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Kanan K. Datta

Publications and source records attributed to Kanan K. Datta.

At least 37 records · Page 2Linked to original sources
arXiv2020

Primordial magnetic fields during the cosmic dawn in light of EDGES 21-cm signal

We study prospects of constraining the primordial magnetic field (PMF) and its evolution during the dark ages and cosmic dawn in light of EDGES 21-cm signal. Our analysis has been carried out on a `colder IGM' background which is one of the promising avenues to interpret the EDGES signal. We consider the dark matter-baryon interactions for the excess cooling. We find that the colder IGM suppresses both the residual free electron fraction and the coupling coefficient between the ionised and neutral components. The Compton heating also gets affected in colder IGM background. Consequently, the IGM heating rate due to the PMF enhances compared to the standard scenario. Thus, a significant fraction of the magnetic energy, for $B_0 \lesssim 0.5 \, {\rm nG}$, gets transferred to the IGM and the magnetic field decays at a much faster rate compared to the simple $(1+z)^2$ scaling during the dark ages and cosmic dawn. This low PMF is an unlikely candidate for explaining the rise of the EDGES absorption signal at lower redshift. We also see that the PMF and DM-baryon interaction together introduces a plateau-like feature in the redshift evolution of the IGM temperature. We find that the upper limit on the PMF depends on the underlying DM-baryon interaction. Higher PMF can be allowed when the interaction cross-section is higher and/or the DM particle mass is lower. Our study shows that the PMF with $B_0$ up to $\sim 0.4 \, {\rm nG}$, which is ruled out in the standard model, can be allowed if DM-baryon interaction with suitable cross-section and DM mass is considered.

astro-ph.CO↗
arXiv2020

Predictions for measuring the 21-cm multi-frequency angular power spectrum using SKA-Low

The light-cone (LC) effect causes the mean as well as the statistical properties of the redshifted 21-cm signal $T_{\rm b}(\hat{\bf n},ν)$ to change with frequency $ν$ (or cosmic time). Consequently, the statistical homogeneity (ergodicity) of the signal along the line of sight (LoS) direction is broken. This is a severe problem particularly during the Epoch of Reionization (EoR) when the mean neutral hydrogen fraction ($\bar{x}_{\rm HI}$) changes rapidly as the universe evolves. This will also pose complications for large bandwidth observations. These effects imply that the 3D power spectrum $P(k)$ fails to quantify the entire second-order statistics of the signal as it assumes the signal to be ergodic and periodic along the LoS. As a proper alternative to $P(k)$, we use the multi-frequency angular power spectrum (MAPS) ${\mathcal C}_{\ell}(ν_1,ν_2)$ which does not assume the signal to be ergodic and periodic along the LoS. Here, we study the prospects for measuring the EoR 21-cm MAPS using future observations with the upcoming SKA-Low. Ignoring any contribution from the foregrounds, we find that the EoR 21-cm MAPS can be measured at a confidence level $\ge 5σ$ at angular scales $\ell \sim 1300$ for total observation time $t_{\rm obs} \ge 128\,{\rm hrs}$ across $\sim 44\,{\rm MHz}$ observational bandwidth. We also quantitatively address the effects of foregrounds on MAPS detectability forecast by avoiding signal contained within the foreground wedge in $(k_\perp, k_\parallel)$ plane. These results are very relevant for the upcoming large bandwidth EoR experiments as previous predictions were all restricted to individually analyzing the signal over small frequency (or equivalently redshift) intervals.

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arXiv2020

Bubble mapping with the Square Kilometer Array -- I. Detecting galaxies with Euclid, JWST, WFIRST and ELT within ionized bubbles in the intergalactic medium at z>6

The Square Kilometer Array is expected to provide the first tomographic observations of the neutral intergalactic medium at redshifts z>6 and pinpoint the locations of individual ionized bubbles during early stages of cosmic reionization. In scenarios where star-forming galaxies provide most of the ionizing photons required for cosmic reionization, one expects the first ionized bubbles to be centered on overdensities of such galaxies. Here, we model the properties of galaxy populations within isolated, ionized bubbles that SKA-1 should be able to resolve at z=7-10, and explore the prospects for galaxy counts within such structures with various upcoming near-infrared telescopes. We find that, for the bubbles that are within reach of SKA-1 tomography, the bubble volume is closely tied to the number of ionizing photons that have escaped from the galaxies within. In the case of galaxy-dominated reionization, galaxies are expected to turn up above the spectroscopic detection threshold of JWST and ELT in even the smallest resolvable bubbles at redshifts z=10 or below. The prospects of detecting galaxies within these structures in purely photometric surveys with Euclid, WFIRST, JWST or ELT are also discussed. While spectroscopy is preferable towards the end of reionization to provide a robust sample of bubble members, multiband imaging may be a competitive option for bubbles at z~10, due to the very small number of line-of-sight interlopers expected at similar redshifts.

