Galaxies

Dark Matter in the Milky Way

The halo is a spherical cloud of thinly scattered stars and globular clusters. It is the largest component of the Milky Way, extending to radius of about 100 kpc. It contains very little dust or gas. No star formation currently takes place there. This means that the halo contains very few young stars. Most of the halo stars are, in fact, 10 - 14 billion years, which is very close to the age of the galaxy itself. Halo stars are extreme Population II stars. They are very old, have very low metal content, and move in randomly tipped, elliptical orbits. The motion of objects in the Milky Way is not consistent with the amount of luminous matter, which is not enough to confine these objects inside the Milky Way boundary. The problem can be reconciled if a lot of dark matter still remains in the halo - the original clump of mass - while the cooling of the hydrogen allows ordinary matter to contract, and settled into the disk.

Figure 05-18 Dark Matter Halo

Observations show that the dark matter in a galaxy surrounding the visible matter in a halo is larger and more nearly spherical than the stars and gas. The visible matter density is higher than the dark matter density near the centers of most galaxies, so the dark matter is not very important there. But it extends well beyond the stars and gas, so the outer parts of galaxies are essentially all dark matter. Figure 05-18 shows an "artist's impression" of a dark halo surrounding an almost edge-on disk galaxy. Figure 05-16f shows the dark matter distribution extends to 100 kpc. The rotation curve in Figure 05-19 provides a very convincing evidence for the pervasive presence of dark matter in the galactic halo (such as shown in M74). It is a plot of the rotational velocity of an object in the galactic plane versus distance to the center. From observations of starlight alone, the rotational velocity would be expected to fall towards the edge (dashed line). In fact the curve flattens (solid line), suggesting that galaxy is surrounded by a halo of unseen, dark matter.

Figure 05-19 Rotation Curve [view large image]

See more about the gamma-ray sky of the Milky Way in the Appendix.

Computer model with dark matter halo surrounding the Milky Way shows that there should be thousands of small galaxies distributed evenly in every direction. Yet there are only a few dozen observed. This case of missing dwarf galaxies is usually explained by their low visibility. There may be thousands of dark satellites, devoid of stars, exist in our vicinity without being detected. Another curious feature is the alignment of the dozens of visible small galaxies. It is found that they are located near a plane roughly perpendicular to the spiral arms (see insert in Figure 20). The same kind of arrangement also occurs in the Andromeda galaxy. A computer simulation on very large scale reveals that these dwarf galaxies were formed within the dark matter filaments, while the large galaxies tend to locate at the node where many such filaments intersected. The peculiar configuration is the result of the movement of these dwarf galaxies along the filament by the gravitational pull of the large galaxy (see Figure 05-20). It seems that both the dark matter halo and dark matter filaments should be considered together in the computer simulation.

Figure 05-20 Dwarf Galaxies [view large image]

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