The Distribution of Dark Matter in the Halo of the early-type galaxy NGC 4636

Abstract

We present the density structure of dark matter in the outer parts (to about 7 effective radii) of the elliptical galaxy NGC 4636, from the radial velocities of 174 globular clusters, using the non-parametric, Bayesian algorithm CHASSIS. We find the galaxy to be rich in dark matter, with $R$-band mass-to-light ($M/L$) ratios rising to about 30, at nearly 4$R_e$; the $K$-band $M/L$ at about 3$R_e$ is found to be nearly 10. The result does not depend on applying the method to the red and blue globular clusters separately. This estimate of $M/L$ is higher than the previous analysis from the same kinematic data. We also find that the dark matter distribution is highly concentrated towards the inner halo.