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The generation mechanism of compressible fluid turbulence at kiloparsec scales in the Interstellar Medium (ISM) is a long-lasting puzzle. In this work, we explore the nature of large-scale turbulence in the external spiral galaxy NGC~6946. We use the Visibility Moment Estimator (VME) to measure the \HI column density and line of sight turbulent velocity power spectra combining the new observations of A array configuration of Karl G. Jansky Very Large Array (VLA) with the VLA B, C, D array observations from The \HI Nearby Galaxy Survey (THINGS). The estimated power spectra are obeying a power law with a slope of $-0.96\pm0.05$ in column density and $-1.81\pm0.07$ in line of sight velocity in length scales ranging from $6$ kpc to $170$ pc. This points towards a forward energy cascade in the plane of the disc with a driving scale at least as large as $6$ kpc. The values of the power law indices indicate a combination of solenoidal and compressive force responsible in driving the measured turbulence. The presence of strong regular magnetic fields from the magnetic spiral arms in the galaxy is possibly contributing to the solenoidal part, while self-gravity or gravitational instability can mostly be the input for the compressive part of the forcing in the driving mechanism., Comment: accepted in MNRAS for publication, Oct 2 2023

One of the major science goals of the Square Kilometre Array (SKA) is to understand the role played by atomic hydrogen (HI) gas in the evolution of galaxies throughout cosmic time. The hyperfine transition line of the hydrogen atom at 21-cm is one of the best tools to detect and study the properties of HI gas associated with galaxies. In this article, we review our current understanding of HI gas and its relationship with galaxies through observations of the 21-cm line both in emission and absorption. In addition, we provide an overview of the HI science that will be possible with SKA and its pre-cursors and pathfinders, i.e. HI 21-cm emission and absorption studies of galaxies from nearby to high redshifts that will trace various processes governing galaxy evolution., Comment: 31 pages, 7 figures, accepted on 27 May 2022 for publication in the Journal of Astrophysics and Astronomy (to appear in the special issue on "Indian participation in the SKA"), figure 4 has been updated

Detection of bending waves is a highly challenging task even in nearby disc galaxies due to their sub-kpc bending amplitudes. However, simulations show that the harmonic bending of a Milky Way like disc galaxy is associated with a harmonic fluctuation in the measured line of sight (los) velocities as well, and can be regarded as a kinematic signature of a manifested bending wave. Here, we look for similar kinematic signatures of bending waves in \HI discs, as they extend to much beyond the optical radii. We present a multipole analysis of the \HI los residual velocity fields of six nearby spiral galaxies from the THINGS sample, which uncovers the bending wave-induced velocity peaks. This allows us to identify the radial positions and amplitudes of the different bending modes present in the galaxies. We find that all of our sample discs show a combined kinematic signature of superposition of a few lower-order bending modes, suggesting that bending waves are a common phenomenon. The identified velocity peaks are found to be of modes $m=2,3$ and $4$, not more than 15 km s$^{-1}$ in amplitude and spread across the entire \HI disc. Interestingly, they appear to be concentrated near the optical edge of their host galaxies. Also, $m=2$ appears to be more common than the other two modes., Comment: Accepted in MNRAS, 12 pages, 11 figures

Turbulence plays an important role in the structure and dynamics of the galaxy and influences various processes therein including star formation. In this work, we investigate the large scale turbulence properties of the external spiral galaxy NGC 5236. We combine the VLA multi-configuration archival data with the new GMRT observation to estimate the column density and line of sight velocity fluctuation power spectra for this galaxy over almost two decades of length scales. The energy input scale to the ISM turbulence is found to be around $6$ kpc. Power spectra of the two-dimensional turbulence in the galaxy's disk follow a power law with a slope $-1.23\pm0.06$ for the column density and $-1.91\pm0.08$ for the line of sight velocity. The measured power spectra slopes strongly suggest in favour of a compressive forcing with a steady energy input of $\sim 7 \times 10^{-11}$ erg cm$^{-2}$ sec$^{-1}$. We conclude that much of these originate from the gravitational instabilities and self-gravity in the disk. This is the first and most comprehensive study of turbulence statistics for any external spiral galaxy., Comment: 8 pages, 3 figures, Accepted for Publication in MNRAS on June 08 2020

Turbulent dynamics generate random fluctuations in density and velocity in the Interstellar Medium (ISM) of spiral and dwarf galaxies. Observationally, H~{\sc i} 21-cm radiation provides a good probe of these stochastic processes and helps us to know more about their nature and generating mechanism. Structure function, auto-correlation function, power spectrum are some of statistical estimators of these fluctuations which have been used in literature, whose studies helps in understanding the generating mechanisms of these structures. In this thesis we are interested in estimating the column density and velocity power spectrum at scales as large as 10 Kpc, to understand the energy associated with these structures and the type of forcing of the turbulence. Estimating power spectrum from observations can be done either by using the directly measured quantity visibility or by reconstructing the sky brightness distribution (or image) from the visibilities. By using simulated observations of a model galaxy with known power spectrum we find that the visibility based estimator reproduce the true power spectrum. The image based estimator has a scale dependent bias that is highly correlated to the incompleteness in the baseline coverage of the interferometer. Interestingly, we found that the noise in the reconstructed image has zero mean and hence the locally averaged quantities estimated from the reconstructed image are unbiased. We implement the visibility moment estimator as discussed in Dutta et al. (2015) and estimate the line of sight velocity fluctuation of the spiral galaxy NGC6946. Our result strongly indicate presence of large scale turbulence in the spiral galaxies. We conclude that we would need to have higher signal to noise H~{\sc i}observations to definitively comment on the generating mechanisms of the large scale turbulence., Comment: 59 pages, 12 figure, 1 table. Final year BS-MS thesis submitted for the completion of BS-MS (Physics) course at IISER Bhopal

Radio interferometric data are used to estimate the sky brightness distributions in radio frequencies. Here we focus on estimators of the large-scale structure and the power spectrum of the sky brightness distribution inferred from radio interferometric observations and assess their efficacy using simulated observations of the model sky. We find that while the large-scale distribution can be unbiasedly estimated from the reconstructed image from the interferometric data, estimates of the power spectrum of the intensity fluctuations calculated from the image are generally biased. The bias is more pronounced for diffuse emission. The visibility based power spectrum estimator, however, gives an unbiased estimate of the true power spectrum. We conclude that for an observation with diffuse emission the reconstructed image can be used to estimate the large-scale distribution of the intensity, while to estimate the power spectrum, visibility based methods should be preferred., Comment: 14 Figures, 16 pages