life. Photo credit: Seth Shostak
If so, Blitz thinks he knows how to find them. "Imagine them plopping through the gas of the outer Milky Way," he says. "They might create some sort of splash or ripple."
These distant reaches are relatively calm, making such disturbances possible to detect. Blitz explains, "It's like throwing darts at a board. As these dark galaxies come at the Milky Way, they're likely going to hit the outer parts because there's more surface area there."
To pinpoint any dark galaxy splashes, Blitz and his research group are mapping the structure of the Milky Way. In the process, they have been able to characterize the warping of our generally flat galaxy is. "It's like hitting cymbals; it's held in the middle and the outer parts are free to vibrate," he says.
Within this structure, Blitz has identified areas of very localized vibrations-an encouraging sign-and is now searching other galaxies for similar characteristics. "That's exactly the kind of signature we look for if the Milky Way were being hit by these dark matter galaxies," he says.
Atomic hydrogen in the nearby galaxy M31 as imaged by the ATA. This is the first radio image of nearly an entire galaxy taken in a single pointing, thanks to the telescope's broad field of view. The hole near the center suggests that the galaxy is running out of gas to fuel future star formation. Image credit: Courtesy Leo Blitz
As promising as the mapping looks, Blitz is hedging his bets with a second approach: seeking gassy cores that could be embedded even in dark galaxies. "We're trying to survey regions of the sky to see if there are concentrations of atomic hydrogen that are not associated with known galaxies," he says. "I'm hoping that by making a large enough survey of the sky, we'll be able to find galaxies that contain only hydrogen and no stars. By looking at the motions of the hydrogen, we'll be able to determine the properties of the dark matter that's within it as well."
The resulting map of interstellar hydrogen could help answer another paradox in astronomy: why today's galaxies haven't yet run out of gas. According to observations, most galaxies have just enough fuel left to make stars for another billion years or so. Yet galaxies have endured for most of the age of the universe, making it unlikely that so many should blink out at once.
Blitz thinks they could be topping up their tanks with interstellar gases. As galaxies interact gravitationally, gases from their edges will get torn loose. These gases may eventually fall onto other galaxies, just as water vapor gets recycled back into rain. "There should be enough material between galaxies to be able to make up for the stars that are currently being formed," he says. "That's measurable with the Allen Telescope."
The ATA's exceptionally wide field of view and superior sensitivity can scan local galaxies efficiently for interstellar hydrogen at its current strength of 42 dishes. Once the telescope attains its full complement of 350 dishes, it will be powerful enough to more fully illuminate two very dark areas of astronomy.