Dark Matter Detection

­The problem of dark matter came about when astronomers began to study galaxies, like our own Milky Way. If we look at the structure of the galaxy as it would ­appear from the outside, most of the Milky Way's 100 billion-plus stars lie in the galactic disk. Most of the stars are concentrated near the center of the dis­k around the nucleus and galactic bulge. Above and below the plane of the disk are a few hundred scattered globular clusters and a large, dim, round region called the halo.

Milky Way
NASA/Photo courtesy Ned Wright
Our home galaxy, the Milky Way

­In studying the Milky Way, astronomers wanted to measure the masses and distributions of masses within the galaxy and star clusters. But you can't just weigh something the size of a galaxy -- you have to find its mass by other methods. One method is to measure the light intensity, or luminosity. Luminosity is related to a star's mass (the more luminous, the more mass -- see How Stars Work). From luminosity measurements, we know that there are about 15 billion solar luminosities (equivalent of sun-masses) between the sun's orbit and the center of the Milky Way.

Another approach to measuring galactic mass is by the rotation of the galactic disk. Imagine that the galaxy is spinning, like a CD or merry-go-round, and that you are looking at it edge-on. Within the galaxy, stars lie at different distances from the center. Some of these stars are moving away from us, while others are moving toward us. We can measure the speed and direction at which stars are moving by measuring the light that comes from them using the Doppler Effect. We can then graph the velocity of stars at different distances from the galaxy's center to get a galactic rotation curve.

Doppler Effect
Much like the high-pitched sound from a fire-truck siren gets lower as the truck moves away, the movement of stars affects the wavelengths of light that we receive from them. This phenomenon is called the Doppler Effect. We can measure the Doppler Effect by measuring lines in a star's spectrum (see How Light Works and How Stars Work) and comparing them to the spectrum of a standard lamp. The amount of the Doppler shift tells us how fast the star is moving relative to us. In addition, the direction of the Doppler shift can tell us the direction of the star's movement. If the spectrum of a star is shifted to the blue end, the star is moving toward us; if the spectrum is shifted to the red end, the star is moving away from us.

The rotation curve tells us about the distribution of mass within the galaxy. If the galaxy is like our solar system, where mass is concentrated in the center, the force of gravity will be greater near the center (the force of gravity decreases with distance). Therefore, objects close to the center orbit faster than those farther away, much like a spinning ice skater who rotates fastest when her arms are tucked in, or closer to her center. So, we would expect stars close to the galactic center to have higher rotational velocities than those farther out and that the galactic rotation curve would decrease exponentially as a function of distance.

But as we'll see on the next page, astronomers discovered that things were not exactly as they expected.