How much dark matter is there inside early-type galaxies?

We study the luminous mass as a function of the dynamical mass inside the effective radius (r_e) of early-type galaxies (ETGs) to search for differences between these masses. We assume Newtonian dynamics and that any difference between these masses is due to the presence of dark matter. We use several samples of ETGs - ranging from 19 000 to 98 000 objects - from the ninth data release of the Sloan Digital Sky Survey. We perform Monte Carlo (MC) simulations (see Appendix A) of galaxy samples and compare them with real samples. The main results are (i) MC simulations show that the distribution of the dynamical versus luminous mass depends on the mass range where the ETGs are distributed (geometric effect). This dependence is caused by selection effects and intrinsic properties of the ETGs. (ii) The amount of dark matter inside r_e is approximately 7 ± 22 per cent. (iii) This amount of dark matter is lower than the minimum estimate (10 per cent) found in the literature and four times lower than the average (30 per cent) of literature estimates. However, if we consider the associated error, our estimate is of the order of the literature average.