|Annu. Rev. Astron. Astrophys. 1994. 32:
Copyright © 1994 by . All rights reserved
2.2. Spiral Galaxies
The best evidence for dark matter in galaxies comes from the rotation curves of spirals, since the dependence of the rotation speed V upon galactocentric distance R is a measure of the density profile (R). An important feature of our own and many other spiral galaxies is that the rotation speed, after an initial rise, remains approximately constant with increasing R (Rubin et al 1980). This implies that the mass within radius R increases like R, which is faster than the increase of visible mass. [Valentjin (1990) has claimed that spiral galaxies have sufficient dust to be opaque, thereby increasing the stellar mass content (cf Disney et al 1989), but Burstein et al (1991) disagree with this and the possibility will be neglected here.] Although the dark matter does not dominate within the optical galaxy (at least for bright galaxies), neutral hydrogen observations suggest that V continues to remain constant well beyond the visible stars (Sancisi & van Albada 1987). In considering the baryonic contribution to galactic halos, the crucial issue is how far the halos extend. For our galaxy the minimum halo radius consistent with rotation curve measurements, the local escape speed, and the kinematics of globular clusters and satellite galaxies is 35 kpc; the dynamics of the Magellanic Stream and the Local Group of galaxies may require a halo radius of 70 kpc (Fich & Tremaine 1991). We will see later that these values are marginally consistent with a baryonic halo. However, Zaritsky et al (1993) argue from observations of satellite systems that spiral galaxies typically have 200 kpc halos and this would be inconsistent with their being composed of baryons.
One indication that halos are dominated by nonbaryonic material may come from the fact that V has the same value in the optical region (where the bulge and disk dominate) as it does well beyond (where the dark matter dominates). This "conspiracy" may require that the ratio of baryonic to nonbaryonic dark mass be comparable to the dimensionless rotation parameter expected for protogalaxies as a result of tidal spinup (Fall & Efstathiou 1981, Blumenthal et al 1986); both are of order 0.1. A recent calculation of this effect, allowing for the response of the dark halo to the dissipative infall of the luminous material, implies a baryonic to nonbaryonic ratio of 0.05 (Flores et al 1993). However, this would not apply if the baryons went dark before galaxy formation. Also, the conspiracy is only required for bright galaxies because only for these is the disk dynamically dominant in the central regions.
Another relevant issue concerns the roundness of galactic halos. If galactic halos are baryonic, one would expect their formation to involve dissipation, in which case they should be flatter than in the nonbaryonic case: N-body experiments show that dissipationless collapse does give some flattening but the resultant triaxial halos are rarely flatter than E6 (Frenk et al 1988, Dubinski & Carlberg 1991). Thus, evidence for halos flatter than this would be evidence for baryonic dark matter. Polar ring galaxies probably provide the best probe of halo shape, and these do seem to indicate triaxiality (Whitmore et al 1987), sometimes (e.g. for NGC 4650A) as high as E6 (Sackett & Sparke 1990). The existence of warped disks may also require triaxial surrounding halos (Teuben 1991), and such disks seem to be ubiquitous (Bosma 1991). Triaxiality in our own halo could also explain the asymmetries of the HI distribution (Blitz & Spergel 1991). Nevertheless, it is not clear whether there is enough triaxiality in these cases to imply baryonic halos.