Chapter 15: Galaxies

This dazzling infrared image from NASA's Spitzer Space Telescope shows hundreds of thousands of stars crowded into the swirling core of our spiral Milky Way galaxy. In visible-light pictures, this region cannot be seen at all because dust lying between Earth and the galactic center blocks our view.

The Milky Way is mostly composed of a surprising form of matter that only reveals itself by its gravitational attraction. Dark matter is spread out over the entire galactic halo. The halo extends far beyond the disk of stars that contains the Sun. A study of the universe shows that there is nothing special or unique about our situation. Our Sun is an average star. Earth is part of a planetary system, and other stars have planetary systems too. Our galaxy is one of many spiral galaxies. It would violate the Copernican idea that we have no privileged position in the universe if dark matter only existed in the Milky Way Galaxy, and astronomers have indeed found, dark matter in other galaxies.

Schematic rotation curve of a galaxy.

The mass distribution of spiral galaxies, like our Milky Way, can be mapped out using a rotation curve; a map of the velocity of different parts of a galaxy with respect to the nucleus. The velocity is measured from the Doppler shift of emission lines produced by gas or absorption lines produced by stars. The striking feature of most spiral galaxy rotation curves is the fact that they are relatively flat; the velocities stay high out to the limits of the visible material in the spiral disk. Kepler's laws dictate that the circular velocity of stars in a galaxy should begin to decrease at the point where the orbit encloses most of the mass of the galaxy, but we don't observe any decline in circular velocity. In many spirals, the gas disk extends further than the stellar disk, and astronomers can use the 21-cm line of neutral hydrogen to map the rotation curve out to very large distances. Since the velocity stays high and roughly constant out to the largest radii studied, we conclude that even these outer orbits do not enclose most of the mass of the galaxy. The bulk of the mass must therefore be invisible material in a dark halo.

Seen from the southern skies, the Large and Small Magellanic Clouds (the LMC and SMC, respectively) are bright patches in the sky. 

The mass of a large galaxy can also be measured using the motions of any dwarf galaxy companions that are in orbit around it. These orbits cannot, of course, be followed in time because they typically take hundreds of millions of years. Astronomers have to make do with a snapshot of the instantaneous velocity of each companion. Since the companions are very small galaxies, their motions are dictated by the mass of the large primary galaxy — just as the motions of the planets in the solar system are dictated by the Sun. This technique has given us striking confirmation of the mass and extent of the dark halo of the Milky Way galaxy. Dwarf ellipticals in orbit around the Milky Way give a total mass for the Milky Way of about 10¹² solar masses, distributed up to 100 kpc from the center of the galaxy. This estimate doubles the size and extent of the Milky Way that was measured by the motions of individual halo stars. Since the starlight in the galaxy only corresponds to about 10¹¹ solar masses, 90% of the total mass of our galaxy is dark matter!

Galaxy cluster Abell 2218 gravitationally lensing distant galaxies

Today, due to gravitational lensing of background galaxies by intervening dark matter, we know that dark matter is some form of stuff that doesn't interact with light except via gravity and has a very small cross-section that prevents it from undergoing frequent collisions. Various possibilities have been ruled out: black holes, dim stars, brown dwarfs, free-floating planets, rocks in space, and microscopic dust grains. The only remaining viable candidate is a tiny subatomic particle, as yet unobserved in physics labs or accelerators. Since dark matter "outweighs" normal matter, this new fundamental particle must be as massive as a proton or neutron and there must be as many of these particles as there are protons and neutrons. We are still working to understand the details of the dark matter distribution, but it appears more smoothly distributed than stars are in galaxies, and there is a nearly smooth component of dark matter in the vast spaces between galaxies.