The VMC Survey - XXI. New star cluster candidates discovered from infrared photometry in the Small Magellanic Cloud

TL;DR

This study used near-infrared VMC data to discover 38 new star cluster candidates in the Small Magellanic Cloud, significantly increasing the known clusters in that region, especially those obscured in optical wavelengths.

Contribution

First application of VMC infrared survey data to identify new star clusters in the SMC, employing automated density estimation and CMD analysis to confirm cluster candidates.

Findings

38 new star cluster candidates identified

Significant increase (~55%) in known clusters in the surveyed area

Most candidates are obscured in optical images and require infrared data for detection

Abstract

We report the first search for new star clusters performed using the VISTA near-infrared YJKs Magellanic Clouds survey (VMC) data sets. We chose a pilot field of ~ 0.4 deg^2 located in the South-West of the Small Magelllanic Cloud (SMC) bar, where the star field is among the densest and highest reddened region in the galaxy. In order to devise an appropriate automatic procedure we made use of dimensions and stellar densities observed in the VMC data sets of the known clusters in this area. We executed different kernel density estimations over a sample of more than 358000 stars with magnitudes measured in the three YJKs filters. We analysed the new cluster candidates whose colour-magnitude diagrams (CMDs), cleaned from field star contamination, were used to assess the clusters' reality and estimate reddenings and ages of the genuine systems. As a result 38 objects (~ a 55% increase in the known star clusters located in the surveyed field) of 0.15 - 0.40 arcmin (2.6 - 7.0 pc) in radius resulted to have near-infrared CMD features which resemble those of star clusters of young to moderate intermediate age (log(t /yr ) ~ 7.5-9.0). Most of the new star cluster candidates are hardly recognizable in optical images without the help of a sound star field decontaminated CMD analysis. For highly reddened star cluster candidates (E(B - V ) >= 0.6 mag) the VMC data sets were necessary in order to recognize them.