arXiv subjects

Prasun Dutta

Publications and source records attributed to Prasun Dutta.

At least 37 records · Page 2Linked to original sources

arXiv2021

Combined lensed estimator to probe the post reionization H I power spectrum

In the post-reionization era, the baryons assembled into the protogalaxies and eventually the present population of the galaxies evolved through merger and evolution. In this work, we discuss a possible probe of the statistical distribution and evolution of the H I density in the post reionization era. We introduce an estimator of the H I power spectrum from the post reionization universe by observing it through the strong gravitational lenses by the nearby galaxy cluster. We also analytically calculate the uncertainties associated with the estimates of the post-EoR power spectrum for the discussed estimator. We access the efficacy of this estimator in the context of 19 galaxy clusters for which the lensing potential has been estimated earlier by various authors. We find that by combining the lensed power spectrum through eight of these cluster lenses, it is possible to estimate the post-reionization H I power spectrum at five-sigma significance for angular multipoles < 4000 for a uGMRT observation of 16 MHz bandwidth from redshifts of 1.25, 1.5 with a total of 400 hours of observation. With the same setup, for a redshift of 3.0, we need 200 hours of total observation time. The estimator also suppresses the diffused galactic foreground, though, the latter is still a dominant contributor to the overall signal and hence need to be estimated and mitigated. We discuss the merits and demerits of the estimator.

astro-ph.CO↗

arXiv2020

First multi-redshift limits on post-Epoch of Reionization (post-EoR) 21 cm signal from z = 1.96 - 3.58 using uGMRT

Measurement of fluctuations in diffuse HI 21 cm background radiation from the post-reionization epoch (z < 6) is a promising avenue to probe the large-scale structure of the Universe and understand the evolution of galaxies. We observe the European Large-Area ISO Survey-North 1 (ELAIS-N1) field at 300-500 MHz using the upgraded Giant Meterwave Radio Telescope (uGMRT) and employ the 'foreground avoidance' technique to estimate the HI 21 cm power spectrum in the redshift range z = 1.96-3.58. Given the possible systematics that may remain in the data, we find the most stringent upper limits on the spherically averaged 21 cm power spectra at k~1.0 Mpc$^{-1}$ are (58.87 mK)$^2$, (61.49 mK)$^2$, (60.89 mK)$^2$, (105.85 mK)$^2$ at z = 1.96,2.19,2.62 and 3.58, respectively. We use this to constrain the product of neutral HI mass density (Omega_HI) and HI bias (b_HI) to the underlying dark matter density field, [Omega_HI*b_HI], as 0.09,0.11,0.12,0.24 at z=1.96,2.19,2.62,3.58, respectively. To the best of our knowledge these are the first limits on the HI 21 cm power spectra at the redshift range z = 1.96 - 3.58 and would play a significant role to constrain the models of galaxy formation and evolution.

astro-ph.CO↗

arXiv2020

Using strong gravitational lensing to probe the post reionization \HI power spectrum

Probing statistical distribution of the neutral hydrogen (\HI) using the redshifted 21-cm hyperfine-transition spectral line holds the key to understand the formation and evolution of the matter density in the universe. The two-point statistics of the \HI distribution can be estimated by measuring the power spectrum of the redshifted 21-cm signal using visibility correlation. A major challenge in this regard is that the expected signal is weak compared to the foreground contribution from the Galactic synchrotron emission and extragalactic point sources in the observing frequencies. In this work, we investigate the possibility of detecting the power spectrum of the redshifted 21-cm signal by using strong gravitational lensing of the galaxy clusters. This method has the advantage that it only enhances the \HI signal and not the diffuse galactic foreground. Based on four simple models of the cluster potentials we show that the strong lenses at relatively lower redshifts with more than one dark matter halo significantly enhances the 21-cm signal from the post reionization era. We discuss the merits and demerits of the method and the future studies required for further investigations.

astro-ph.CO↗

arXiv2020

Evidence of large scale energy cascade in the spiral galaxy NGC 5236

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.

