4.1 The nature of galactic dark matter

Galaxies are born out of primordial fluctuations with an evolution probably driven by gravitation as the dominant effect. Gravitation, as a geometric concept, has the same effect on the different types of particles. Some forces other than gravitation, such as the interaction with photons, dissipative effects, magnetic fields, etc., could also have an influence and act on the involved particles differentially, but an overall trend for galaxies and clusters to have a similar composition to the general composition of the Universe is to be expected.

Our knowledge about the composition of the Universe has changed in recent times with respect to the classical view, summarized, for instance, by Schramm (1992). This new conception has been reviewed, for instance, by Turner (1999a, b). The dominant matter is considered to be cold dark matter (CDM), consisting of particles moving slowly, so that the CDM energy density is mainly due to the particle's rest mass, there being a large series of candidates for CDM particles, but axions and neutralinos being the most attractive possibilities.

Big Bang nucleosynthesis studies have been able to accurately determine the baryon density as (0.019±0.0012)h^-2. The cluster baryon density has also been accurately determined by X-ray and the Sunyaev-Zeldovich effect to be f_B = (0.07±0.007)h^-3/2 and, assuming that rich clusters provide a fair sample of matter in the Universe, also $\Omega_{B}^{}$/$\Omega_{M}^{}$ = f_B, from which, it follows $\Omega_{M}^{}$ = (0.27±0.05)h^-1/2. The Universe is however flat, $\Omega$ = 1, with the CMB spectrum being a sensitive indicator. Therefore $\Omega$ = 1 = $\Omega_{M}^{}$ + $\Omega_{\Lambda}^{}$, where $\Omega_{\Lambda}^{}$ ~ 0.7 represents the contribution of the vacuum energy, or rather, the contribution of the cosmological term $\Lambda$. With this high value of $\Omega_{\Lambda}^{}$ the Universe should be in accelerating expansion, which has been confirmed by the study of high-redshift supernovae, which also suggest $\Omega_{\Lambda}^{}$ ~ 0.7 (Perlmutter, Turner and White, 1999; Perlmutter et al. 1999). The stellar or visible matter is estimated to be $\Omega_{V}^{}$ = 0.003 - 0.006. All these values can be written in a list easier to remember, with values compatible with the above figures, adopting the values of H₀ = 65kms^-1Mpc^-1; h=0.65:

$$\Omega_{V}^{}$$ = 0.003

$$\Omega_{B}^{}$$ = 0.03

$$\Omega_{M}^{}$$ = 0.3

$$\Omega$$ = 1

less precise but useful for exploratory fast calculations.

A large cluster should have more or less this composition, including the halo of course, even if a halo could contain several baryonic concentrations or simply none. Therefore, a first direct approach to the problem suggests that halos are non baryonic, with baryonic matter being a minor constituent.

This is also the point of view assumed by most current theoretical models (this will be considered later, in Section 4.2.2), which follow the seminal papers by Press and Schechter (1974) and White and Rees (1978). We advance the comment that, in these models, a dominant collisionless non dissipative cold dark matter is the main ingredient of halos while baryons, probably simply gas, constitute the dissipative component, able to cool, concentrate, fragment and star-producing. Some gas can be retained mixed in the halo, and therefore halos would be constituted of non-baryonic matter plus small quantities of gas, its fraction decreasing with time, while mergers and accretion would provide increasing quantities to the visible disks and bulges. Therefore, a first approach suggests that galactic dark matter is mainly non-baryonic, which would be considered as the standard description. Baryons, and therefore visible matter, may not have condensed completely within a large DM halo, and therefore the baryon/DM ratio should be similar in the largest halos and in the whole Universe, although this ratio could be different in normal galaxies.

However, other interesting possibilities have also been proposed. The galactic visible/dark matter fraction depends very much on the type of galaxy, but a typical value could be 0.1. This is also approximately the visible/baryon matter fraction in the Universe, which has led some authors to think that the galactic dark matter is baryonic (e.g. Freeman, 1997) in which case the best candidates would be gas clouds, stellar remnants or substellar objects. The stellar remnants present some problems: white dwarfs require unjustified initial mass functions; neutron stars and black holes would have produced much more metal enrichment. We cannot account for the many different possibilities explored. Substellar objects, like brown dwarfs, are an interesting identification of MACHOs, the compact objects producing microlensing of foreground stars. Alcock et al. (1993), Aubourg et al. (1993) and others have suggested that MACHOSs could provide a substantial amount of the halo dark matter, as much as 50-60% for masses of about 0.25 M_$\odot$, but the results very much depend on the model assumed for the visible and dark matter components, and are still uncertain. Honma and Kan-ya (1998) argued that if the Milky Way does not have a flat rotation curve out to 50 kpc, brown dwarfs could account for the whole halo, and in this case the Milky Way mass is only 1.1 × 10¹¹M_$\odot$.

