5.2 Trends in Dark Matter Properties

Broeils (DMW) is involved in an ongoing project to obtain mass models for a large sample of low-luminosity disk galaxies in order to investigate relations between DM properties and galaxy properties. He finds that DDO 168 and DDO 105 have dark-to-luminous mass ratios that are less extreme than some dwarf irregulars and are more characteristic of normal spirals. This raises the interesting question of whether the gas-rich dwarfs like DDO 154 with their high DM fractions are typical, or whether most dwarf irregulars are, like DDO 168 and DDO 105, more similar to bright spirals.

A related issue is the central density of the halo around disk galaxies. Since an observed rotation curve can often be fit by a range of mass models, there is some uncertainty in the derived central density of the dark halo. (It is worth noting that this uncertainty is usually less than that associated with density determinations in dwarf spheroidals.) However, the studies of dwarf irregulars described above indicate that central DM densities are generally higher than those in normal spirals. Typically, the halos of dwarf irregulars have central densities in the range 0.01 to 0.1 M pc^-3, whereas the halos of bright spirals ordinarily have values between 0.001 and 0.01 M pc^-3.

As is apparent from the above discussion, there is a good deal of scatter in these trends. Thus while faint disk galaxies usually have higher DM fractions and central densities, other parameters may also be important. It is worth noting, however, that the increase in DM fraction with decreasing luminosity in disk galaxies seems to be confirmed by studies of normal spirals specifically aimed at detecting such relationships. This is discussed further in the next Section.