Prasun Dutta

The scaleheight of NGC 1058 measured from its Hi power spectrum

We have measured the HI power spectrum of the nearly face-on spiral galaxy NGC 1058 from radio-interferometric observations using a visibility based estimator. The power spectrum is well fitted by two different power laws P(U) = AU � , one with α = − 2.5 ± 0.6 at small length-scales (600pcto1.5kpc) and another with α = − 1.0 ± 0.2 at large length-scales (1.5kpcto10.0kpc). We interpret this change in the slope of the power spectrum as a transition from 3D turbulence at small length-scales to 2D turbulence in the plane of the galaxy’s disk at large length-scales. We use the observed break in the power spectrum to estimate the galaxy’s scale-height, which we find to be 490 ± 90 pc.

H i power spectrum of the spiral galaxy NGC 628

We have measured the H I power spectrum of the nearly face-on spiral galaxy NGC 628 (M74) using a visibility-based estimator. The power spectrum is well fitted by a power law P(U) =AU α, with α=−1.6 ± 0.2 over the length-scale 800 pc to 8 kpc. The slope is found to be independent of the width of the velocity channel. This value of the slope is a little more than one in excess of what has been seen at considerably smaller length-scales in the Milky Way, the Small Magellanic Cloud, the Large Magellanic Cloud (LMC) and the dwarf galaxy DDO 210. We interpret this difference as indicating a transition from three-dimensional turbulence at small scales to two-dimensional turbulence in the plane of the galaxy disc at length-scales larger than the H i scaleheight of the galaxy. The slope measured here is similar to that found at large scales in the LMC. Our analysis also places an upper limit to the galaxy scaleheight at 800 pc.

H i 21 cm opacity fluctuations power spectra towards Cassiopeia A

The angular power spectrum of H I 21 cm opacity fluctuations is a useful statistic for quantifying the observed opacity fluctuations as well as for comparing these with theoretical models. We present here the H I 21 cm opacity fluctuation power spectrum towards the supernova remnant Cas A from interferometric data with spacial resolution of 5 arcsec and spectral resolution of 0.4 km s −1 . The power spectrum has been estimated using a simple but robust visibility-based technique. We find that the power spectrum is well fitted by a power law P τ (U) =Uα with a power-law index of α ~ ;−2.86 ± 0.10 (3s error) over the scales of 0.07-2.3 pc for the gas in the Perseus spiral arm and 0.002-0.07 pc (480-15 730 au) for that in the local arm. This estimated power-law index is consistent with earlier observational results based on both H I emission over larger scales and absorption studies over a similar range of scales. We do not detect any statistically significant change in the power-law index with the velocity width of the frequency channels. This constrains the power-law index of the velocity structure function to be β= 0.2 ± 0.6 (3σ error).

The role of feedback in shaping the structure of the interstellar medium

We present an analysis of the role of feedback in shaping the neutral hydrogen (H I) content of simulated disc galaxies. For our analysis, we have used two realizations of two separate Milky Way-like (similar to L star) discs - one employing a conservative feedback scheme (McMaster Unbiased Galaxy Survey), the other significantly more energetic [Making Galaxies In a Cosmological Context (MaGICC)]. To quantify the impact of these schemes, we generate zeroth moment (surface density) maps of the inferred H I distribution; construct power spectra associated with the underlying structure of the simulated cold interstellar medium, in addition to their radial surface density and velocity dispersion profiles. Our results are compared with a parallel, self-consistent, analysis of empirical data from The H I Nearby Galaxy Survey (THINGS). Single power-law fits (P proportional to k(gamma)) to the power spectra of the stronger feedback (MaGICC) runs (over spatial scales corresponding to similar to 0.5 to similar to 20 kpc) result in slopes consistent with those seen in the THINGS sample (gamma similar to -2.5). The weaker feedback (MUGS) runs exhibit shallower power-law slopes (gamma similar to -1.2). The power spectra of the MaGICC simulations are more consistent though with a two-component fit, with a flatter distribution of power on larger scales (i.e. gamma similar to -1.4 for scales in excess of similar to 2 kpc) and a steeper slope on scales below similar to 1 kpc (gamma similar to -5), qualitatively consistent with empirical claims, as well as our earlier work on dwarf discs. The radial H I surface density profiles of the MaGICC discs show a clear exponential behaviour, while those of the MUGS suite are essentially flat; both behaviours are encountered in nature, although the THINGS sample is more consistent with our stronger (MaGICC) feedback runs.

Steiner trees and spanning trees in six-pin soap films

The problem of finding minimum (local as well as absolute) path lengths joining given points (or terminals) on a plane is known as the Steiner problem. The Steiner problem arises in finding the minimum total road length joining several towns and cities. We study the Steiner tree problem using six-pin soap films. Experimentally, we observe spanning trees as well as Steiner trees partly by varying the pin diameter. We propose a possibly exact expression for the length of a spanning tree or a Steiner tree, which fails mysteriously in certain cases.

Magnetohydrodynamic turbulence in supernova remnants

We present estimates of the angular power spectra of the synchrotron radiation intensity fluctuations at 6 and 20 cm for the shell-type supernova remnant Cas A and the filled-centre Crab supernova remnant. We find that the intensity fluctuations of both the sources have a power-law power spectrum with index −3.24 ± 0.03. This power-law power spectrum is consistent with the magnetohydrodynamic turbulence in the synchrotron emitting plasma. For Cas A, there is a break in the power spectrum and the power-law index changes from −3.2 to −2.2 at large angular scale. This transition occurs at an angular scale that corresponds to the shell thickness of Cas A. We interpret this as a transition from three- to two-dimensional turbulence on scales that are, respectively, smaller and larger than the shell thickness.

The H i column density power spectrum of six nearby spiral galaxies

We propose a method to determine the power spectrum of \HI column density fluctuations using radio-interferometric observations of 21-cm emission from the ISM of galaxies. We have used this to estimate the power spectra of six nearly face on nearby spiral galaxies. Earlier work has shown that these power spectra are well fitted by power laws with slopes around -1.6 across length-scales

The effect of the w‐term on the visibility correlation and power spectrum estimation

\sim 1 \, {\rm kpc}

A study of interstellar medium of dwarf galaxies using H i power spectrum analysis

to

Probing interstellar turbulence in spiral galaxies using H i power spectrum analysis

\sim 10 \, {\rm kpc}