Suman Majumdar
Imperial College London, Physics, Post-Doc
- Physics, Cosmology (Physics), Computational Physics, Observational Astronomy, Numerical Simulations, Radio Astronomy, and 15 moreTheoretical astrophysics, Galaxy Formation and Evolution, Extragalactic Astronomy, Physical Cosmology, Intergalactic Medium, Data Analysis, Cosmology (Physics) (Physics), HII Regions, Quasar, Astronomy, Astrophysics, Epoch of Reionization, Cosmological Reionisation, Ionized Bubble, and Finite light travel timeedit
- Dr. Jonathan R. Pritchard, Prof. Garrelt Mellema, Prof. Somnath Bharadwaj, Prof. Sugata Pratik Khastgiredit
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The evolution of topology and morphology of ionized or neutral hydrogen during different stages of the Epoch of Reionization (EoR) have the potential to provide us a great amount of information about the properties of the ionizing sources... more
The evolution of topology and morphology of ionized or neutral hydrogen during different stages of the Epoch of Reionization (EoR) have the potential to provide us a great amount of information about the properties of the ionizing sources during this era. We compare a variety of reionization source models in terms of the geometrical properties of the ionized regions. We show that the percolation transition in the ionized hydrogen, as studied by tracing the evolution of the Largest Cluster Statistics (LCS), is a robust statistic that can distinguish the fundamentally different scenarios — inside-out and outside-in reionization. Particularly, the global neutral fraction at the onset of percolation is significantly higher for the inside-out scenario as compared to that for the outside-in reionization. In complementary to percolation analysis, we explore the shape and morphology of the ionized regions as they evolve in different reionization models in terms of the Shapefinders (SFs) tha...
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The dependence of the bispectrum on the size and shape of the triangle contains a wealth of cosmological information. Here we consider a triangle parameterization which allows us to separate the size and shape dependence. We have... more
The dependence of the bispectrum on the size and shape of the triangle contains a wealth of cosmological information. Here we consider a triangle parameterization which allows us to separate the size and shape dependence. We have implemented an FFT based fast estimator for the three dimensional (3D) bin averaged bispectrum, and we demonstrate that it allows us to study the variation of the bispectrum across triangles of all possible shapes (and also sizes). The computational requirement is shown to scale as ∼ N g 3 log N g 3 where N g is the number of grid points along each side of the volume. We have validated the estimator using a non-Gaussian field for which the bispectrum can be analytically calculated. The estimated bispectrum values are found to be in good agreement (< 10 % deviation) with the analytical predictions across much of the triangle-shape parameter space. We also introduce linear redshift space distortion, a situation where also the bispectrum can be analytically...
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C ii line intensity mapping (LIM) is a potential technique to probe the early galaxies from the Epoch of Reionization (EoR). Several experiments e.g. CONCERTO, TIME, CCAT-p are underway to map the C ii LIM signal fluctuations from the... more
C ii line intensity mapping (LIM) is a potential technique to probe the early galaxies from the Epoch of Reionization (EoR). Several experiments e.g. CONCERTO, TIME, CCAT-p are underway to map the C ii LIM signal fluctuations from the EoR, enabling us to estimate the C ii power-spectrum and C ii×21-cm cross-power spectrum. Observed LIM signal will have its time evolution embedded in it along the Line of Sight (LoS) due to the finite travel time of the signal from its origin to the observer. We have investigated this so-called light-cone effect on the observed statistics of our semi-numerically simulated C ii signal from the EoR. Using a suit of simulated C ii and neutral hydrogen 21-cm maps and corresponding light-cone boxes, we have shown that the light-cone effect can impact the C ii power spectrum by more than 15% at large scales (k ∼ 0.1Mpc−1, at z = 6.8). We have also observed that the impact of light-cone effect on the C ii power spectrum drops with decreasing redshift within ...
