Structural Parameters for 10 Halo Globular Clusters in M33

TL;DR

This study characterizes the structural and dynamical properties of 10 halo globular clusters in M33 using Hubble Space Telescope data, revealing their similarity to clusters in M31 and the Milky Way, and identifying a universal fundamental plane.

Contribution

First detailed analysis of structural and dynamical parameters of M33 halo globular clusters using HST imaging and model fitting, highlighting their similarities to other galaxy clusters.

Findings

Most clusters are well fitted by King and Wilson models.

Cluster properties show a universal fundamental plane.

Predicted velocity dispersions agree with observations.

Abstract

In this paper, we present the properties of 10 halo globular clusters with luminosities $L\simeq 5-7\times 10^5{L_\odot}$ in the Local Group galaxy M33 using the images of {\it Hubble Space Telescope} Wide Field Planetary Camera 2 in the F555W and F814W bands. We obtained ellipticities, position angles and surface brightness profiles for them. In general, the ellipticities of M33 sample clusters are similar to those of M31 clusters. The structural and dynamical parameters are derived by fitting the profiles to three different models combined with mass-to-light ratios ($M/L$ values) from population-synthesis models. The structural parameters include core radii, concentration, half-light radii {\bf and} central surface brightness. The dynamical parameters include the integrated cluster mass, integrated binding energy, central surface mass density {\bf and} predicted line-of-sight velocity dispersion at the cluster center. The velocity dispersions of four clusters predicted here agree well with the observed dispersions by Larsen et al. The results here showed that the majority of the sample halo globular clusters are well fitted by King model as well as by Wilson model, and better than by S\'ersic model. In general, the properties of clusters in M33, M31 and the Milky Way fall in the same regions of parameter spaces. The tight correlations of cluster properties indicate a "fundamental plane" for clusters, which reflects some universal physical conditions and processes operating at the epoch of cluster formation.