An extended star cluster at the outer edge of the spiral galaxy M33

TL;DR

This paper reports the discovery of an extended, old, and metal-poor star cluster at the outskirts of galaxy M33, suggesting complex galaxy merging history.

Contribution

It presents the first identification of an extended globular-like cluster in M33, expanding understanding of galaxy assembly and cluster properties.

Findings

Cluster is located 12.5 kpc from M33 center

Cluster's luminosity and size are comparable to similar clusters in the Local Group

Indicates a complex merging history for M33

Abstract

We report a discovery of an extended globular-like star cluster, M33-EC1, at the outer edge of the spiral galaxy M33. The distance to the cluster is 890 kpc, and it lies at a 12.5 kpc projected distance from the center of M33. Old age (>~7 Gyr) and low metallicity ([M/H] <~ -1.4) are estimated on the basis of isochrone fits. Color-magnitude diagrams of stars, located in the cluster's area, photometric and structural parameters of the cluster are presented. Cluster's luminosity (M_V = -6.6) and half-light radius (r_h = 20.3 pc) are comparable to those of the extended globular clusters, discovered in more luminous Local Group galaxies, the Milky Way and M31. Extended globular clusters are suspected to be remnants of accreted dwarf galaxies, and the finding of such a cluster in the late-type dwarf spiral galaxy M33 would imply a complex merging history in the past.