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The H i 21-cm optical depth (τb) can be considerably large as the kinetic and spin temperature of the inter-galactic medium(IGM) is expected to be very low during cosmic dawn. It will be particularly higher at regions with H i... more
The H i 21-cm optical depth (τb) can be considerably large as the kinetic and spin temperature of the inter-galactic medium(IGM) is expected to be very low during cosmic dawn. It will be particularly higher at regions with H i over-density. We revisit the validity of the widely used linearized equation for estimating the H i 21-cm differential brightness temperature (Tb) which assumes τb < <1 and approximates [1 − exp ( − τb)] as τb. We consider two scenarios, one without any additional cooling mechanism or radio background ( referred as standard scenario) and the other (referred as excess-cooling scenario) assumes the EDGES like absorption profile and an excess cooling mechanism. We find that given a measured global absorption signal, consistent with the standard(excess-cooling) scenario, the linearized equation overestimates the spin temperature by $\sim 5{{\ \rm per\ cent}}(10{{\ \rm per\ cent}})$. Further, using numerical simulations, we study impact that the large optical...
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We study the spherically averaged bispectrum of the 21-cm signal from the Epoch of Reionization (EoR). This metric provides a quantitative measurement of the level of non-Gaussianity of the signal, which is expected to be high. We focus... more
We study the spherically averaged bispectrum of the 21-cm signal from the Epoch of Reionization (EoR). This metric provides a quantitative measurement of the level of non-Gaussianity of the signal, which is expected to be high. We focus on the impact of the light-cone (LC) effect on the bispectrum and its detectability with the future SKA-Low telescope. Our investigation is based on a single reionization LC model and an ensemble of 50 realizations of the 21-cm signal to estimate the cosmic variance errors. We calculate the bispectrum with a new, optimized direct estimation method, DviSukta, which calculates the bispectrum for all possible unique triangles. We find that the LC effect becomes important on scales $k_1 \lesssim 0.1\, {\rm Mpc}^{-1}$, where, for most triangle shapes, the cosmic variance errors dominate. Only for the squeezed limit triangles, the impact of the LC effect exceeds the cosmic variance. Combining the effects of system noise and cosmic variance we find that ∼3σ...
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We consider a sample of 82 non-repeating fast radio bursts (FRBs) detected at Parkes, ASKAP, CHIME, and UTMOST, each of which operates over a different frequency range and has a different detection criterion. Using simulations, we perform... more
We consider a sample of 82 non-repeating fast radio bursts (FRBs) detected at Parkes, ASKAP, CHIME, and UTMOST, each of which operates over a different frequency range and has a different detection criterion. Using simulations, we perform a maximum-likelihood analysis to determine the FRB population model that best fits these data. Our analysis shows that models where the pulse scatter broadening increases moderately with redshift (z) are preferred to those where this increases very sharply or where scattering is absent. Further, models where the comoving event rate density is constant over z are preferred to those where it follows the cosmological star formation rate. Two models for the host dispersion measure (DMHost) distribution (a fixed and a random DMHost) are found to predict comparable results. We obtain the best-fitting parameter values $\alpha =-1.53^{+0.29}_{-0.19}$, $\overline{E}_{33}=1.55^{+0.26}_{-0.22}$, and γ = 0.77 ± 0.24. Here α is the spectral index, γ is the expo...
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C ii line intensity mapping (LIM) is a potential technique to probe the early galaxies from the Epoch of Reionization (EoR). Several experiments e.g. CONCERTO, TIME, CCAT-p are underway to map the C ii LIM signal fluctuations from the... more
C ii line intensity mapping (LIM) is a potential technique to probe the early galaxies from the Epoch of Reionization (EoR). Several experiments e.g. CONCERTO, TIME, CCAT-p are underway to map the C ii LIM signal fluctuations from the EoR, enabling us to estimate the C ii power-spectrum and C ii × 21-cm cross-power spectrum. The observed LIM signal will have its time evolution embedded in it along the line of sight due to the finite traveltime of the signal from its origin to the observer. We have investigated this so-called light-cone effect on the observed statistics of our seminumerically simulated C ii signal from the EoR. Using a suit of simulated C ii and neutral hydrogen 21-cm maps and corresponding light-cone boxes, we have shown that the light-cone effect can impact the C ii power spectrum by more than 15 per cent at large scales ($k\sim 0.1\, \text{Mpc}^{-1}$, at $z$ = 6.8). We have also observed that the impact of light-cone effect on the C ii power spectrum drops with de...
