Evidence for significant radial increase of the mass-to-light ratio based on phenomenological analysis of eight early-type galaxies

In this paper we study the sample of eight nearby early-type galaxies for which we have reliable estimates of their total dynamical mass in their interior and exterior parts based on the observed globular clusters. We use a phenomenological approach in the study of the gradient of the mass-to-light ratio of the galaxies in the sample. Since the outermost point for which we have the estimates of the mass-to-light ratios is fixed at 5 effective radii, this provides the opportunity to study the dark matter content of early-type galaxies which is expected to dominate in their outer parts, i.e., beyond similar to 2-3 effective radii. We find that all the galaxies in our sample show the increase of the cumulative mass-to-light ratio which indicates various amount of additional, dark, component in their mass content. We show that galaxies with higher values of alpha + beta (where alpha and beta are slope parameters) have higher virial masses. We show that two galaxies which are slow rotators (NGC 1407 and NGC 5846) have alpha+ beta > 1 whereas the remaining 6 galaxies are all fast rotators, and for these objects we found that alpha + beta <= 1. We also compare our findings with the theoretical expectations coming from numerical simulations.