A dynamical study of Galactic globular clusters under different relaxation conditions

TL;DR

This study systematically compares photometric and kinematic data of globular clusters under different relaxation states using King and f^( u) models, revealing model-dependent structural properties and emphasizing the need for more comprehensive data.

Contribution

It introduces the first application of f^( u) models to globular clusters and assesses their effectiveness relative to King models across different relaxation conditions.

Findings

King models fit photometric data well but less so for kinematic profiles.

f^( u) models better describe less relaxed clusters' profiles.

Structural parameters vary significantly depending on the model used.

Abstract

We perform a systematic combined photometric and kinematic analysis of a sample of globular clusters under different relaxation conditions, based on their core relaxation time (as listed in available catalogs), by means of two well-known families of spherical stellar dynamical models. Systems characterized by shorter relaxation time scales are expected to be better described by isotropic King models, while less relaxed systems might be interpreted by means of non-truncated, radially-biased anisotropic f^(\nu) models, originally designed to represent stellar systems produced by a violent relaxation formation process and applied here for the first time to the study of globular clusters. The comparison between dynamical models and observations is performed by fitting simultaneously surface brightness and velocity dispersion profiles. For each globular cluster, the best-fit model in each family is identified, along with a full error analysis on the relevant parameters. Detailed structural properties and mass-to-light ratios are also explicitly derived. We find that King models usually offer a good representation of the observed photometric profiles, but often lead to less satisfactory fits to the kinematic profiles, independently of the relaxation condition of the systems. For some less relaxed clusters, f^(\nu) models provide a good description of both observed profiles. Some derived structural characteristics, such as the total mass or the half-mass radius, turn out to be significantly model-dependent. The analysis confirms that, to answer some important dynamical questions that bear on the formation and evolution of globular clusters, it would be highly desirable to acquire larger numbers of accurate kinematic data-points, well distributed over the cluster field.