On the dark matter formation scenario of NGC 4147

TL;DR

This study measures the velocity dispersion profile of NGC 4147, revealing a flat or rising profile at large radii, which challenges recent models and supports a dark matter halo origin for the cluster.

Contribution

It provides observational evidence against recent predictions of declining velocity dispersion in globular clusters without tidal tails, suggesting a dark matter component in NGC 4147.

Findings

Velocity dispersion profile is flat or slightly rising at large radii.

Outer stars are kinematically hot, contradicting recent models.

Supports formation of NGC 4147 in a dark matter-rich dwarf galaxy.

Abstract

We report results on the radial velocity dispersion profile built out to the outskirts of NGC 4147, a Milky Way globular cluster with detected strong tidal tails. The cluster was chosen to probe, from an observational point of view, recent simulations that suggest that rising velocity dispersion profiles at large distance from the clusters' centers would be seen in globular clusters without tidal tails. From GEMINI@GMOS spectra, centered in the infrared CaII triplet region, of selected stars located along the onset of NGC 4147's tidal tails, we measured their radial velocities and overall metallicities. The derived metallicities were used to ultimately assessing on the highly-ranked cluster candidates of 9 stars, located between ~ 7 and 33 pc from the cluster's center, suitable for testing the aforementioned simulations. We complemented the present radial velocities with others available in the literature for cluster's members, and built a cluster velocity dispersion profile which suggests a mostly flat or slightly rising profile at large distances from the cluster's center. This outcome confirms that kinematically hot outermost cluster's stars are seen in NGC 4147, which disproves the recent model predictions. Nevertheless, the mean velocity dispersion of the outermost cluster's stars agrees with NGC 4147 being formed in a 10^8-10^9Mo dwarf galaxy with a cored dark matter profile that later was accreted on to the Milky Way.