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We present a study of the 21-cm signal bispectrum (which quantifies the non-Gaussianity in the signal) from the Cosmic Dawn (CD). For our analysis, we have simulated the 21-cm signal using radiative transfer code grizzly, while... more
We present a study of the 21-cm signal bispectrum (which quantifies the non-Gaussianity in the signal) from the Cosmic Dawn (CD). For our analysis, we have simulated the 21-cm signal using radiative transfer code grizzly, while considering two types of sources (mini-QSOs and HMXBs) for Ly α coupling and the X-ray heating of the IGM. Using this simulated signal, we have, for the first time, estimated the CD 21-cm bispectra for all unique k-triangles and for a range of k modes. We observe that the redshift evolution of the bispectrum magnitude and sign follow a generic trend for both source models. However, the redshifts at which the bispectrum magnitude reaches their maximum and minimum values and show their sign reversal depends on the source model. When the Ly α coupling and the X-ray heating of the IGM occur simultaneously, we observe two consecutive sign reversals in the bispectra for small k-triangles (irrespective of the source models). One arising at the beginning of the IGM h...
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The light-cone effect causes the mean as well as the statistical properties of the redshifted 21-cm signal ${T_{\rm b}}(\hat{\boldsymbol {n}}, \nu)$ to change with frequency ν (or cosmic time). Consequently, the statistical homogeneity... more
The light-cone effect causes the mean as well as the statistical properties of the redshifted 21-cm signal ${T_{\rm b}}(\hat{\boldsymbol {n}}, \nu)$ 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 ($\bar{x}_{\rm {H\,{\small I}}}$) 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 multifrequency angular power spectrum (MAPS) ${\mathcal {C}}_{\ell }(\nu _1,\nu _2)$, which does not assume the signal to be ergodic and periodic along the LoS. Here, we study the prospects for...
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The matched filtering technique is an efficient method to detect H ii bubbles and absorption regions in radio interferometric observations of the redshifted 21-cm signal from the epoch of reionization and the Cosmic Dawn. Here, we present... more
The matched filtering technique is an efficient method to detect H ii bubbles and absorption regions in radio interferometric observations of the redshifted 21-cm signal from the epoch of reionization and the Cosmic Dawn. Here, we present an implementation of this technique to the upcoming observations such as the SKA1-low for a blind search of absorption regions at the Cosmic Dawn. The pipeline explores four dimensional parameter space on the simulated mock visibilities using a MCMC algorithm. The framework is able to efficiently determine the positions and sizes of the absorption/H ii regions in the field of view.
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Research Interests: Physics, Dust (Astronomy & Astrophysics), HII Regions, Astrophysics, Statistical Methods in Archaeology, and 15 moreComputational Methods, Epoch of Reionization, Matter, Physical sciences, Simulations, Fluctuations, Gaussian, Science Technology, large scale structure of the Universe, Redshift Space Distortions, EOR, Spectral Density, Peculiar Velocities, Covariance, and MHz
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Research Interests: Physics, Statistics, Intergalactic Medium, Dust (Astronomy & Astrophysics), Astrophysics, and 15 moreNumerical Methods, Computational Methods, Epoch of Reionization, Astronomy, Universe, Galaxy, Physical sciences, Simulations, Power Spectrum, Science Technology, large scale structure of the Universe, Radiative Transfer, Peculiar Velocities, Hydrogen line, and Redshift
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The nature of dark matter sets the timeline for the formation of first collapsed halos and thus affects the sources of reionization. Here, we consider two different models of dark matter: cold dark matter (CDM) and thermal warm dark... more
The nature of dark matter sets the timeline for the formation of first collapsed halos and thus affects the sources of reionization. Here, we consider two different models of dark matter: cold dark matter (CDM) and thermal warm dark matter (WDM), and study how they impact the epoch of reionization (EoR) and its 21-cm observables. Using a suite of simulations, we find that in the WDM scenarios, the structure formation on small scales gets suppressed resulting in a smaller number of low mass dark matter halos compared to the CDM scenario. Assuming that the efficiency of sources in producing ionizing photons remain the same, this leads to a lower number of total ionizing photons produced at any given cosmic time and thus in a delay in the reionization process. We also find visual differences in the neutral hydrogen (HI) topology and in 21-cm maps in case of the WDM compared to the CDM. However, differences in the 21-cm power spectra, at the same neutral fraction, are found to be small....
