A Search for Old Star Clusters in the Large Magellanic Cloud

TL;DR

This study conducted a detailed survey of candidate old star clusters in the Large Magellanic Cloud, finding no new old clusters and providing insights into the galaxy's cluster formation history.

Contribution

First comprehensive color-magnitude diagram survey of candidate old LMC clusters revealing their true intermediate ages and refining the understanding of the LMC's cluster formation timeline.

Findings

No new old clusters were identified among the candidates.

Most clusters are of intermediate age (1-3 Gyr), not old.

Cluster formation in the LMC was absent from 3-8 Gyr ago, with a peak around 1.5 Gyr ago.

Abstract

We report the first results of a color-magnitude diagram survey of 25 candidate old LMC clusters. For almost all of the sample, it was possible to reach the turnoff region, and in many clusters we have several magnitudes of the main sequence. Age estimates based on the magnitude difference $\delta T_1$ between the giant branch clump and the turnoff revealed that no new old clusters were found. The candidates turned out to be of intermediate age (1-3 Gyr) We show that the apparently old ages as inferred from integrated UBV colors can be explained by a combination of stochastic effects produced by bright stars and by photometric errors for faint clusters lying in crowded fields. The relatively metal poor candidates from the CaII triplet spectroscopy also turned out to be of intermediate age. This, combined with the fact that they lie far out in the disk, yields interesting constraints regarding the formation and evolution of the LMC disk. We also study the age distribution of intermediate age and old clusters This homogeneous set of accurate relative ages allows us to make an improved study of the history of cluster formation/destruction for ages $>1$Gyr. We confirm previous indications that there was apparently no cluster formation in the LMC during the period from 3-8 Gyr ago, and that there was a pronounced epoch of cluster formation beginning 3 Gyrs ago that peaked at about 1.5 Gyrs ago. Our results suggest that there are few, if any, genuine old clusters in the LMC left to be found.