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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.

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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.

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arXiv2018

A method to determine the evolution history of the mean neutral Hydrogen fraction

The light-cone (LC) effect imprints the cosmological evolution of the redshifted 21-cm signal $T_{\rm b} ({\hat{\bf{n}}}, ν)$ along the frequency axis which is the line of sight (LoS) direction of an observer. The effect is particularly pronounced during the Epoch of Reionization (EoR) when the mean hydrogen neutral fraction ${\bar{x}_{\rm HI}}(ν)$ falls rapidly as the universe evolves. The multi-frequency angular power spectrum (MAPS) ${{\mathcal C}_{\ell}}(ν_1,ν_2)$ quantifies the entire second-order statistics of $T_{\rm b} ({\hat{\bf{n}}}, ν)$ considering both the systematic variation along $ν$ due to the cosmological evolution and also the statistically homogeneous and isotropic fluctuations along all the three spatial directions encoded in ${\hat{\bf{n}}}$ and $ν$. Here we propose a simple model where the systematic frequency $(ν_1,ν_2)$ dependence of ${{\mathcal C}_{\ell}}(ν_1,ν_2)$ arises entirely due to the evolution of ${\bar{x}_{\rm HI}}(ν)$. This provides a new method to observationally determine the reionization history. Considering a LC simulation of the EoR 21-cm signal, we use the diagonal elements $ν_1=ν_2$ of ${{\mathcal C}_{\ell}}(ν_1,ν_2)$ to validate our model. We demonstrate that it is possible to recover the reionization history across the entire observational bandwidth provided we have the value ${\bar{x}_{\rm HI}}$ at a single frequency as an external input.

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arXiv2018

Impact of inhomogeneous CMB heating of gas on the HI 21-cm signal during dark ages

Observations of redshifted 21-cm signal from neutral hydrogen (HI) appear to be the most promising probe of the cosmic dark ages. The signal carries information about the thermal state along with density distribution of the intergalactic medium (IGM). The cosmic microwave background radiation (CMBR), through its interaction with charged particles, plays a major role in determining the kinetic and spin temperature of HI gas in the IGM during dark ages. A Spatially fluctuating ionization fraction, which is caused by inhomogeneous recombinations, causes heat transfer from the CMBR to the IGM gas inhomogeneous. We revisit the impact of this inhomogeneous heat transfer on spatial fluctuations in the observed HI 21-cm signal over a large redshift range during dark ages. Our study shows that the effect negatively impacts fluctuations in the HI spin temperature and results in an enhanced HI 21-cm power spectrum. We find that the effect is particularly important during the transition of the gas kinetic temperature being coupled to the CMBR to fully decoupled from it, i.e., in the redshift range $30 \lesssim z \lesssim 300$. It is found that, on the average the HI power spectrum, $P_{T_b}(k, z)$ is enhanced by $\sim 4\%$, $\sim10 \%$ , $\sim 20\%$, and $\sim 30 \%$ at redshifts $60$, $90$, $140$, and $200$ respectively at $k=0.1 \, {\rm Mpc}^{-1}$. The effect becomes even more significant at lower values of $k_{\parallel}^2/k^2$ due to the reduced dominance of the peculiar velocity. It is observed that the power spectrum is enhanced by $\sim 49\%$ and $\sim 93\%$ at redshifts $140$ and $200$ respectively at $k=0.1 \, {\rm Mpc}^{-1}$ for $k_{\parallel}^2/k^2=0$. This enhancement has a weak $k$-mode dependence.