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This white paper highlights the crucial and urgent synergies required between WFIRST, Subaru Hyper Suprime-Cam or other >25m-class telescopes galaxy observations and SKA 21cm measurements to constrain the nature of reionization... more
This white paper highlights the crucial and urgent synergies required between WFIRST, Subaru Hyper Suprime-Cam or other >25m-class telescopes galaxy observations and SKA 21cm measurements to constrain the nature of reionization (ionization history and topology) and its sources.
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With the advent of the first luminous sources at Cosmic Dawn (CD), the redshifted 21-cm signal, from the neutral hydrogen in the Inter-Galactic Medium (IGM), is predicted to undergo a transition from absorption to emission against the... more
With the advent of the first luminous sources at Cosmic Dawn (CD), the redshifted 21-cm signal, from the neutral hydrogen in the Inter-Galactic Medium (IGM), is predicted to undergo a transition from absorption to emission against the CMB. Using simulations, we show that the redshift evolution of the sign and the magnitude of the 21-cm bispectrum can disentangle the contributions from Ly$\alpha$ coupling and X-ray heating of the IGM, the two most dominant processes which drive this transition. This opens a new avenue to probe the first luminous sources and the IGM physics at CD.
This white paper highlights the crucial and urgent synergies required between WFIRST, Subaru Hyper Suprime-Cam or other >25m-class telescopes galaxy observations and SKA 21cm measurements to constrain the nature of reionization... more
This white paper highlights the crucial and urgent synergies required between WFIRST, Subaru Hyper Suprime-Cam or other >25m-class telescopes galaxy observations and SKA 21cm measurements to constrain the nature of reionization (ionization history and topology) and its sources.
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We apply two methods to estimate the 21-cm bispectrum from data taken within the Epoch of Reionisation (EoR) project of the Murchison Widefield Array (MWA). Using data acquired with the Phase II compact array allows a direct bispectrum... more
We apply two methods to estimate the 21-cm bispectrum from data taken within the Epoch of Reionisation (EoR) project of the Murchison Widefield Array (MWA). Using data acquired with the Phase II compact array allows a direct bispectrum estimate to be undertaken on the multiple redundantly spaced triangles of antenna tiles, as well as an estimate based on data gridded to the uv-plane. The direct and gridded bispectrum estimators are applied to 21 h of high-band (167–197 MHz; z = 6.2–7.5) data from the 2016 and 2017 observing seasons. Analytic predictions for the bispectrum bias and variance for point-source foregrounds are derived. We compare the output of these approaches, the foreground contribution to the signal, and future prospects for measuring the bispectra with redundant and non-redundant arrays. We find that some triangle configurations yield bispectrum estimates that are consistent with the expected noise level after 10 h, while equilateral configurations are strongly foreg...
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C ii line intensity mapping (LIM) is a potential technique to probe the early galaxies from the Epoch of Reionization (EoR). Several experiments e.g. CONCERTO, TIME, CCAT-p are underway to map the C ii LIM signal fluctuations from the... more
C ii line intensity mapping (LIM) is a potential technique to probe the early galaxies from the Epoch of Reionization (EoR). Several experiments e.g. CONCERTO, TIME, CCAT-p are underway to map the C ii LIM signal fluctuations from the EoR, enabling us to estimate the C ii power-spectrum and C ii×21-cm cross-power spectrum. Observed LIM signal will have its time evolution embedded in it along the Line of Sight (LoS) due to the finite travel time of the signal from its origin to the observer. We have investigated this so-called light-cone effect on the observed statistics of our semi-numerically simulated C ii signal from the EoR. Using a suit of simulated C ii and neutral hydrogen 21-cm maps and corresponding light-cone boxes, we have shown that the light-cone effect can impact the C ii power spectrum by more than 15% at large scales (k ∼ 0.1Mpc−1, at z = 6.8). We have also observed that the impact of light-cone effect on the C ii power spectrum drops with decreasing redshift within ...
