UV studies of star formation in different extragalactic environments

Abstract

Star formation is crucial for galaxy evolution and can be driven by both internal processes and external interactions with neighboring galaxies. By studying the properties of star formation, we can infer the potential causes behind it. This complex process also depends on the surrounding environment, which influences the physical properties of both the stellar and gaseous components that support star formation. Therefore, it is essential to understand how the environment impacts star formation and how this process, in turn, affects the environment. Within galaxies, star-forming environments can vary from their nuclei to their outer disks. It also varies over different galaxy types, from dwarfs to massive spirals. Analyzing these variations offers insights into both local and global star formation processes. Our thesis focuses on UV studies of star formation in various extragalactic environments and its impact on the interstellar medium (ISM) and structural complexity of a galaxy. We primarily utilize observations from the Ultra-Violet Imaging Telescope (UVIT), along with multiwavelength data, for a comprehensive analysis. Using UVIT and other high-resolution observations, we conducted a spatially resolved study. We detected star-forming complexes (SFCs) ranging from a few parsecs to hundreds of parsecs in size. Our studies include a nearby massive galaxy (NGC 628), a sample of 16 starforming dwarf galaxies (SFDGs), and an ultra-faint dwarf galaxy (UFDG; Leo T). In our study, we estimated an approximate high-mass Initial Mass Function (IMF) for the SFCs across NGC 628 using Hα and UV emissions. Our results indicate that the IMF derived from B-type stars is steeper and more reliable. We observed little variation in the high-mass IMF between arms and spurs within NGC 628. Additionally, we analyzed the properties of polyaromatic hydrocarbons (PAHs), noting an increase in small PAH molecules as SFCs become top heavy and blue. We also examined emissions in bubble and superbubble regions, highlighted in James Webb Space Telescope (JWST) images. To quantify the morphological complexity, we conducted a multiwavelength multiscale analysis of NGC 628, employing statistical measures such as permutation entropy, disequilibrium, and statistical complexity. This analysis yielded a characteristic smoothing scale of about 196 parsecs, which marks a transition from star formation-driven scales to those influenced by galaxy dynamics. Dwarf galaxies have completely different star-forming environments; they are characterized by low mass, small size, and a metal-poor composition, and are very different compared to the massive galaxy discussed above. We studied SFDGs and one UFDG using UVIT observations. We classified the SFDGs in our sample into dwarf spirals, irregulars, and blue compact dwarfs (BCDs), and found that their properties, including star formation rate and SFC sizes, depend on morphology. We also examined the FUV emission in Leo T, a UFDG, to gain insight into star formation in extreme environments. By comparing FUV SFCs with high-resolution optical data, we also identified UV-emitting stars. Thus, this thesis provides a comprehensive study of star formation across diverse environments within galaxies and over different galaxy types, utilizing our own UV observations and multiwavelength archival data.