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arXiv2017

Towards simulating and quantifying the light-cone EoR 21-cm signal

The light-cone (LC) effect causes the Epoch of Reionization (EoR) 21-cm signal $T_{\rm b} (\hat{\bf{n}}, ν)$ to evolve significantly along the line of sight (LoS) direction $ν$. In the first part of this paper, we present a method to properly incorporate the LC effect in simulations of the EoR 21-cm signal that include peculiar velocities. Subsequently, we discuss how to quantify the second order statistics of the EoR 21-cm signal in the presence of the LC effect. We demonstrate that the 3D power spectrum $P({\bf{k}})$ fails to quantify the entire information because it assumes the signal to be ergodic and periodic, whereas the LC effect breaks these conditions along the LoS. Considering a LC simulation centered at redshift $8$ where the mean neutral fraction drops from $0.65$ to $0.35$ across the box, we find that $P({\bf{k}})$ misses out $\sim 40 \%$ of the information at the two ends of the $17.41 \, {\rm MHz}$ simulation bandwidth. The multi-frequency angular power spectrum (MAPS) ${\mathcal C}_{\ell}(ν_1,ν_2)$ quantifies the statistical properties of $T_{\rm b} (\hat{\bf{n}}, ν)$ without assuming the signal to be ergodic and periodic along the LoS. We expect this to quantify the entire statistical information of the EoR 21-cm signal. We apply MAPS to our LC simulation and present preliminary results for the EoR 21-cm signal.

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arXiv2017

Prediction of the 21-cm signal from reionization: comparison between 3D and 1D radiative transfer schemes

Three-dimensional radiative transfer simulations of the epoch of reionization can produce realistic results, but are computationally expensive. On the other hand, simulations relying on one-dimensional radiative transfer solutions are faster but limited in accuracy due to their more approximate nature. Here, we compare the performance of the reionization simulation codes grizzly and C2-ray which use 1D and 3D radiative transfer schemes respectively. The comparison is performed using the same cosmological density fields, halo catalogues and source properties. We find that the ionization maps, as well as the 21-cm signal maps from these two simulations are very similar even for complex scenarios which include thermal feedback on low mass halos. The comparison between the schemes in terms of the statistical quantities such as the power spectrum of the brightness temperature fluctuation agree with each other within 10% error throughout the entire reionization history. grizzly seems to perform slightly better than the semi-numerical approaches considered in Majumdar et al. (2014) which are based on the excursion set principle. We argue that grizzly can be efficiently used for exploring parameter space, establishing observations strategies and estimating parameters from 21-cm observations.

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arXiv2016

Line of sight anisotropies in the Cosmic Dawn and EoR 21-cm power spectrum

The line of sight direction in the redshifted 21-cm signal coming from the cosmic dawn and the epoch of reionization is quite unique in many ways compared to any other cosmological signal. Different unique effects, such as the evolution history of the signal, non-linear peculiar velocities of the matter etc will imprint their signature along the line of sight axis of the observed signal. One of the major goals of the future SKA-LOW radio interferometer is to observe the cosmic dawn and the epoch of reionization through this 21-cm signal. It is thus important to understand how these various effects affect the signal for it's actual detection and proper interpretation. For more than one and half decades, various groups in India have been actively trying to understand and quantify the different line of sight effects that are present in this signal through analytical models and simulations. In many ways the importance of this sub-field under 21-cm cosmology have been identified, highlighted and pushed forward by the Indian community. In this article we briefly describe their contribution and implication of these effects in the context of the future surveys of the cosmic dawn and the epoch of reionization that will be conducted by the SKA-LOW.

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arXiv2016

Probing individual sources during reionization and cosmic dawn using SKA HI 21-cm observations

Detection of individual luminous sources during the reionization epoch and cosmic dawn through their signatures in the HI 21-cm signal is one of the direct approaches to probe the epoch. Here, we summarize our previous works on this and present preliminary results on the prospects of detecting such sources using the SKA1-low experiment. We first discuss the expected HI 21-cm signal around luminous sources at different stages of reionization and cosmic dawn. We then introduce two visibility based estimators for detecting such signal: one based on the matched filtering technique and the other relies on simply combing the visibility signal from different baselines and frequency channels. We find that that the SKA1-low should be able to detect ionized bubbles of radius $R_b \gtrsim 10$ Mpc with $\sim 100$ hr of observations at redshift $z \sim 8$ provided that the mean outside neutral Hydrogen fraction $ x_{\rm HI} \gtrsim 0.5$. We also investigate the possibility of detecting HII regions around known bright QSOs such as around ULASJ1120+0641 discovered by Mortlock et al. 2011. We find that a $5 σ$ detection is possible with $600$ hr of SKA1-low observations if the QSO age and the outside $ x_{\rm HI} $ are at least $\sim 2 \times 10^7$ Myr and $\sim 0.2$ respectively. Finally, we investigate the possibility of detecting the very first X-ray and Ly-$α$ sources during the cosmic dawn. We consider mini-QSOs like sources which emits in X-ray frequency band. We find that with a total $\sim 1000$ hr of observations, SKA1-low should be able to detect those sources individually with a $\sim 9 σ$ significance at redshift $z=15$. We summarize how the SNR changes with various parameters related to the source properties.