The bispectrum can quantify the non-Gussianity present in the redshifted 21-cm signal produced by the neutral hydrogen ($\rm {H \small {I}}$) during the Epoch of Reionization (EoR). Motivated by this, we perform a comprehensive study of... more
The bispectrum can quantify the non-Gussianity present in the redshifted 21-cm signal produced by the neutral hydrogen ($\rm {H \small {I}}$) during the Epoch of Reionization (EoR). Motivated by this, we perform a comprehensive study of the EoR 21-cm bispectrum using simulated signals. Given a model of reionization, we demonstrate the behaviour of the bispectrum for all unique triangles in k space. For ease of identification of the unique triangles we parametrize the k-triangle space with two parameters, namely the ratio of the two arms of the triangle (n = k2/k1) and the cosine of the angle between them (cos θ). Furthermore, for the first time we quantify the impact of the redshift space distortions (RSD) on the spherically averaged EoR 21-cm bispectrum in the entire unique triangle space. We find that the real space signal bispectra for small and intermediate k1-triangles ($k_1 \le 0.6 \, \, {\rm Mpc}^{-1}$) is negative in most of the unique triangle space. It takes a positive sig...
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... more
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 thres...
The line of sight direction in the redshifted 21-cm signal coming from the cosmic dawn and the epoch of reioniza-tion is quite unique in many ways compared to any other cos-mological signal. Different unique effects, such as the evolution... more
The line of sight direction in the redshifted 21-cm signal coming from the cosmic dawn and the epoch of reioniza-tion is quite unique in many ways compared to any other cos-mological 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 cos-mology 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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Studying the cosmic dawn and the epoch of reionization through the redshifted 21 cm line are among the major science goals of the SKA1. Their significance lies in the fact that they are closely related to the very first stars in the... more
Studying the cosmic dawn and the epoch of reionization through the redshifted 21 cm line are among the major science goals of the SKA1. Their significance lies in the fact that they are closely related to the very first stars in the universe. Interpreting the upcoming data would require detailed modelling of the relevant physical processes. In this article, we focus on the theoretical models of reionization that have been worked out by various groups working in India with the upcoming SKA in mind. These models include purely analytical and semi-numerical calculations as well as fully numerical radiative transfer simulations. The predictions of the 21 cm signal from these models would be useful in constraining the properties of the early galaxies using the SKA data. Keywords: intergalactic medium – cosmology: theory – dark ages, reionization, first stars – diffuse radiation – large-scale structure of Universe – methods: numerical –methods: statistical
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The EoR 21-cm signal is expected to become highly non-Gaussian as reionization progresses. This severely affects the error-covariance of the EoR 21-cm power spectrum which is important for predicting the prospects of a detection with... more
The EoR 21-cm signal is expected to become highly non-Gaussian as reionization progresses. This severely affects the error-covariance of the EoR 21-cm power spectrum which is important for predicting the prospects of a detection with ongoing and future experiments. Most earlier works have assumed that the EoR 21-cm signal is a Gaussian random field where (1) the error-variance depends only on the power spectrum and the number of Fourier modes in the particular k bin, and (2) the errors in the different k bins are uncorrelated. Here we use an ensemble of simulated 21-cm maps to analyze the error-covariance at various stages of reionization. We find that even at the very early stages of reionization (¯ x H i ∼ 0.9) the error-variance significantly exceeds the Gaussian predictions at small length-scales (k > 0.5 Mpc −1) while they are consistent at larger scales. The errors in most k bins (both large and small scales), are however found to be correlated. Considering the later stages (¯ x H i = 0.15), the error-variance shows an excess in all k bins within k 0.1 Mpc −1 , and it is around 200 times larger than the Gaussian prediction at k ∼ 1 Mpc −1. The errors in the different k bins are all also highly correlated, barring the two smallest k bins which are anti-correlated with the other bins. Our results imply that the predictions for different 21-cm experiments based on the Gaussian assumption underestimate the errors, and it is necessary to incorporate the non-Gaussianity for more realistic predictions.
