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2.6 Dwarf galaxy halos

The phase space evolution of dark matter can put a constraint on the nature of it, as follows (cf. Tremaine & Gunn 1979) : it can be shown that there is a minimum mass for neutrino's which they should have if they are to constitute the dark matter in dwarf galaxies. This has spurred the quest for a thorough study of dwarf galaxy dynamics. Not only dwarf spheroidals can give an answer, also gas rich dwarfs can help here. The ability of dark matter candidates to cluster or not on small scales has led to the important distinction of cold dark matter (CDM), which can cluster on small scales, and hot dark matter (HDM), which cannot, but might be important e.g. on the scales of clusters and superclusters. (cf. Blumenthal et al. 1984).

For dwarf spheroidals, kinematics of individual bright stars allows estimates of the mass-to-light ratios. A recent update (Mateo 1994) shows that for the Draco and Ursa Minor dwarf spheroidals, the mass-to-light ratio is about 100. Better surface photometry is now available from the work of Irwin & Hadzidimitriou (1995), and more velocities are being collected for individual stars in all dwarf spheroidals orbiting our Galaxy, with ever greater accuracy (cf. Olszewski 1998). The conclusion that some of these systems have a high central density of dark matter (as high as 1 Msmsun pc-3 for the Ursa Minor dwarf) seems fairly secure, although tidal effects from the Galaxy remain an important source of uncertainty.

For gas rich dwarfs, 21-cm HI line studies using the WSRT or the VLA, have yielded results for e.g. DDO 154 (Carignan & Freeman 1985), DDO 170 (Lake et al. 1990), NGC 3109 (Jobin & Carignan 1990) and DDO 105 (Broeils 1992). All these galaxies are dark matter dominated, and their dark halos, when modeled as isothermal spheres, or with Hernquist profiles, appear to be concentrated enough to exclude massive neutrinos as their constituents. Recent work by De Blok and his collaborators (e.g. De Blok & McGaugh 1997) on low surface brightness late type disk galaxies leads to a similar conclusion.

However, a new problem arises related to numerical simulations of cosmological models. Moore (1994) and also Navarro et al. (1996) find core radii for dwarf galaxies from their numerical simulations, which are even smaller than those observed in the above mentioned four systems, although Kravtsov et al. (1998) claim to have solved this problem. In any case, the rotation curve data for dwarf galaxies can be used to constrain cosmological models via such simulations, and it may well be possible that a standard CDM Omega = 1 model is ruled out by them (e.g. Navarro 1998).

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