Stellar population in the Magellanic Clouds: Star formation and evolution in diverse environments

Abstract

The Magellanic Clouds (MCs) are the closest interacting satellite dwarf galaxies of the Milky Way (MW). The Magellanic system comprises the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC), which are connected by the Magellanic Bridge, a structure composed of gas and stars. The recent proper motion (PM) studies of the MCs suggest that they are interacting not only with each other but also with the MW. The interactions experienced by the MCs have also produced a prominent tail of neutral hydrogen gas, known as the Magellanic Stream, which stretches across ∼ 200◦ of the southern sky. In addition, filamentary leading arms extend ahead of the Clouds’ motion, arching over the MW disk. Interactions between the MCs trigger star formation and impact the internal kinematics of both galaxies. Therefore, studying the stellar populations residing within these galaxies is essential to unraveling their evolution and interaction history. Star clusters serve as valuable tracers for unraveling the star formation history of galaxies. In the MCs, over 4000 clusters have been cataloged. Determining their ages provides insights into the cluster formation history (CFH) of the MCs. However, a comprehensive age estimation from the center to the outskirts of the LMC and SMC is still lacking. Such global age dating is essential for identifying the episodes of enhanced star/ cluster formation (CF) triggered by the LMC-SMC interactions. Also, modeling the internal kinematics of the LMC and SMC is crucial for revealing the disk response of the MCs to their mutual interaction. The goal of this thesis is to trace the CFH and model the kinematics of the MCs to gain insights into their interaction history. The analysis in our studies is based entirely on Gaia (Global Astrometric Interferometer for Astrophysics) Data Release 3 (DR3) data sets. In this study, we present a detailed view of CF to trace the evolution and interaction history of the MCs in the last 3.5 Gyr. We parameterized 1710 and 280 star clusters in the LMC and the SMC, where 847 and 113 clusters are newly characterized in the outer LMC and SMC, respectively. We estimated the age-extinction metallicitydistance parameters using an automated fitting of the color-magnitude diagram (CMD) after field star removal, followed by a Markov Chain Monte Carlo (MCMC) technique. We report a first-time detection of two synchronized CF peaks in the MCs at 1.5 }0.12 Gyr and 800 }60 Myr. We recommend that the choice of the metallicity (Z) values of isochrones for clusters with age ≤ 1 - 2 Gyr are ZLMC = 0.004 - 0.008 and ZSMC = 0.0016 - 0.004 for the LMC and SMC, respectively. We found evidence for spiral arms in the LMC, as traced by the cluster count profiles over the last 3.5 Gyr. The density maps provide evidence of ram-pressure stripping in the North-East of the LMC, a severe truncation of CF in the South of the LMC, and a radial shrinkage of CF in the SMC in the last 450 Myr. The last SMC-LMC interaction (∼ 150 Myr) resulted in a substantial CF in the North and Eastern SMC, with a marginal impact on the LMC. This study brings out the CF episodes in the MCs and their connection to the LMC-SMC-MW interactions. The internal kinematics of the LMC disk have been modeled by several studies using different tracers with varying coverage, resulting in a range of parameters. In this study, we modeled the LMC disk using 1705 star clusters and field stars, based on a robust MCMC method. The dependency of model parameters on the age, coverage, and strength of the clusters are also presented. This is the first comprehensive 2D kinematic study using star clusters. Red clump (RC) stars and young main-sequence stars are also modeled for comparison. The clusters and field stars are found to have distinctly different kinematic centers, disk inclination, position angle of the line of nodes, and scale radius. We also note a significant radial variation of the disk parameters. Clusters and young stars are found to have a large residual PM and a relatively large velocity dispersion when compared to the RC field population, which could be due to perturbation from the bar and spiral arms. We traced the presence of large residual PM and non-circular motion among clusters likely to be due to the bar and detected a decrease in the scale radius as a result of the possible evolution of the bar. The kinematically deviant clusters point to a spatio-temporal disturbance in the LMC disk, matching the expected impact parameter and time of the recent collision between the LMC and the SMC. Similar to the LMC, we modeled the kinematics of the SMC by analyzing the PM from nine different stellar populations, which include young main sequence stars (< 2 Gyr), RGB stars, RC stars, red giants with line-of-sight (LOS) velocities, and three groups of star clusters. We trace the evolution from a non-rotating flattened elliptical system as mapped by the old population to a rotating highly stretched disk structure as denoted by the young main sequence stars and clusters (< 400 Myr). We estimated that the inclination, i (∼ 58◦ to 82◦) decreases and the position angle, Θ (∼ 180◦ to 240◦) increases with age. We estimated an asymptotic velocity of ∼ 49 - 89 km s−1 with scale-radius of ∼ 6 - 9 kpc for the young main sequence populations with velocity dispersion of ∼ 11 km s−1, suggesting a rotation-supported disk structure. Our models estimate a LOS extension of ∼ 30 kpc, in agreement with observations. We identified four regions of the SMC showing anomalies in the residual PM, the East Anomaly (EA), South East Anomaly (SEA), South Anomaly (SA), and West Anomaly (WA). The SEA appears like an infalling feature and is identified for the first time. The tidal imprints observed in the residual PM of the SMC suggest that the recent interaction with the LMC considerably shapes its evolution.