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ABSTRACT The recent discovery of the z = 7.085 quasar has opened up a new window to peep into the inter galactic medium during the epoch of reionization (EoR). The detection of the ionized region around such a high redshift quasar is in... more
ABSTRACT The recent discovery of the z = 7.085 quasar has opened up a new window to peep into the inter galactic medium during the epoch of reionization (EoR). The detection of the ionized region around such a high redshift quasar is in principle capable of constraining the neutral fraction of the IGM and the quasar&#39;s age. Here we present the possibility of detection of such an ionized region around a quasar at z = 8 using redshifted 21-cm observations of the neutral hydrogen and possible ways to constrain IGM neutral fraction and quasar&#39;s age through this detection.
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In this chapter we provide an overview of the current status of the simulations and modelling of the Cosmic Dawn and Epoch of Reionization. We discuss the modelling requirements as dictated by the characteristic scales of the problem and... more
In this chapter we provide an overview of the current status of the simulations and modelling of the Cosmic Dawn and Epoch of Reionization. We discuss the modelling requirements as dictated by the characteristic scales of the problem and the SKA instrumental properties and the planned survey parameters. Current simulations include most of the relevant physical processes. They can follow the full nonlinear dynamics and are now reaching the required scale and dynamic range, although small-scale physics still needs to be included at sub-grid level. However, despite a significant progress in developing novel numerical methods for efficient utilization of current hardware they remain quite computationally expensive. In response, a number of alternative approaches, particularly semi-analytical/semi-numerical methods, have been developed. While necessarily more approximate, if appropriately constructed and calibrated on simulations they could be used to quickly explore the vast parameter space available. Further work is still required on including some physical processes in both simulations and semi-analytical modelling. This hybrid approach of fast, approximate modelling calibrated on numerical simulations can then be used to construct large libraries of reionization models for reliable interpretation of the observational data.
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We provide an overview of 21cm tomography of the Cosmic Dawn and Epoch of Reionization as possible with SKA-Low. We show why tomography is essential for studying CD/EoR and present the scales which can be imaged at different frequencies... more
We provide an overview of 21cm tomography of the Cosmic Dawn and Epoch of Reionization as possible with SKA-Low. We show why tomography is essential for studying CD/EoR and present the scales which can be imaged at different frequencies for the different phases of SKA- Low. Next we discuss the different ways in which tomographic data can be analyzed. We end with an overview of science questions which can only be answered by tomography, ranging from the characterization of individual objects to understanding the global processes shaping the Universe during the CD/EoR
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The EoR 21-cm signal is expected to become increasingly non-Gaussian as reionization proceeds. We have used semi-numerical simulations to study how this affects the error predictions for the EoR 21-cm power spectrum. We expect SNR=Nk−−−√... more
The EoR 21-cm signal is expected to become increasingly non-Gaussian as reionization proceeds. We have used semi-numerical simulations to study how this affects the error predictions for the EoR 21-cm power spectrum. We expect SNR=Nk−−−√ for a Gaussian random field where Nk is the number of Fourier modes in each k bin. We find that non-Gaussianity is important at high SNR where it imposes an upper limit [SNR]l. For a fixed volume V, it is not possible to achieve SNR>[SNR]l even if Nk is increased. The value of [SNR]l falls as reionization proceeds, dropping from ∼500 at x¯HI=0.8−0.9 to ∼10 at x¯HI=0.15 for a [150.08Mpc]3 simulation. We show that it is possible to interpret [SNR]l in terms of the trispectrum, and we expect [SNR]l∝V−−√ if the volume is increased. For SNR≪[SNR]l we find SNR=Nk−−−√/A with A∼0.95−1.75, roughly consistent with the Gaussian prediction. We present a fitting formula for the SNR as a function of Nk, with two parameters A and [SNR]l that have to be determined using simulations. Our results are relevant for predicting the sensitivity of different instruments to measure the EoR 21-cm power spectrum, which till date have been largely based on the Gaussian assumption.
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We also study different redshift space observables of the 21-cm signal from EoR: the variance, power spectrum and its various angular multipole moments. We find that both semi-numerical models perform reasonably well in predicting these... more
We also study different redshift space observables of the 21-cm signal from EoR: the variance, power spectrum and its various angular multipole moments. We find that both semi-numerical models perform reasonably well in predicting these observables at length scales relevant for present and future experiments. However, Sem-Num performs slightly better than CPS+GS in producing the reionization history, which is necessary for interpreting the future observations.