astro-ph.GA↗

arXiv2020

Calibration Requirements for Epoch of Reionization 21-cm signal observations -- I. Effect of time-correlated gains

The residual gain errors add to the systematics of the radio interferometric observations. In case of the high dynamic range observations, these systematic effects dominates over the thermal noise of the observation. In this work, we investigate the effect of time-correlated residual gain errors in the estimation of the power spectrum of the sky brightness distribution in high dynamic range observations. Particularly, we discuss a methodology to estimate the bias in the power spectrum estimator of the redshifted 21-cm signal from neutral hydrogen in the presence of bright extragalactic compact sources. We find, that for the visibility based power spectrum estimators, particularly those use nearby baseline correlations to avoid noise bias, the bias in the power spectrum arises mainly from the time correlation in the residual gain error. The bias also depends on the baseline distribution for a particular observation. Analytical calculations show that the bias is dominant for certain types of baseline pairs used for the visibility correlation. We perform simulated observation of extragalactic compact sources in the presence of residual gain errors with the GMRT like array and estimate the bias in the power spectrum. Our results indicate that in order to estimate the redshifted 21-cm power spectrum, better calibration techniques and estimator development are required.

astro-ph.IM↗

arXiv2020

The study of the angular and spatial distribution of radio selected AGNs and star-forming galaxies in the ELAIS N1 field

The cosmic evolution of bias of different source populations with underlying dark matter density field in post reionization era can shed light on large scale structures. Studying the angular and spatial distribution of different compact sources using deep radio catalogue at low-frequency is essential to understand the matter distribution of the present Universe. Here, we investigate the relationship of luminous matter with their host dark matter haloes by measuring the angular and spatial clustering of sources (two-point statistics), using deep radio observation of ELAIS N1 (EN1) field with upgraded Giant Metrewave Radio Telescope (uGMRT) at 300-500 MHz. We also analyze the 612 MHz GMRT archival data of the same field to understand the cosmic evolution of clustering of different source populations. We classify the sources as star-forming galaxies (SFGs) and active galactic nuclei (AGN) based on their radio luminosity. We find that the spatial clustering length and bias to the dark matter density field of SFGs are smaller than AGNs at both frequencies. This proves that AGNs are mainly hosted by massive haloes and hence strongly clustered. However, a small decrease in the bias for both kind of sources at higher frequency indicates that we are most likely tracing the faint objects residing in less massive haloes at higher frequencies. Our results are in excellent agreement with previous findings at radio and multi-frequency surveys. However, comparison with SKADS simulation suggests that the halo mass for different populations used in the simulation is systematically lower. This work quantifies the spatial distribution of extragalactic compact objects in EN1 field and bridges the gap between shallow and deep surveys.

astro-ph.GA↗

arXiv2019

Detailed study of ELAIS N1 field with the uGMRT -- II. Source Properties and Spectral Variation Of Foreground Power Spectrum from 300-500 MHz Observations

Understanding the low-frequency radio sky in depth is necessary to subtract foregrounds in order to detect the redshifted 21 cm signal of neutral hydrogen from the Cosmic Dawn, Epoch of Reionization (EoR) and post-reionization era. In this second paper of the series, we present the upgraded Giant Metrewave Radio Telescope (uGMRT) observation of the ELAIS N1 field made at 300-500 MHz. The image covers an area of $\sim 1.8$ $\mathrm{deg}^{2}$ and has a central background rms noise of $\sim$ 15 $μ\mathrm{Jy}$ $\mathrm{beam}^{-1}$. We present a radio source catalog containing 2528 sources (with flux densities > 100 $μ$Jy) and normalized source counts derived from that. The detailed comparison of detected sources with previous radio observations is shown. We discuss flux scale accuracy, positional offsets, spectral index distribution and correction factors in source counts. The normalized source counts are in agreement with previous observations of the same field, as well as model source counts from the Square Kilometre Array Design Study (SKADS) simulation. It shows a flattening below $\sim$1 mJy which corresponds to rise in population of star forming galaxies and radio-quiet AGN. For the first time, we estimated the spectral characteristics of the angular power spectrum or Multi-Frequency Angular Power Spectrum (MFAPS) of diffuse Galactic synchrotron emission (DGSE) over the wide frequency bandwidth of $300-500$~MHz from radio interferometric observations. This work demonstrates the improved capabilities of the uGMRT.

