Photometric survey of stellar clusters in the outer part of M33. II. Analysis of HST/ACS images

TL;DR

This study used HST/ACS images to confirm and discover stellar clusters in M33, providing detailed properties and age estimates, including some candidates for old globular clusters and a young cluster in the outer disk.

Contribution

It presents the first detailed HST-based analysis of stellar clusters in M33, confirming ground survey candidates and identifying new clusters with age estimations.

Findings

Confirmed 24 genuine clusters from ground survey candidates.

Discovered 91 new stellar clusters in M33.

Identified clusters with a range of ages, including young and old globular candidates.

Abstract

We have used deep ACS/WFC images of M33 to check nature of extended objects detected by the ground based survey of Zloczewski et al. (2008). A total of 24 candidates turned out to be genuine compact stellar clusters. In addition we detected 91 new clusters. Equatorial coordinates, integrated magnitudes and angular sizes are listed for all 115 objects. Fourty-two clusters have sufficiently red colors to be candidates for old globulars. For four clusters we extracted resolved stellar photometry. Object 33-3-018 located in the outer disk of M33 turned out to be a young cluster with an age estimated at 200-350 Myr. Cluster ZK-90 has an age of 3-5 Gyr. The remaining two clusters have intermediate ages ranging from one to a few Gyr.