Unravelling the low-frequency radio secrets of Galactic star-forming regions with the uGMRT

Abstract

Star-forming regions are dynamic environments shaped by the complex interplay of various astrophysical processes and objects. On one hand, massive stars during their evolutionary phases ionise the surrounding interstellar medium (ISM), forming H II regions that emit thermal free-free radiation. On the other hand, energetic activities such as supernova explosions, jets and outflows from young stellar objects (YSOs), and stellar winds from massive stars, etc., inject energy into the medium through nonthermal processes, giving rise to nonthermal emission. The interaction between these phenomena contributes to the rich and intricate structure of the ISM in star-forming regions. Notably, the imprints of these processes are particularly bright at sub-GHz (< 1GHz) frequencies yet remain relatively unexplored. This thesis presents a comprehensive investigation of sub-GHz radio emission in star- forming environments such as the Orion Nebula and the W43/W44 region, using high-sensitivity, broadband continuum observations from the upgraded Giant Metrewave Radio Telescope (uGMRT), complemented by multiwavelength data across the optical, submillimetre, infrared, X-ray, and gamma-ray regimes. A key methodological contribution is a systematic study on the reliability of in-band and broadband spectral index measurements, quantifying the dependence of spectral index accuracy on the signal-to-noise ratio. This framework enables the robust identification and separation of thermal and nonthermal components within complex radio morphologies. We investigate the Orion Nebula—a prototypical massive star-forming region—revealing new nonthermal emission in the periphery of this H II region, possibly associated with Herbig- Haro objects and protostellar outflows. The observed synchrotron-like spectral indices are found to be co-spatial with optical shock tracers, suggesting localised particle acceleration in jet-driven shocks. As an alternative or complementary scenario, we explore the physical conditions of molecular clouds correlated with nonthermal radio features, considering cloud–cloud collisions as a possible mechanism for shock generation and energy dissipation. High-energy data from X-ray and gamma ray observations provide additional support for the role of these shocks in producing relativistic particle populations. In addition, this thesis contributes to the pilot study of the Metrewave Galactic Plane with the uGMRT (MeGaPluG) survey, through detailed mapping of the W43/W44 region. This demonstrates the capability to conduct large-scale Galactic plane surveys and illustrates the separation of known thermal and nonthermal emission components. The results presented in this thesis enhance our understanding of thermal and nonthermal processes in Galactic star-forming regions and lay the groundwork for a future full-scale wide-field survey of the Galactic plane at sub-GHz frequencies.