astro-ph.CO↗

arXiv2019

Detailed study of the ELAIS N1 field with the uGMRT - I. Characterizing the 325 MHz foreground for redshifted 21 cm observations

In this first paper of the series, we present initial results of newly upgraded Giant Meterwave Radio Telescope (uGMRT) observation of European Large-Area ISO Survey-North 1 (ELAIS-N1) at 325 MHz with 32 MHz bandwidth. Precise measurement of fluctuations in Galactic and extragalactic foreground emission as a function of frequency as well as angular scale is necessary for detecting redshifted 21-cm signal of neutral hydrogen from Cosmic Dawn, Epoch of Reionization (EoR) and post-reionization epoch. Here, for the first time we have statistically quantified the Galactic and extragalactic foreground sources in the ELAIS-N1 field in the form of angular power spectrum using the newly developed Tapered Gridded Estimator (TGE). We have calibrated the data with and without direction-dependent calibration techniques. We have demonstrated the effectiveness of TGE against the direction dependent effects by using higher tapering of field of view (FoV). We have found that diffuse Galactic synchrotron emission (DGSE) dominates the sky, after point source subtraction, across the angular multipole range $ 1115 \leqslant \mathcal{\ell} \leqslant 5083 $ and $ 1565 \leqslant \mathcal{\ell} \leqslant 4754 $ for direction-dependent and -independent calibrated visibilities respectively. The statistical fluctuations in DGSE has been quantified as a power law of the form $\mathcal{C}_{\mathcal{\ell}}= A \mathcal{\ell}^{-β} $. The best fitted values of (A, $β$) are ($ 62 \pm 6$ $mK^{2}$, $2.55 \pm 0.3 $) and ($ 48 \pm 4$ $mK^{2}$, $2.28 \pm 0.4 $ ) for the two different calibration approaches. For both the cases, the power law index is consistent with the previous measurements of DGSE in other parts of sky.

astro-ph.CO↗

arXiv2018

A Tapered Gridded Estimator (TGE) for the Multi-Frequency Angular Power Spectrum (MAPS) and the Cosmological HI 21-cm Power Spectrum

In this work we present a new approach to estimate the power spectrum $P({\bf k})$ of redshifted HI 21-cm brightness temperature fluctuations. The MAPS $C_{\ell}(ν_a,ν_b)$ completely quantifies the second order statistics of the sky signal under the assumption that the signal is statistically homogeneous and isotropic on the sky. Here we generalize an already existing visibility based estimator for $C_{\ell}$, namely TGE, to develop an estimator for $C_{\ell}(ν_a,ν_b)$. The 21-cm power spectrum is the Fourier transform of $C_{\ell}(Δν)$ with respect to $Δν=\mid ν_a-ν_b \mid$, and we use this to estimate $P({\bf k})$. Using simulations of $150 \, {\rm MHz}$ GMRT observations, we find that this estimator is able to recover $P(k)$ with an accuracy of $5-20 \%$ over a reasonably large $k$ range even when the data in $80 \%$ randomly chosen frequency channels is flagged.

astro-ph.CO↗

arXiv2018

An Image-based Tapered Gridded Estimator (ITGE) for the angular power spectrum

We present the Image-based Tapered Gridded Estimator (ITGE) to measure the angular power spectrum $(C_{\ell})$ of the sky signal directly from the visibilities measured in radio-interferometric observations. The ITGE allows us to modulate the sky response through a window function which is implemented in the image plane, and it is possible to choose a wide variety of window functions. In the context of the cosmological HI 21-cm signal, this is useful for masking out the sky signal from specific directions which have strong residual foregrounds. In the context of the ISM in external galaxies, this is useful to separately estimate the $C_{\ell}$ of different parts of the galaxy. The ITGE deals with gridded data, hence it is computationally efficient. It also calculates the noise bias internally and exactly subtracts this out to give an unbiased estimate of $C_{\ell}$. We validate the ITGE using realistic VLA simulations at $1.4 {\rm GHz}$. We have applied the ITGE to estimate the $C_{\ell}$ of HI 21-cm emission from different regions of the galaxy NGC 628. We find that the slope of the measured $C_{\ell}$ in the outer region is significantly different as compared with the inner region. This indicates that the statistical properties of ISM turbulence possibly differ in different regions of the galaxy.