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arXiv2016

Imaging the redshifted 21-cm pattern around the first sources during the cosmic dawn using the SKA

Understanding properties of the first sources in the Universe using the redshifted \HI ~21-cm signal is one of the major aims of present and upcoming low-frequency experiments. We investigate the possibility of imaging the redshifted 21-cm pattern around the first sources during the cosmic dawn using the SKA1-low. We model the \HI ~21-cm image maps, appropriate for the SKA1-low, around the first sources consisting of stars and X-ray sources within galaxies. In addition to the system noise, we account also for the astrophysical foregrounds by adding them to the signal maps. We find that after subtracting the foregrounds using a polynomial fit and suppressing the noise by smoothing the maps over $10^{'} - 30^{'}$ angular scale, the isolated sources at $z \sim 15$ are detectable with $\sim 4 - 9 \, σ$ confidence level in 2000 h of observation with the SKA1-low. Although the 21-cm profiles around the sources get altered because of the Gaussian smoothing, the images can still be used to extract some of the source properties. We account for overlaps in the patterns of the individual sources by generating realistic \HI ~21-cm maps of the cosmic dawn that are based on $N$-body simulations and a one-dimensional radiative transfer code. We find that these sources should be detectable in the SKA1-low images at $z = 15$ with an SNR of $\sim 14 (4)$ in 2000 (200) h of observations. One possible observational strategy thus could be to observe multiple fields for shorter observation times, identify fields with SNR $\gtrsim 3$ and observe these fields for much longer duration. Such observations are expected to be useful in constraining the parameters related to the first sources.

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arXiv2016

21-cm signature of the first sources in the Universe: Prospects of detection with SKA

Currently several low-frequency experiments are being planned to study the nature of the first stars using the redshifted 21-cm signal from the cosmic dawn and epoch of reionization. Using a one-dimensional radiative transfer code, we model the 21-cm signal pattern around the early sources for different source models, i.e., the metal-free Population III (PopIII) stars, primordial galaxies consisting of Population II (PopII) stars, mini-QSOs and high-mass X-ray binaries (HMXBs). We investigate the detectability of these sources by comparing the 21-cm visibility signal with the system noise appropriate for a telescope like the SKA1-low. Upon integrating the visibility around a typical source over all baselines and over a frequency interval of 16 MHz, we find that it will be possible to make a $\sim 9-σ$ detection of the isolated sources like PopII galaxies, mini-QSOs and HMXBs at $z \sim 15$ with the SKA1-low in 1000 hours. The exact value of the signal to noise ratio (SNR) will depend on the source properties, in particular on the mass and age of the source and the escape fraction of ionizing photons. The predicted SNR decreases with increasing redshift. We provide simple scaling laws to estimate the SNR for different values of the parameters which characterize the source and the surrounding medium. We also argue that it will be possible to achieve a SNR $\sim 9$ even in the presence of the astrophysical foregrounds by subtracting out the frequency-independent component of the observed signal. These calculations will be useful in planning 21-cm observations to detect the first sources.

