Papers by Meera Nandakumar
Journal of Astrophysics and Astronomy
One of the major science goals of the Square Kilometre Array (SKA) is to understand the role play... more One of the major science goals of the Square Kilometre Array (SKA) is to understand the role played by atomic hydrogen (H i) 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 H i gas associated with galaxies. In this article, we review our current understanding of H i 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 H i science that will be possible with SKA and its precursors and pathfinders, i.e. H i 21-cm emission and absorption studies of galaxies from nearby to high redshifts that will trace various processes governing galaxy evolution.
arXiv (Cornell University), Sep 19, 2022
One of the major science goals of the Square Kilometre Array (SKA) is to understand the role play... more One of the major science goals of the Square Kilometre Array (SKA) is to understand the role played by atomic hydrogen (H i) 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 H i gas associated with galaxies. In this article, we review our current understanding of H i 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 H i science that will be possible with SKA and its precursors and pathfinders, i.e. H i 21-cm emission and absorption studies of galaxies from nearby to high redshifts that will trace various processes governing galaxy evolution.
Monthly Notices of the Royal Astronomical Society
Detection of bending waves is a highly challenging task even in nearby disc galaxies due to their... more 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 H i discs, as they extend too much beyond the optical radii. We present a multipole analysis of the H i 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 ...
Research in Astronomy and Astrophysics, 2019
Radio interferometric data are used to estimate the sky brightness distributions in radio frequen... more 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.
arXiv: Astrophysics of Galaxies, 2018
Turbulent dynamics generate random fluctuations in density and velocity in the Interstellar Mediu... more 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...
Turbulence plays an important role in the structure and dynamics of the galaxy and influences var... more 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±0.06 for the column density and -1.91±0.08 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 ∼ 7 × 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...
Large scale coherent density and velocity structure measurement in the Interstellar medium of NGC 5236
2020 URSI Regional Conference on Radio Science ( URSI-RCRS), 2020
We present here our preliminary results on measurement of H I coherent density and velocity struc... more We present here our preliminary results on measurement of H I coherent density and velocity structures in the ISM of external spiral galaxy NGC 5236. Our estimations are totally unbiased, as it is using moments of directly measured visibility from radio interferometer. Our result on density power spectrum estimation indicates the presence of scale invariant structures over a large length scales of two order of magnitude. Also our results suggest that the average scale height of NGC 5236 is less than 200pc. These result are giving insight to the generating mechanism behind the ISM turbulence which is still not known yet.
Evidence of large-scale energy cascade in the spiral galaxy NGC 5236
Monthly Notices of the Royal Astronomical Society
Turbulence plays an important role in the structure and dynamics of the galaxy and influences var... more 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 Very Large Array (VLA) multiconfiguration 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 interstellar medium turbulence is found to be around 6 kpc. Power spectra of the two-dimensional turbulence in the galaxy’s disc follow a power law with a slope −1.23 ± 0.06 for the column density and −1.91 ± 0.08 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 ∼7 × 10−11 erg cm −2 s−1. We conclude that much of these originate from the gravitational instabilities and...
Evidence of large-scale energy cascade in the spiral galaxy NGC 5236
Monthly Notices of the Royal Astronomical Society
Turbulence plays an important role in the structure and dynamics of the galaxy and influences var... more 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 Very Large Array (VLA) multiconfiguration 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 interstellar medium turbulence is found to be around 6 kpc. Power spectra of the two-dimensional turbulence in the galaxy’s disc follow a power law with a slope −1.23 ± 0.06 for the column density and −1.91 ± 0.08 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 ∼7 × 10−11 erg cm −2 s−1. We conclude that much of these originate from the gravitational instabilities and...
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Papers by Meera Nandakumar