Study of star cluster populations in the magellanic clouds

Abstract

The Magellanic Clouds (MCs), neighbouring galaxies to the Milky Way (MW), consist of a pair of irregular type of galaxies : Large Magellanic Cloud (LMC) and Small Mag- ellanic Cloud (SMC). The recent proper motion study of MCs suggested that they are interacting with each other, as well as with the MW. The Magellanic Bridge connecting both Clouds and the Magellanic Stream are the witnesses to these interactions. The interactions between the MCs could have triggered star formation in both the galaxies, which can be confirmed by studying formation and evolution history of star clusters in the MCs. Proximity of MCs and their location at high galactic latitude enables us to resolve their individual populations as well as detect faint stellar populations within them using ground based telescopes under good seeing conditions. Though ~4000 clusters in the MCs have been catalogued, studies suggest the catalogue is still in- complete. Therefore, it is necessary to age-date the identified clusters, including the less massive clusters, to understand the demographics of cluster formation and their correlation in the L&SMC. In this study, we estimated parameters (age and reddening) of a large number of clusters to understand demographics of cluster formation and signatures of interactions using the Optical Gravitational Experiment III data. We developed a semi-automated method and parameterized more than 1200 clusters to produce two online catalogues. The new tool developed and implemented to characterize ~1200 star clusters in the MCs will be useful to age-date the star cluster population when large survey data of MCs get published in the future. We also classified these clusters and grouped them into 5 groups, based on their mass and strength(total number of cluster members). In our sample, 90% of the SMC clusters and 80% of the LMC clusters studied here belong to the open cluster like population in our Galaxy, with mass < 1700 Mʘ, suggesting similar population of clusters in these three galaxies. We found that both the clouds have gone through a recent burst of cluster formation at ~125 Myr ago. We suggest the most recent interaction between the Clouds around 100-250 Myr ago could have triggered the cluster formation. We find evidence for an outside to inside propagation of cluster formation in the LMC, whereas the cluster formation is found to propagate from South-West to North-east in the SMC. The bar region of the LMC is found to have formed clusters in the age range 60-250 Myr ago, suggesting that the LMC bar was active in the recent past. We estimated stellar mass function (MF) and total mass of 66 LMC clusters, as well as the cluster mass function in the LMC. The peak value of the slope, -2.26 ± 0.44 is similar to Salpeter's initial MF slope value (-2.35) for the Solar neighbourhood. Recent studies reveal that, a good number of massive intermediate age (~ a few Gyr old) star clusters (>50%) in the LMC have multiple populations and/or extended main-sequence turn-off (MSTO) stars. These cannot be explained by photometric errors or stellar binarity, whereas age spread and effects due to stellar evolution, such as stellar rotation are the suggested possibilities. This topic has received a lot of attention in the recent years, where most of the studies were focused on star clusters in the LMC and in the optical wavelengths. Here we studied Kron 3, an intermediate age rich star cluster, located in the southern SMC. We used multi-wavelength data from ultraviolet (UV) to infrared to study the stellar population in Kron 3. We used the near-UV observations made using the UltraViolet Imaging Telescope (UVIT) on AstroSat (the first Indian space observatory) for this study. The combined data from UVIT, HST, Gaia and VISTA help us to derive the radius of the cluster and identify an extended red clump population in the field de-contaminated CMD. Analysis using three different methods suggest a large spread in metallicity among the red clump stars as the possible reason for the extended red clump population. Kron 3 is thus one of the younger clusters in the SMC to show multiple population and detailed high resolution spectroscopic studies are needed to confirm this result. This study demonstrated the advantage of superior resolution UVIT data in the near-UV pass-band in addressing metallicity differences among stars in a cluster.