THE PROBLEM OF THE MASS-TO-LIGHT RATIO IN EARLY-TYPE GALAXIES

In this contribution I discuss the problem of the mass-to-light ratio in early-type galaxies. The observational evidence based on different and independent techniques suggest that interior to similar to 3R(e) (where R-e is the effective radius) from the center of these galaxies dark matter does not play important dynamical role and the mass is dominated by the visible, stellar component. The long-slit spectra observations enable us to deduce the full line-of-sight velocity distribution of a given galaxy, but are limited to similar to 3 - 4R(e). Fortunately, the kinematics can also be inferred beyond this limit using planetary nebulae (PNe) and globular clusters (GCs), but the anisotropies of orbits in this approach are not well constrained. I will show that some hints can nevertheless be obtained even using small samples available at this stage. Finally, I will use the X-rays methodology to infer the mass-to-light ratios in different early-type galaxies and to compare the results obtained with the aforementioned techniques while emphasizing the problems inherent in this methodology (e.g. possible lack of hydrostatic equilibrium).