Age Determination of Six Intermediate-age SMC Star Clusters with HST/ACS

TL;DR

This study uses deep HST/ACS photometry to determine ages and metallicities of six intermediate-age SMC star clusters, revealing a complex age-metallicity relation and spatial distribution.

Contribution

It provides a homogeneous set of low-metallicity isochrones and detailed age estimates for six SMC clusters using multiple models.

Findings

NGC419 has an age of 1.2-1.6 Gyr.

SMC clusters show a metallicity spread of ~0.6 dex.

The SMC has a large depth of ~17 kpc.

Abstract

We present a photometric analysis of the star clusters Lindsay 1, Kron 3, NGC339, NGC416, Lindsay 38, and NGC419 in the Small Magellanic Cloud (SMC), observed with the Hubble Space Telescope Advanced Camera for Surveys (ACS) in the F555W and F814W filters. Our color magnitude diagrams (CMDs) extend ~3.5 mag deeper than the main-sequence turnoff points, deeper than any previous data. Cluster ages were derived using three different isochrone models: Padova, Teramo, and Dartmouth, which are all available in the ACS photometric system. Fitting observed ridgelines for each cluster, we provide a homogeneous and unique set of low-metallicity, single-age fiducial isochrones. The cluster CMDs are best approximated by the Dartmouth isochrones for all clusters, except for NGC419 where the Padova isochrones provided the best fit. The CMD of NGC419 shows several main-sequence turn-offs, which belong to the cluster and to the SMC field. We thus derive an age range of 1.2-1.6 Gyr for NGC419. Interestingly, our intermediate-age star clusters have a metallicity spread of ~0.6 dex, which demonstrates that the SMC does not have a smooth, monotonic age-metallicity relation. We find an indication for centrally concentrated blue straggler star candidates in NGC416, while for the other clusters these are not present. Using the red clump magnitudes, we find that the closest cluster, NGC419 (~50kpc), and the farthest cluster, Lindsay 38 (~67kpc), have a relative distance of ~17kpc, which confirms the large depth of the SMC.