The Net Advance of Physics: The Nature of Dark Matter, by Kim Griest -- Section 2A.

Spiral Galaxies

The most robust evidence for dark matter comes from the rotation

curves of spiral galaxies. Using 21 cm emission, the velocities of

clouds of neutral hydrogen can be measured as a function of r, the

distance from the center of the galaxy. In almost all cases, after a

rise near r=0, the velocities remain constant out as far as can be

measured. By Newton's law for circular motion ,

this implies that the density drops like at large radius and that

the mass at large radii. Once r becomes greater than the

extent of the mass, one expects the velocities to drop , but

this is not seen, implying that we do not know how large the

extended dark halos around spirals are. For example, the rotation

curve of NGC3198 [2] implies , or . The large

discrepancy between this number and is seen in many external

galaxies and is the strongest evidence for dark matter.

It is fortunate that the most secure evidence for dark matter is in

spiral galaxies, since searches for dark matter can be made only in

spiral galaxies: in fact only in our spiral, the Milky Way.

Unfortunately, the rotation curve of the Milky Way is not well

constrained, with recent measurements extending only to 15 to 20

kpc, and having differing amplitudes and shapes [3,4]. This leads to

substantial uncertainty in the amount of dark matter in our Galaxy.

There are other indicators of the mass of the Milky Way. By

studying the motion of dwarf galaxies (especially Leo I at a distance

of 230 kpc) Zaritsky et al. [5] find a mass of the Milky Way of

, for , and

(assuming the Universe is like the Milky Way). A very recent study

by Kochanek [6] does a maximum likelihood analysis including

constraints from satellite velocities, the distribution of high velocity

stars (local escape velocity), the rotation curve, and the tidal effects

of M31, to find a mass of the Milky Way inside 50 kpc of

. It is interesting that this value is just what one

expects from a flat rotation curve with v=220 km/sec out to 50 kpc,

so the Milky Way is very likely a typical spiral with a large dark

halo.

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