Measurements of the HI 21-cm power spectra from the reionization epoch will be influenced by the evolution of the signal along the line-of-sight direction of any observed volume. We use numerical as well as semi-numerical simulations of... more
Measurements of the HI 21-cm power spectra from the reionization epoch will be influenced by the evolution of the signal along the line-of-sight direction of any observed volume. We use numerical as well as semi-numerical simulations of reionization in a cubic volume of 607 Mpc across to study this so-called light cone effect on the HI 21-cm power spectrum. We find that the light cone effect has the largest impact at two different stages of reionization: one when reionization is ∼20% and other when it is ∼80% completed. We find a factor of ∼4 amplification of the power spectrum at the largest scale available in our simulations. We do not find any significant anisotropy in the 21-cm power spectrum due to the light cone effect. We argue that for the power spectrum to become anisotropic, the light cone effect would have to make the ionized bubbles significantly elongated or compressed along the line-of-sight, which would require extreme reionization scenarios. We also calculate the two-point correlation functions parallel and perpendicular to the line-of-sight and find them to differ. Finally, we calculate an optimum frequency bandwidth below which the light cone effect can be neglected when extracting power spectra from observations. We find that if one is willing to accept a 10% error due to the light cone effect, the optimum frequency bandwidth for k=0.056Mpc−1 is ∼7.5 MHz. For k=0.15 and 0.41Mpc−1 the optimum bandwidth is ∼11 and ∼16 MHz respectively.
The reionization of the Universe, is believed to be occurred through the growth of ionized regions (bubbles) in the neutral intergalactic medium (IGM). The detection of individual ionized bubbles around high redshift quasars in... more
The reionization of the Universe, is believed to be occurred through the growth of ionized regions (bubbles) in the neutral intergalactic medium (IGM). The detection of individual ionized bubbles around high redshift quasars in redshifted 21-cm maps is one of the most promising, direct probes of the epoch of reionization (EoR). The signal from such bubbles will be buried in noise and foregrounds, the latter being at least a few orders of magnitude stronger than the signal. We discuss the possibility of detecting these bubbles in radio-interferometric observations of redshifted neutral hydrogen (HI) 21 cm radiation, using a visibility based matched filter technique.
We use different simulations to analyze the impact of HI fluctuations outside the bubble on the detectability of the bubble using this technique. Also, we find that there is a fundamental limitation on the smallest bubble that can be detected arising from the statistical fluctuations in the HI distribution.
Due to the effect of finite light travel time (FLTT), a growing ionized bubble around a quasar will appear distorted to a present day observer. There are two distinct FLTT distortions in the bubble's image: (i) it's apparent center is shifted along the line of sight (LOS) towards the observer from the quasar; (ii) it's shape is anisotropic along the LOS. We study the impact of this distortion in matched filter bubble detection, where it is most convenient to use a spherical template for the filter. The mismatch between the spherical filter and the anisotropic image causes a 10 - 20% degradation in the SNR relative to that of a spherical bubble. We conclude that a spherical filter is adequate for bubble detection and the FLTT distortions do not affect the lower limits for bubble detection with 1000 hr of GMRT observations.
We use different simulations to analyze the impact of HI fluctuations outside the bubble on the detectability of the bubble using this technique. Also, we find that there is a fundamental limitation on the smallest bubble that can be detected arising from the statistical fluctuations in the HI distribution.
Due to the effect of finite light travel time (FLTT), a growing ionized bubble around a quasar will appear distorted to a present day observer. There are two distinct FLTT distortions in the bubble's image: (i) it's apparent center is shifted along the line of sight (LOS) towards the observer from the quasar; (ii) it's shape is anisotropic along the LOS. We study the impact of this distortion in matched filter bubble detection, where it is most convenient to use a spherical template for the filter. The mismatch between the spherical filter and the anisotropic image causes a 10 - 20% degradation in the SNR relative to that of a spherical bubble. We conclude that a spherical filter is adequate for bubble detection and the FLTT distortions do not affect the lower limits for bubble detection with 1000 hr of GMRT observations.
