Next Contents Previous


The mass-to-light ratio of an astronomical object is defined as Υ ≡ M / L. Stellar populations exhibit values Υ = 1 - 10 in solar units, in the solar neighborhood Υ = 2.5 - 7, in the Galactic disk Υ = 1.0 - 1.7 from C. Flynn & al. [38].

Dwarf spheroidal galaxies (dSph) are the smallest stellar systems containing dark matter and exhibit very high M/L ratios, Υ = 10 - 100. In Andromeda IX Υ = 93 +120/-50, in Draco Υ = 330 ± 125. The dwarf spheroidals have radii of ≈ 100 pc and central velocity dispersions ≈ 10 km s-1 which is larger than expected for self-gravitating, equilibrium stellar populations. The generally accepted picture has been, that dwarf galaxies have slowly rising rotation curves and are dominated by dark matter at all radii.

However, R. A. Swaters & al. [39] have reported observations of H I rotation curves for a sample of 73 dwarf galaxies, among which eight galaxies have sufficiently extended rotation curves to permit reliable determination of the core radius and the central density. They found that dark matter only becomes important at radii larger than three or four disk scale lengths. Their conclusion is, that the stellar disk can explain the mass distribution over the optical parts of the galaxy, and dark matter only becomes relevant at large radii. However, the required stellar mass-to-light ratios are high, up to 15 in the R-band.

Comparing the properties of dwarf galaxies in both the core and outskirts of the Perseus Cluster, Penny and Conselice [40] found a clear correlation between mass-to-light ratio and the luminosity of the dwarfs, such that the faintest dwarfs require the largest fractions of dark matter to remain bound. This is to be expected, as the fainter a galaxy is, the less luminous mass it will contain, therefore the higher its dark matter content must be to prevent its disruption. Dwarfs are more easily influenced by their environment than more massive galaxies

The distance to the Perseus Cluster prevents an easy determination of Υ, so S. J. Penny & C. J. Conselice [40] instead determined the dark matter content of the dwarfs by calculating the minimum mass needed in order to prevent tidal disruption by the cluster potential, using their sizes, the projected distance from the cluster center to each dwarf and the mass of the cluster interior. Three of 15 dwarfs turned out to have mass-to-light ratios smaller than 3, indicating that they do not require dark matter.

Ultra-compact dwarf galaxies (UCDs) are stellar systems with masses of around 107 - 108 Msun and half-mass radii of 10–100 pc. A remarkable properties of UCDs is that their dynamical mass-to-light ratios are on average about twice as large as those of globular clusters of comparable metallicity, and also tend to be larger than what one would expect based on simple stellar evolution models. UCDs appear to contain very little or no dark matter.

H. Baumgardt & S. Mieske [42] have presented collisional N-body simulations which study the coevolution of a system composed of stars and dark matter. They find that DM gets removed from the central regions of such systems due to dynamical friction and mass segregation of stars. The friction timescale is significantly shorter than a Hubble time for typical globular clusters, while most UCDs have friction times much longer than a Hubble time. Therefore, a significant dark matter fraction may remain within the half-mass radius of present-day UCDs, making dark matter a viable explanation for their elevated mass-to-light ratios.

A different type of systems are the ultra-faint dwarf galaxies (UFDs). When interpreted as steady state objects in virial equilibrium by V. Belokurov & al. [41], they would be the most DM dominated objects known in the Universe. Their half-light radii range from 70 pc to 320 pc.

A special case is the UFD disk galaxy Segue 1, studied by M. Xiang-Gruess & al. [43], which has a baryon mass of only about 1000 solar masses. One interpretation is that this is a thin non-rotating stellar disk not accompanied by a gas disk, embedded in an axisymmetric DM halo and with a ratio fMhalo / Mb ≈ 200. But if the disk rotates, f could be as high as 2000. If Segue 1 also has a magnetized gas disk, the dark matter halo has to confine the effective pressure in the stellar disk and the magnetic Lorentz force in the gas disk as well as possible rotation. Then f could be very large [43]. Another interpretation is that Segue 1 is an extended globular cluster rather than an UFD [41].

Next Contents Previous