Deciphering the details of recent interactions in the magellanic clouds

Abstract

The Small Magellanic Cloud (SMC), a dwarf galaxy in the local neighborhood, offers an accessible laboratory for exploring star formation processes in a metal-poor environment.The SMC’s structure, stellar populations, and kinematics bear clear evidence of its recent dynamical encounters with the Large Magellanic Cloud (LMC) and the Milky Way. In particular, the young massive stars formed in these disturbed conditions serve as crucial tracers of galaxy interactions, stellar feedback, and hierarchical star formation. However, until recently, the far ultraviolet (FUV) view of the SMC has been limited by spatial resolution and incomplete coverage. In this thesis, we employ data from the Ultra Violet Imaging Telescope (UVIT) onboard AstroSat together with complementary optical and near-infrared surveys (Gaia, SMASH, VMC) to present the most comprehensive FUV-based study of young stellar populations in the SMC to date. As the first step, we constructed a catalog of ∼76,800 SMC FUV stars, of which ∼62,900 are identified as probable SMC main body members. This catalog includes UV, optical, and IR fluxes, enabling the study of stellar populations across multiple evolutionary stages. Based on the Gaia optical color–magnitude diagram (CMD), FUV stars were classified into four young populations: Young 1, Young 2, Young 3, and Blue Loop.Their spatial distribution reveals a highly irregular and clumpy morphology, withstructures such as a broken bar, a shell-like feature, and the inner Wing. The Young1, Young 2, and Young 3 populations are concentrated in the eastern and northeasternSMC, while the Blue Loop, Young 2 and Young 3 dominate the southwest. Proper motion analysis demonstrates that stars younger than ∼150 Myr show evidence of east–west kinematic stretching, consistent with signatures of the recent LMC–SMC encounter, while no strong perturbation is seen along declination. To probe star formation closely, we analyzed clustering of stars with ages below 150 Myr, identifying 236 stellar structures whose sizes span from two parsecs to three hundred parsecs. Their irregular morphologies,perimeter-area dimension (Dp = 1.46± 0.04), fractal dimensions (D2 ∼ 1.3–1.6), and log-normal surface density distributionclosely resemble the properties of a turbulent interstellar medium. These results provide strong evidence that star formation in the SMC is hierarchical in nature and regulated by supersonic turbulence, with galaxy interactions supplying the driving mechanism for this turbulent regime. Expanding the analysis to the northeastern SMC Shell region, we combined UVIT data with Gaia EDR3 to create an FUV–optical catalog of ∼14,400 stars. Overlaying isochrones on FUV–optical CMD reveals multiple star formation episodes, most notablyat ∼260 Myr, linked to the last close LMC–SMC encounter, and at ∼60 Myr,associated with the SMC’s close approach to the Milky Way. The FUV stellar surface density, together with the dispersion in proper motion, indicates that the inner SMC extends northeastward to about 2.2°. In the FUV stellar density map, we identify arm-like and arc-like features whose kinematics are comparable to those of its main body. These outer extensions represent spatial overdensities of stars formed during multiple episodes of star formation, but they do not exhibit any clear kinematic distinction.The median proper motion and velocity dispersion are comparable to those of the SMC main body, suggesting that this region has not undergone significant tidal influence. Expanding the analysis to the northeastern SMC Shell region, we combined UVIT data with Gaia EDR3 to create an FUV–optical catalog of ∼14,400 stars. Overlaying isochrones on FUV–optical CMD reveals multiple star formation episodes, most notably at ∼260 Myr, linked to the last close LMC–SMC encounter, and at ∼60 Myr,associated with the SMC’s close approach to the Milky Way. The FUV stellar surface density, together with the dispersion in proper motion, indicates that the inner SMC extends northeastward to about 2.2°. In the FUV stellar density map, we identify arm-like and arc-like features whose kinematics are comparable to those of its main body. These outer extensions represent spatial over densities of stars formed during multiple episodes of star formation, but they do not exhibit any clear kinematic distinction.The median proper motion and velocity dispersion are comparable to thoseof the SMC main body, suggesting that this region has not undergone significant tidal influence. Finally, to characterize the properties of the SMC’s young stars, we choose a sample region within the Shell where multifilter UVIT data are available for robustness. We constructed spectral energy distributions (SEDs) for 1348 stars in the Shell region,spanning 18 photometric bands from UV to IR. Using single- and double-component fits, we derived effective temperatures, luminosities, and radii, identifying 1242 single systems which are mainly main sequence B- and A-type stars (10,000–30,000 K, 3–8 M⊙) and 85 systems which are double systems. These double systems include 18 stripped star binary systems, 9 subgiant-giant binary systems, and 20 candidate binaries. We also found 38 double systems, which could be non-contact binary or a star with a circumstellar disk or line-of-sight projections within the SMC. The characterized single and double systems include known eclipsing binaries, emission line stars, photometric variables, and pulsating variables. Taken together, the results presented in this thesis deliver the most extensive FUV catalog of the SMC to date and provide a detailed view of the distribution, clustering, kinematics, and properties of its young stars. Our findings establish that the morphology and dynamics of massive stars are strongly shaped by LMC–SMC interactions,while their spatial substructure reflects turbulence-driven hierarchical star formation. By identifying binary systems and stripped stars, this work also lays critical groundworkfor spectroscopic follow-up studies, which are essential to fully constrain the role of binarity and feedback in metal-poor environments. Looking forward, future UV missions such as UVEX and INSIST will further expand upon these results, offering deeper insight into how massive stars regulate the galaxies’ evolution in the early Universe.