Extending the formalism of Datta, Bharadwaj & Choudhury for detecting ionized bubbles in redshifted 21-cm maps using a matched filtering technique, we use different simulations to analyse the impact ofHI fluctuations outside the bubble... more
Extending the formalism of Datta, Bharadwaj & Choudhury for detecting ionized bubbles in redshifted 21-cm maps using a matched filtering technique, we use different simulations to
analyse the impact ofHI fluctuations outside the bubble on the detectability of the bubble. In the first three kinds of simulations there is a spherical bubble of comoving radius Rb, the one that we are trying to detect, located at the centre, and the neutral hydrogen (H I) outside the bubble
traces the underlying dark matter distribution. We consider three different possible scenarios of re-ionization, i.e. (i) there is a single bubble (SB) in the field of view (FoV) and the hydrogen neutral fraction is constant outside this bubble, (ii) patchy re-ionization (PR) with many small ionized bubbles in the FoV (PR1) and (iii) many spherical ionized bubbles of the same radius Rb (PR2). The centres of the extra bubbles trace the dark matter distribution. The fourth kind of simulation uses more realistic maps based on seminumeric modelling (SM) of ionized regions.We make predictions for the currently functioning Giant Metrewave Radio Telescope
(GMRT) and a forthcoming instrument, the Murchison Widefield Array (MWA) at a redshift of 6 (corresponding to an observed frequency 203 MHz) for 1000 h observations.We find that for both the SB and PR1 scenarios the fluctuating intergalactic medium restricts bubble detection to size Rb ≤ 6Mpc and Rb ≤ 12 Mpc for the GMRT and the MWA, respectively, however large be the integration time. These results are well explained by analytical predictions. In
the PR2 scenario, we find that bubble detection is almost impossible for neutral fraction xHI < 0.6 because of large uncertainty due to the HI fluctuations. Applying the matched filter technique to the SMscenario, we find that it works well even when the targeted ionized bubble is non-spherical due to surrounding bubbles and inhomogeneous recombination. We find that determining the size and positions of the bubbles is not limited by the HI fluctuations in the SB and PR1 scenario but limited by the instrument’s angular resolution instead, and this can be done more precisely for larger bubble. We also find that for bubble detection the GMRT configuration is somewhat superior to the proposed MWA.
analyse the impact ofHI fluctuations outside the bubble on the detectability of the bubble. In the first three kinds of simulations there is a spherical bubble of comoving radius Rb, the one that we are trying to detect, located at the centre, and the neutral hydrogen (H I) outside the bubble
traces the underlying dark matter distribution. We consider three different possible scenarios of re-ionization, i.e. (i) there is a single bubble (SB) in the field of view (FoV) and the hydrogen neutral fraction is constant outside this bubble, (ii) patchy re-ionization (PR) with many small ionized bubbles in the FoV (PR1) and (iii) many spherical ionized bubbles of the same radius Rb (PR2). The centres of the extra bubbles trace the dark matter distribution. The fourth kind of simulation uses more realistic maps based on seminumeric modelling (SM) of ionized regions.We make predictions for the currently functioning Giant Metrewave Radio Telescope
(GMRT) and a forthcoming instrument, the Murchison Widefield Array (MWA) at a redshift of 6 (corresponding to an observed frequency 203 MHz) for 1000 h observations.We find that for both the SB and PR1 scenarios the fluctuating intergalactic medium restricts bubble detection to size Rb ≤ 6Mpc and Rb ≤ 12 Mpc for the GMRT and the MWA, respectively, however large be the integration time. These results are well explained by analytical predictions. In
the PR2 scenario, we find that bubble detection is almost impossible for neutral fraction xHI < 0.6 because of large uncertainty due to the HI fluctuations. Applying the matched filter technique to the SMscenario, we find that it works well even when the targeted ionized bubble is non-spherical due to surrounding bubbles and inhomogeneous recombination. We find that determining the size and positions of the bubbles is not limited by the HI fluctuations in the SB and PR1 scenario but limited by the instrument’s angular resolution instead, and this can be done more precisely for larger bubble. We also find that for bubble detection the GMRT configuration is somewhat superior to the proposed MWA.