Dark matter in the Milky Way

3.3 Dark matter in the Milky Way

Experimental searches for dark matter invariably are trying to detect cold dark matter within our own galaxy. Thus, it is useful to review at this stage the current thoughts about the distribution of cold dark matter within the Milky Way and, for terrestrial based experiments, the likely cold dark matter presence near Earth. Figure 5

shows both the observational data on the rotation curve [50

] and a recent determination of various mass components [72

Figure 5: The rotation curve of the Milky Way. In the left-hand panel are the measured rotation speeds given by the average values from a number of measurements on different objects [50 ]. The right hand panel shows the various mass components that combine together to reproduce the observed curve between 5 and 25 kpc [72 ]. The dotted lines are the bulge and disk contributions, and the short-dashed curve is the dark matter contribution. The solid curve shows the combined effect of all three, and this is compared to the long-dashed curve which approximates the measured data in the left-hand panel below 25 kpc.

At the position of the Sun, 7.5 – 8 kpc, it can be seen that the contributions to the enclosed mass from the bulge, the disk, and the dark matter halo are comparable. In these types of studies, the dark matter halo is assumed to be in a quasi-spherically symmetric distribution in virialised equilibrium. The halo is usually taken to be non-rotating and the local density comes out as $∼ 0.3 GeV ∕cm3$ . The velocity distribution of the DM particles is assumed to be Maxwellian with an upper cut-off at the Galactic escape velocity. Most calculations of event rates and energy deposits in detectors are done assuming this straight forward type of DM halo [78 ]. Possible modifications to this simple DM geometry include:

A modified radial CDM density profile giving a much lower CDM local density. This has been implied recently by analysis of microlensing events towards the Galactic bulge. A much larger number of events have been seen than expected and this suggests an unseen stellar population within the solar radius that can apparently account for the local rotation speeds without the need for dark matter [56 ].
Gravitational clustering of CDM particles at the centres of massive objects [124 ], such as the Sun, the Earth, or the galactic centre.
CDM halos with non-zero angular momentum.
Clumpy CDM galaxy halos [57 , 89 , 91 ].
Non-equilibrium situations with on-going CDM infall into the Galaxy [116 , 115 ].
Non-equilibrium situations with on-going CDM infall into the local cluster/supercluster.
CDM scattered into stable orbits around the Sun [40 ].