• J. S. Bagla

      Articles written in Journal of Astrophysics and Astronomy

    • TreePM: A code for cosmological N-body simulations

      J. S. Bagla

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      We describe the TreePM method for carrying out large N-Body simulations to study formation and evolution of the large scale structure in the Universe. This method is a combination of Barnes and Hut tree code and Particle-Mesh code. It combines the automatic inclusion of periodic boundary conditions of PM simulations with the high resolution of tree codes. This is done by splitting the gravitational force into a short range and a long range component. We describe the splitting of force between these two parts. We outline the key differences between TreePM and some other N-Body methods.

    • Prospects of Detecting HI using Redshifted 21-cm Radiation at $z \sim 3$

      Bharat Kumar Gehlot J. S. Bagla

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      Distribution of cold gas in the post-reionization era provides an important link between distribution of galaxies and the process of star formation. Redshifted $21$-cm radiation from the hyperfine transition of neutral hydrogen allows us to probe the neutral component of cold gas, most of which is to be found in the interstellar medium of galaxies. Existing and upcoming radio telescopes can probe the large scale distribution of neutral hydrogen via HI intensity mapping. In this paper, we use an estimate of the HI power spectrum derived using an ansatz to compute the expected signal from the large scale HI distribution at $z \sim 3$. We find that the scale dependence of bias at small scales makes a significant difference to the expected signal even at large angular scales. We compare the predicted signal strength with the sensitivity of radio telescopes that can observe such radiation and calculate the observation time required for detecting neutral hydrogen at these redshifts. We find that OWFA (Ooty Wide Field Array) offers the best possibility to detect neutral hydrogen at $z \sim 3$ before the SKA (Square Kilometer Array) becomes operational. We find that the OWFA should be able to make a $3 \sigma$ or a more significant detection in $2000$ hours of observations at several angular scales. Calculations done using the Fisher matrix approach indicate that a $5\sigma$ detection of the binned HI power spectrum via measurement of the amplitude of the HI power spectrum is possible in $1000$ h (Sarkar et al. 2017).

    • Reconstruction of dynamical dark energy potentials: Quintessence, tachyon and interacting models


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      Dynamical models for dark energy are an alternative to the cosmological constant. It is important to investigate properties of perturbations in these models and go beyond the smooth FRLW cosmology. This allows us to distinguish different dark energy models with the same expansion history. For this, one oftenneeds the potential for a particular expansion history. We study how such potentials can be reconstructed by obtaining closed formulae for potential or reducing the problem to quadrature. We consider three classes of models here: tachyons, quintessence and interacting dark energy.We present results for the constant $\omega$ and the CPL parametrization. The method given here can be generalized to any arbitrary form of $\omega(z)$.

    • Upcoming SKA precursor surveys and sensitivity to HI mass function


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      We describe a simulation for the distribution of galaxies focusing on the atomic hydrogen content. Our aim is to make predictions for surveys of galaxies using the redshifted 21-cm line emission. We take the expected distribution of HI masses, circular velocities, sizes of galaxies and orientations into account for this simulation. We use the sensitivity of ASKAP and MeerKAT radio telescopes to estimate the number of detections of HI galaxies in upcoming surveys.We validate our simulation with earlier estimates carried out by using some of these considerations. We show that unlike earlier simulations that take some of the factors into account, the predicted number of galaxies and their distribution across masses changes significantly when allthese are accounted for. We describe our predictions for the MIGHTEE-HI and WALLABY surveys for blind detection of galaxies using the redshifted 21-cm radiation. We study the dependence of the predicted number of detections on the HI mass function. We also describe our future plans to improve the simulation.

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    • Continuous Article Publication

      Posted on January 27, 2016

      Since January 2016, the Journal of Astrophysics and Astronomy has moved to Continuous Article Publishing (CAP) mode. This means that each accepted article is being published immediately online with DOI and article citation ID with starting page number 1. Articles are also visible in Web of Science immediately. All these have helped shorten the publication time and have improved the visibility of the articles.

    • Editorial Note on Continuous Article Publication

      Posted on July 25, 2019

      Click here for Editorial Note on CAP Mode

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