My research interests fall under two broad areas: understanding conditions in the interstellar medium (ISM) of the Milky Way, and the process of galaxy evolution. Given below is a brief summary of different research projects I have done so far.

Low-z damped Lyman-alpha systems

Damped Lyman &alpha systems (DLAs) are the highest HI column density systems detected in absorption in the optical and ultraviolet (UV) spectra of background quasars (N(HI) > 2E20 per sq cm). Even though they are presumed to be the high-_z_ counterparts of today’s normal galaxies their HI content is insufficient (by about a factor of 2) to entirely fuel the stars observed in today’s galaxies. But they still contain the bulk of the neutral hydrogen at high z (~80%). Understanding the nature of the galaxies that give rise to DLAs and their redshift evolution has been an important question in galaxy evolution for nearly 30 years. We carried a deep search for redshifted HI 21cm emission from three DLAs and sub-DLAs at z~0.1 with the Green Bank Telescope (GBT). No evidence for a redshifted HI 21cm emission signal was obtained in their GBT spectra. The non-detection of HI 21cm emission yielded strong constraints on the HI masses of the associated galaxies continuing the trend of low HI masses found in all low-_z_ DLAs and sub-DLAs that have been searched for redshifted HI 21cm emission.

Sub-millimeter observations of shocked molecular gas associated with W28F

Supernovae (SNe) inject large amounts of energy and chemically enriched materials into their surrounding interstellar medium and, in some instances, into molecular clouds. The interaction of an supernova remnant (SNR) with an associated molecular cloud plays a crucial role in the evolution of its physical and chemical properties. Despite their importance, only a handful of studies have been made addressing the molecular richness in molecular clouds impacted by SNRs. Sub-mm observations of SNRs can be used to build a census of their molecular richness, which in turn can motivate various chemical and physical models aimed at explaining the chemical evolution in such that condition. We carried out a spectral line observation in 230 and 345 GHz atmospheric windows towards one of the 1720 MHz OH maser sites (an indicator of interaction of shock with the molecular cloud) of the supernova remnant W28F with APEX with an aim of creating a molecular inventory in this region. Thermal emission of methanol was detected for the first time in a post-shock region. Additionally, multiple molecular species like H2CO, SO, SiO, CS, CO, C-18-O, C-17-O, HCO+, HC-13-O+, HNC, HCN, NNH+, CN, CCH, and NO were detected with many of them showing multiple J transitions. Detected lines of methanol and formaldehyde were used to constrain the physical conditions existent in the region using RADEX, a radiative transfer code.

Pathfinder to LAsMAGal: A high resolution survey of the inner galaxy

Many large scale CO surveys already exist today which provide first hand information on the molecular gas distribution in the Galactic plane. This information can be used to test various existing Galactic models. But, most of these surveys use the low lying CO(2-1 or 1-0) line which becomes optically thick in most clouds with moderately high density (~ 1E3 per cubic cm at 25K for the 1-0 transition). Additionally, all the surveys towards the 4th quadrant of the Galactic plane also have a poor angular resolution. These factors make it difficult to study sub-parsec (clump) scales and hence, in turn, the process of star formation. To remedy this, we conducted a pathfinder study for a high resolution Galactic plane survey of the inner Galaxy in the 4th quadrant using 3-2 transition of 12/13-CO isotopologues. The Large APEX sub-Millimeter Array (LAsMA) 7-pixel receiver on the APEX telescope was used for this in the 345GHz atmospheric window where, it has a spatial resolution of 19”. So far we have observed a longitude range of 345-350 degree (the far end of the southern galactic bar). The data from this region shows a factor of ~4 improvement on noise levels compared to SEDIGISM. This means that we could detect fainter clouds and hence also have a higher probability to detect outflows.

Effect of feedback from OB stars on the G305 star forming complex

I am interested in the process of high mass star formation. Although the process of low mass star formation is much better understood, how high mass stars (M>8 Msun) form is still a field of active research. These stars usually form inside giant molecular clouds (GMCs) as the dominant members of young stellar clusters. They are short-lived (<30Myr), but are known to inject large amounts of feedback into the interstellar medium in the form of stellar winds, ionizing radiation and supernovae. These feedback mechanisms can sweep up the surrounding gas and create parsec-scale cavities around them, forming dense shells of gas as a result and thereby triggering star formation. Conversely, they can also completely disperse their surrounding molecular gas suppressing star formation. This ability to both constructively and destructively affect star formation means that high mass stars play a significant role in driving the evolution of GMCs. We studied the effect of feedback from massive stars in the G305 HII region and molecular cloud complex. The LAsMA array on APEX telescope was used to observe the G305 complex in the (3-2) rotational transition of both 12-CO and its 13-CO isotopologue. Line profiles of excitation maps along radially outward directions demonstrate that the excitation temperature and 13-CO 3-2/2-1 ratio (obtained by using SEDIGISM data with ours) increase steeply by factors of ~2-3 at the edge of the denser gas traced by 13-CO that faces the hot stars at the center of the complex and steadily decreases away from it. Regions with a higher 8-micron flux (which traces feedback strength) have higher median excitation temperatures, column densities, and 13-CO 3-2/2-1 ratio. The centroid velocity probability distribution function of the region shows exponential wings, indicative of turbulence driven by strong stellar winds. A novel stacked spectral analysis of the line profiles showed that regions with stronger feedback have higher skewness and narrower peaks with pronounced wings compared to regions with weaker feedback. The region was decomposed into clumps using dendrogram analysis and a catalog of the clump properties was created. The surface mass densities of clumps showed a positive correlation with the incident 8-micron flux. Clumps also demonstrated much higher fragmentation under the effect of feedback indicating that the clumps have been triggered. The CDFs of clump masses and their L/M ratios are both flatter than that of the Galactic average, indicating that clumps are heavier and more efficient at forming stars in G305 compared to the Galactic average. All these evidences provide definite evidence in support of triggering and rule out the destructive effect of feedback on the star formation of the GMC.