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

TL;DR

This study maps the dark matter density in the outer halo of galaxy NGC 4636 using globular cluster velocities, revealing a high concentration of dark matter with increasing mass-to-light ratios at larger radii.

Contribution

It provides a non-parametric Bayesian analysis of dark matter distribution in NGC 4636, extending previous work with higher estimated mass-to-light ratios.

Findings

Dark matter is highly concentrated towards the inner halo.

Mass-to-light ratios increase to about 30 at 4 effective radii.

Results are consistent across red and blue globular clusters.

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.