Let us then briefly comment on the possibility of dark gas clouds, as defended by Pfenniger and Combes (1994), Pfenniger, Combes and Martinet (1994) and Pfenniger (1997). They have proposed that spiral galaxies evolve from Sd to Sa, i.e. the bulge and the disk both increase and at the same time the M/L ratio decreases. Sd are gas-richer than Sa. It is then tempting to conclude that dark matter gradually transforms into visible matter, i.e. into stars. Then, the dark matter should be identified with gas. Why, then, cannot we see that gas? Such a scenario could be the case if molecular clouds possessed a fractal structure from 0.01 to 100 pc. Clouds would be fragmented into smaller, denser and colder sub-clumps, with the fractal dimension being 1.6-2. Available millimeter radiotelescopes are unable to detect such very small clouds. This hypothesis would also explain Bosma's relation between dark matter and gas (Section 2.3), because dark matter would, in fact, be gas (the observable HI disk could be the observable atmosphere of the dense molecular clouds). In this case, the dark matter should have a disk distribution.

The identification of disk gas as galactic dark matter was first proposed by Valentijn (1991) and was later analyzed by González-Serrano and Valentijn (1991), Lequeux, Allen and Guilloteau (1993), Pfenniger, Combes and Martinet (1994), Gerhard and Silk (1996) and others. H₂ could be associated to dust, producing a colour dependence of the radial scale length compatible with large amounts of H₂. Recently, Valentijn and van der Werf (1999) detected rotational lines of H₂ at 28.2 and 17.0 $\mu$m in NGC 891 on board ISO, which are compatible with the required dark matter. If confirmed, this experiment would be crucial, demonstrating that a disk baryonic visible component is responsible for the anomalous rotation curve and the fragility of apparently solid theories. Confirmation in other galaxies could be difficult as H₂ in NGC 891 seems to be exceptionally warm (80-90 K).

A disk distribution is, indeed, the most audacious statement of this scenario. Olling (1996) has deduced that the galaxy NGC 4244 has a flaring that requires a flattened halo. However, this analysis needs many theoretical assumptions; for example, the condition of vertical hydrostatic equilibrium requires further justification, particularly considering that NGC 4244 is a Scd galaxy, with vertical outflows being more important in late type galaxies. Warps have also been used to deduce the shape of the halo. Again, Hofner and Sparke (1994) found that only one galaxy NGC 2903, out of the five studied, had a flattened halo. In this paper, a particular model of warps is assumed (Sparke and Casertano, 1988), but there are other alternatives (Binney 1991, 1992). The Sparke and Casertano model seems to fail once the response of the halo to the precession of the disk is taken into account (Nelson and Tremaine, 1995; Dubinski and Kuijken, 1995). Kuijken (1997) concludes that "perhaps the answer lies in the magnetic generation of warps" (Battaner, Florido and Sanchez-Saavedra 1990). On the other hand, if warps are a deformation of that part of the disk that is already gravitationally dominated by the halo, the deformation of the disk would be a consequence of departures from symmetry in the halo. To isolate disk perturbations embedded in a perfect unperturbed halo is unrealistic. Many other proposals have been made to study the shape of the halo, most of which are reviewed in the cited papers by Olling, and in Ashman (1982), but very different shapes have been reported (see section 3.4).

There is also the possibility that a visible halo component could have been observed (Sackett et al. 1994; Rausher et al. 1997) but due to the difficulty of working at these faint levels, this finding has yet to be confirmed.

Many other authors propose that the halo is baryonic, even if new models of galactic formation and evolution should be developed (de Paolis et al. 1997). This is in part based on the fact that all dark matter "observed" in galaxies and clusters could be accounted for by baryonic matter alone. Under the interpretation of de Paolis et al. (1995) small dense clouds of H₂ could also be identified with dark matter, and even be responsible for microlensing, but instead of being distributed in the disk, they would lie in a spherical halo.