astro-ph.CO↗

arXiv2018

Estimating statistics of sky brightness using radio interferometric observations

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.

astro-ph.IM↗

arXiv2017

Validating a novel angular power spectrum estimator using simulated low frequency radio-interferometric data

The "Tapered Gridded Estimator" (TGE) is a novel way to directly estimate the angular power spectrum from radio-interferometric visibility data that reduces the computation by efficiently gridding the data, consistently removes the noise bias, and suppresses the foreground contamination to a large extent by tapering the primary beam response through an appropriate convolution in the visibility domain. Here we demonstrate the effectiveness of TGE in recovering the diffuse emission power spectrum through numerical simulations. We present details of the simulation used to generate low frequency visibility data for sky model with extragalactic compact radio sources and diffuse Galactic synchrotron emission. We then use different imaging strategies to identify the most effective option of point source subtraction and to study the underlying diffuse emission. Finally, we apply TGE to the residual data to measure the angular power spectrum, and assess the impact of incomplete point source subtraction in recovering the input power spectrum $C_{\ell}$ of the synchrotron emission. This estimator is found to successfully recovers the $C_{\ell}$ of input model from the residual visibility data. These results are relevant for measuring the diffuse emission like the Galactic synchrotron emission. It is also an important step towards characterizing and removing both diffuse and compact foreground emission in order to detect the redshifted $21\, {\rm cm}$ signal from the Epoch of Reionization.

astro-ph.CO↗

arXiv2015

Insights into the evolution of symbiotic recurrent novae from radio synchrotron emission: V745 Scorpii and RS Ophiuchi

We present observations at 610 MHz and 235 MHz using the Giant Metrewave Radio Telescope (GMRT) of the recurrent nova V745 Scorpii which recorded its last outburst on 6 February 2014. This is the second symbiotic recurrent nova whose light curve at low frequencies has been followed in detail, the first being RS Ophiuchi in 2006. We fitted the 610 MHz light curve by a model of synchrotron emission from an expanding shell being modified by radiative transfer effects due to local absorbing gas consisting of a uniformly distributed and a clumpy component. Using our model parameters, we find that the emission at 235 MHz peaked around day 35 which is consistent with our GMRT observations. The two main results of our study are: (1) The radio emission at a given frequency is visible sooner after the outburst in successive outbursts of both V745 Scorpii and RS Ophiuchi. The earlier detection of radio emission is interpreted to be caused by decreasing foreground densities. (2) The clumpy material, if exists, is close to the white dwarf and can be interpreted as being due to the material from the hot accretion disk. The uniform density gas is widespread and attributed to the winds blown by the white dwarf. We present implications of these results on the evolution of both novae. Such studies alongwith theoretical understanding have the potential of resolving several outstanding issues such as why all recurrent novae are not detectable in synchrotron radio and whether recurrent novae are progenitor systems of type 1a supernova.

astro-ph.HE↗

arXiv2015

Visibility moments and power spectrum of turbulence velocity

Here we introduce moments of visibility function and discuss how those can be used to estimate the power spectrum of the turbulent velocity of external spiral galaxies. We perform numerical simulation to confirm the credibility of this method and found that for galaxies with lower inclination angles it works fine. This is the only method to estimate the power spectrum of the turbulent velocity fluctuation in the ISM of the external galaxies.

astro-ph.GA↗

arXiv2015

Effect of turbulent velocity on the \HI intensity fluctuation power spectrum from spiral galaxies