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arXiv2015

Cross-correlating 21cm intensity maps with Lyman Break Galaxies in the post-reionization era

We investigate the cross-correlation between the spatial distribution of Lyman Break Galaxies (LBGs) and the 21cm intensity mapping signal at $z\sim[3-5]$. At these redshifts, galactic feedback is supposed to only marginally affect the matter power spectrum, and the neutral hydrogen distribution is independently constrained by quasar spectra. Using a high resolution N-body simulation, populated with neutral hydrogen a posteriori, we forecast for the expected LBG-21cm cross-spectrum and its error for a 21cm field observed by the Square Kilometre Array (SKA1-LOW and SKA1-MID), combined with a spectroscopic LBG survey with the same volume. The cross power can be detected with a signal-to-noise ratio (SNR) up to ~10 times higher (and down to ~4 times smaller scales) than the 21cm auto-spectrum for this set-up, with the SNR depending only very weakly on redshift and the LBG population. We also show that while both the 21cm auto- and LBG-21cm cross-spectra can be reliably recovered after the cleaning of smooth-spectrum foreground contamination, only the cross-power is robust to problematic non-smooth foregrounds like polarized synchrotron emission.

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arXiv2015

Effects of the sources of reionization on 21-cm redshift-space distortions

The observed 21-cm signal from the epoch of reionization will be distorted along the line-of-sight by the peculiar velocities of matter particles. These redshift-space distortions will affect the contrast in the signal and will also make it anisotropic. This anisotropy contains information about the cross-correlation between the matter density field and the neutral hydrogen field, and could thus potentially be used to extract information about the sources of reionization. In this paper, we study a collection of simulated reionization scenarios assuming different models for the sources of reionization. We show that the 21-cm anisotropy is best measured by the quadrupole moment of the power spectrum. We find that, unless the properties of the reionization sources are extreme in some way, the quadrupole moment evolves very predictably as a function of global neutral fraction. This predictability implies that redshift-space distortions are not a very sensitive tool for distinguishing between reionization sources. However, the quadrupole moment can be used as a model-independent probe for constraining the reionization history. We show that such measurements can be done to some extent by first-generation instruments such as LOFAR, while the SKA should be able to measure the reionization history using the quadrupole moment of the power spectrum to great accuracy.

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arXiv2015

21-cm signal from cosmic dawn - II: Imprints of the light-cone effects

Details of various unknown physical processes during the cosmic dawn and the epoch of reionization can be extracted from observations of the redshifted 21-cm signal. These observations, however, will be affected by the evolution of the signal along the line-of-sight which is known as the "light-cone effect". We model this effect by post-processing a dark matter $N-$body simulation with an 1-D radiative transfer code. We find that the effect is much stronger and dramatic in presence of inhomogeneous heating and Ly$α$ coupling compared to the case where these processes are not accounted for. One finds increase (decrease) in the spherically averaged power spectrum up to a factor of 3 (0.6) at large scales ($k \sim 0.05\, \rm Mpc^{-1}$) when the light-cone effect is included, though these numbers are highly dependent on the source model. The effect is particularly significant near the peak and dip-like features seen in the power spectrum. The peaks and dips are suppressed and thus the power spectrum can be smoothed out to a large extent if the width of the frequency band used in the experiment is large. We argue that it is important to account for the light-cone effect for any 21-cm signal prediction during cosmic dawn.

astro-ph.CO↗
arXiv2015

On using large scale correlation of the Ly-$α$ forest and redshifted 21-cm signal to probe HI distribution during the post reionization era

We investigate the possibility of detecting the 3D cross correlation power spectrum of the Ly-$α$ forest and HI 21 cm signal from the post reionization epoch. The cross-correlation signal is directly dependent on the dark matter power spectrum and is sensitive to the 21-cm brightness temperature and Ly-$α$ forest biases. These bias parameters dictate the strength of anisotropy in redshift space. We find that the cross-correlation power spectrum can be detected using $400 ~ \, \rm hrs$ observation with SKA-mid (phase 1) and a futuristic BOSS like experiment with a quasar (QSO) density of $30 ~ \rm deg^{-2}$ at a peak SNR of $15$ for a single field experiment at redshift $z = 2.5$. We also study the possibility of constraining various bias parameters using the cross power spectrum. We find that with the same experiment $1 σ$ conditional errors on the 21-cm linear redshift space distortion parameter $β_T$ and $β_{F}$ corresponding to the Ly-$α$ forest are $\sim 2.7\%$ and $\sim 1.4 \%$ respectively for $10$ independent pointings of the SKA-mid (phase 1). This prediction indicates a significant improvement over existing measurements. We claim that the detection of the 3D cross correlation power spectrum will not only ascertain the cosmological origin of the signal in presence of astrophysical foregrounds but will also provide stringent constraints on large scale HI biases. This provides an independent probe towards understanding cosmological structure formation.

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