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(Jayaram N. Chengalur, Subhashis Roy, Nissim Kanekar, Visweshwar Ram Marthi, Previous members: Swarna Kanti Ghosh, Nimisha Kantharia, Narendra Nath Patra)

The life cycle of a galaxy consists of an interplay between its two main constituents, the stars and the interstellar medium (ISM), with the dark matter halo providing the background stage. Stars initially form by the collapse of molecular clouds in the ISM, and then pollute the ISM with metals, both during their life (via stellar winds) and especially towards the end of their life, via supernovae. These metals cause further cooling of the ISM gas, which gives rise to further star formation. Understanding physical conditions in the ISM of galaxies, and the evolution of the ISM, is hence a major area of research in astronomy and at NCRA-TIFR.

The ISM consists of various gas phases, including neutral atomic, ionized, and molecular gas, at different temperatures, pressures, and densities. The neutral atomic ISM is best probed with the hyperfine spectral line of neutral atomic hydrogen (HI) at a rest frequency of 1420.40575 MHz (21cm). NCRA-TIFR astronomers have long used this HI 21cm spectral transition to study physical conditions in the neutral gas in the Milky Way. For example, HI 21cm absorption studies with the GMRT have found evidence for a phase transition in neutral hydrogen in the Milky Way, above a critical HI surface density. At surface densities higher than this threshold, the hydrogen is present in both the cold and the warm phases (at temperatures ~100 K, and >1000 K, respectively); however, at surface densities below this threshold, the hydrogen appears to exist predominantly in the warm phase. Such GMRT HI 21cm absorption studies have also been used to find evidence that the standard two-phase models of neutral hydrogen may not be applicable in the Milky Way, as a significant fraction of the neutral gas appears to lie in the thermally unstable phase. NCRA-TIFR astronomers continue to use the HI 21cm transition as a critical tool to probe physical conditions in the Milky Way.

Besides HI 21cm studies, NCRA-TIFR astronomers have also used radio recombination lines to probe the temperature and density of ionized gas in the Milky Way. Studies of complex molecules like acetaldehyde have been used to determine the spatial extent of organic molecules, and to show that such organic molecules are widespread in the star-forming regions and not confined to tiny hot cores. Radio continuum studies of ionized regions around massive stars (HII regions) and supernova remnants have also been used to glean information about physical conditions in the ionized gas.