Investigation of dark matter and modified Newtonian dynamics in early-type galaxies through globular cluster systems

Astronomical Observatory, Volgina 7, 11060 Belgrade, Serbia
e-mail: srdjan@aob.bg.ac.rs

Received: 12 March 2013
Accepted: 31 July 2014

Abstract

Context. The departures from Newtonian dynamics based on the mass-follows-light approach discovered in the outer parts of some early-type galaxies imply the existence of dark matter and/or necessary modifications to the Newtonian approach. We study dynamical models of a sample of ten early-type galaxies in both Newtonian and MOND approaches.

Aims. The measurements of the radial velocities of the globular clusters in ten massive early-type galaxies are used to test the predictions of dynamical models with and without dark matter assuming Newtonian and MOND approaches out to several effective radii.

Methods. The globular clusters taken from the SLUGGS database are used as tracers of the gravitational potential of the galaxies in a sample. We solve the Jeans equation for both the Newtonian (mass-follows-light and dark matter models) and the MOND approaches by assuming spherical symmetry and compare the resulting mass-to-light ratios with stellar population synthesis models. For both approaches, we apply various assumptions on velocity anisotropy.

Results. We find that the Newtonian mass-follows-light models without a significant amount of dark matter can provide successful fits for only one galaxy (NGC 2768), and for the remaining nine early-type galaxies, various amounts of dark matter are required in the outer parts beyond 2 − 3R_e. With MOND models, we find that four early-type galaxies could be fit without dark matter and that the remaining six galaxies require an additional dark component to successfully fit the line-of-sight observed velocity dispersions; the galaxy NGC 4486 (M 87) is the only galaxy for which dark matter is required in the inner regions, and MOND cannot fit the data without additional dark matter. In the inner region, the galaxy NGC 4365 requires higher mass-to-light ratios than the stellar values from population synthesis, but a reasonable mass-to-light ratio can be reached for MOND assuming slightly tangential orbits. The ten galaxies can be split into two classes: those with concentrations at (NGC 1407) or above the ΛCDM concentration-mass relation, given their measured virial masses, and those below this relation. The former generally require dark matter in both Newtonian and MOND approaches, while the latter do not require appreciable amounts of dark matter.