We use numerical simulations to investigate effect of turbulent velocity on the power spectrum of \HI intensity from external galaxies when (a) all emission is considered, (b) emission with velocity range smaller than the turbulent velocity dispersion is considered. We found that for case (a) the intensity fluctuation depends directly only on the power spectrum of the column density, whereas for case (b) it depends only on the turbulent velocity fluctuation. We discuss the implications of this result in real observations of \HI fluctuations.

astro-ph.GA↗

arXiv2015

The structure function of Galactic \HI opacity fluctuations on AU scales based on MERLIN, VLA and VLBA data

We use MERLIN, VLA and VLBA observations of Galactic \HI absorption towards 3C~138 to estimate the structure function of the \HI opacity fluctuations at AU scales. Using Monte Carlo simulations, we show that there is likely to be a significant bias in the estimated structure function at signal-to-noise ratios characteristic of our observations, if the structure function is constructed in the manner most commonly used in the literature. We develop a new estimator that is free from this bias and use it to estimate the true underlying structure function slope on length scales ranging $5$ to $40$~AU. From a power law fit to the structure function, we derive a slope of $0.81^{+0.14}_{-0.13}$, i.e. similar to the value observed at parsec scales. The estimated upper limit for the amplitude of the structure function is also consistent with the measurements carried out at parsec scales. Our measurements are hence consistent with the \HI opacity fluctuation in the Galaxy being characterized by a power law structure function over length scales that span six orders of magnitude. This result implies that the dissipation scale has to be smaller than a few AU if the fluctuations are produced by turbulence. This inferred smaller dissipation scale implies that the dissipation occurs either in (i) regions with densities $\gtrsim 10^3 $cm$^-3$ (i.e. similar to that inferred for "tiny scale" atomic clouds or (ii) regions with a mix of ionized and atomic gas (i.e. the observed structure in the atomic gas has a magneto-hydrodynamic origin).

astro-ph.GA↗

arXiv2013

The \HI 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 $\sim 1 \, {\rm kpc}$ to $\sim 10 \, {\rm kpc}$, the amplitude however was undetermined. In the present work we have determined the amplitude of the \HI column density power spectrum. We find that the \HI column density $N_{\rm HI}$ expressed in units of $10^{20} \, {\rm cm}^{-2}$ has mean square fluctuations in the range $\sim 0.03$ to $\sim 20$. The amplitude of the power spectrum is found to be tightly correlated with the \HI mass fraction of the galaxies. The physical process responsible for these scale-invariant fluctuations is, however, at present not known.

astro-ph.GA↗

arXiv2012

Probing interstellar turbulence in spiral galaxies using HI power spectrum analysis

We estimate the \HI intensity fluctuation power spectrum for a sample of 18 spiral galaxies chosen from THINGS. Our analysis spans a large range of length-scales from $\sim 300 {\rm pc}$ to $\sim 16 {\rm kpc}$ across the entire galaxy sample. We find that the power spectrum of each galaxy can be well fitted by a power law $P_{\rm HI}(U) = A U^α$, with an index $α$ that varies from galaxy to galaxy. For some of the galaxies the scale-invariant power-law power spectrum extends to length-scales that are comparable to the size of the galaxy's disk. The distribution of $α$ is strongly peaked with 50% of the values in the range $α=-1.9$ to 1.5, and a mean and standard deviation of -1.3 and 0.5 respectively. We find no significant correlation between $α$ and the star formation rate, dynamical mass, \HI mass or velocity dispersion of the galaxies. Several earlier studies that have measured the power spectrum within our Galaxy on length-scales that are considerably smaller than $500 {\rm pc}$ have found a power-law power spectrum with $α$ in the range $\approx -2.8$ to -2.5. We propose a picture where we interpret the values in the range $\approx -2.8$ to -2.5 as arising from three dimensional (3D) turbulence in the Interstellar Medium (ISM) on length-scales smaller than the galaxy's scale-height, and we interpret the values in the range $\approx -1.9$ to -1.5 measured in this paper as arising from two-dimensional ISM turbulence in the plane of the galaxy's disk. It however still remains a difficulty to explain the small galaxy to galaxy variations in the values of $α$ measured here.

astro-ph.GA↗