Nimisha Kantharia

Star-forming Galaxies:

Galaxies evolve in isolation or in denser environments such as small groups or large clusters. As expected, the evolution of these galaxies in terms of star formation rate, gas content and other internal properties will depend on the environment. In the denser environments, the galaxies are subject to tidal interactions due to the presence of other nearby galaxies which can modify star-forming properties and to hydrodynamic processes due to intra-cluster gas like ram pressure stripping and viscous stripping which can lead to gas depletion. On the other hand, when galaxies evolve in isolation, they are only subject to internal perturbations. Thus, it is important to study both (1) the physical processes that act on these galaxies in dense environments and (2) the evolutionary paths of galaxies in both environments. This is particularly important for small groups since more than 60% of galaxies are believed to evolve in small groups. At GMRT, research includes the study of star-forming galaxies ranging from small to large spirals evolving in a range of environments in the near Universe. Research is actively being pursued on normal disk galaxies, low surface brightness galaxies, and HII galaxies in terms of understanding their low radio frequency spectral energy distribution, magnetic field distribution, halo emission, atomic gas distribution and kinematics, interactions and star formation properties in different environments. Research also involves combining radio results with results from other wavebands and studying the global properties of these galaxies.

Novae at GMRT frequencies:

It is believed that about half the stars in the Milky Way are found in multiple or binary systems with the number being higher for massive OB stars and lower for low mass red dwarf stars. Stars evolving in binaries will follow a distinct evolutionary track from those evolving as singles. There are binary systems which consist of a white dwarf and a red giant or main sequence star as the companion. In several such binaries, the white dwarf accretes matter which overflows the Roche lobe of the companion star and the material accumulates on the surface of the white dwarf. A cataclysmic thermonuclear reaction is ignited in this material when sufficient material is deposited on the surface leading to the ejection of mass and energy into the surrounding medium. These cataclysmic systems known as novae brighten by several magnitudes in the optical within a short interval and are observed to emit in the entire electromagnetic spectrum due to a range of physical phenomena. The fast-moving ejecta from the explosion set up shocks in the interstellar medium as they encounter the ambient material, and this leads to the emission of synchrotron radiation which can be ideally observed at GMRT frequencies. There is a class of novae known as recurrent novae (e.g RS Ophiuchi) where outbursts recur on timescales of a few years and these systems are believed to harbor a white dwarf whose mass is close to the Chandrasekhar limit. These, then, are strong contenders for progenitor systems of Type 1a supernovae. Studying the evolution of the radio synchrotron emission in successive recurrent nova outbursts can shed light on the evolution of these intriguing systems, the progenitor scenario and the possibility of them evolving into Type 1a supernovae, in addition to shock physics. The GMRT is used to study and model the early light curves and the longer term evolution of the synchrotron emission. There are also works on studying the early evolution of supernovae including Type 1a supernovae and gamma-ray bursts.

Probing ionised regions using radio recombination lines:

The ultraviolet radiation from stars ionises their surrounding gaseous regions containing hydrogen, helium or carbon. Subsequently, an equilibrium is established wherein the ionisation is balanced by recombination. These regions around stars are called Stromgren spheres. There are also regions in the Milky Way where hydrogen, carbon or helium can be ionized by the interstellar radiation field. In all these regions, electrons can recombine to excited quantum levels in the atom and cascade down to lower levels giving rise to recombination lines at radio frequencies. This spectral line emission can be mapped from the ionized regions and physical properties such as temperature, electron density, and size can be modeled. Moreover, the distribution of this gas and its kinematics can also be studied using these lines. These results are combined with other diagnostics from the same gas to better constrain the physical parameter space. Thus, studies of HII regions around stars and the photodissociation regions using these spectral lines are carried out at NCRA-TIFR.