2.3 Improved local disk model

Crézé et al. (1998) analyzed the new HIPPARCOS data and the associated Coravel radial velocity databases, and created a proper sample of stars for which they could reanalyze the galactic force law in the z-direction. Their best solution is ₀ = 0.076 ± 0.015 M pc^-3, which does not leave any room for disk dark matter.

This result has consequences for an idea of Pfenniger & Combes (1994), i.e. that the dark matter is in the form of cold gas, which is almost undetectable due to its fractal structure. This idea is partly designed to explain the evolutionary sequence from Sd (gas rich, dark matter important) to Sa galaxies (gas poor, and apparently less dark matter within the optical radius) (cf. Pfenniger et al. 1994). It also explains the coincidence noted by Bosma (1978) that the ratio of total mass to gas mass surface density becomes constant in the outer parts. Carignan et al. (1990) restated that result by noting the similarity in shape between the computed rotation curve for the HI component with that of the dark halo component.

For the solar neighbourhood, the rotation curve for the disk component has to rise to a least about 180 km s^-1 to make a roughly flat total rotation curve with bulge and disk alone. For an exponential disk which places the Sun roughly at the position of turnover of the rotation curve of the disk component (~ 2.2 times the scalelength), this corresponds to a surface density of about 100 M pc^-2. Gould et al. (1996) evaluate the ``visible'' components as follows : 13 M pc<sup>-2</sup> due to the (ordinary) gas, 14 M pc<sup>-2</sup> due to young stars, and 12 M pc<sup>-2</sup> due to dwarfs. Thus if all matter is in a thin disk, about 60 M pc<sup>-2</sup> is ``missing''. If this matter is distributed as cold gas in a disk, it is hard to see why such a gas rich disk does not go unstable and forms stars. Gerhard & Silk (1996) propose instead that such matter is distributed in a flattened halo.