Supervisors: Dr. Somnath Bharadwaj Phone: +917797575301 Address: Department of Physics and Meteorology(Center for Theoretical Study),
Indian Institute of Technology Kharagpur,
Kharagpur, India-721302
Studying the cosmic dawn and the epoch of reionization through the redshifted 21 cm line are amon... 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
The line of sight direction in the redshifted 21-cm signal coming from the cosmic dawn and the ep... 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.
The EoR 21-cm signal is expected to become highly non-Gaussian as reionization progresses. This s... 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.
The particle nature of dark matter remains a mystery. In this paper, we consider two dark matter
... more The particle nature of dark matter remains a mystery. In this paper, we consider two dark matter models—Late Forming Dark Matter (LFDM) and Ultra-Light Axion (ULA) models—where the matter power spectra show novel effects on small scales. The high redshift universe offers a powerful probe of their pa- rameters. In particular, we study two cosmological observables: the neutral hydrogen (HI) redshifted 21-cm signal from the epoch of reionization, and the evolution of the collapsed fraction of HI in the redshift range 2 < z < 5. We model the theoretical predictions of the models using CDM-like N-body simulations with modified initial conditions, and generate reionization fields using an excursion-set model. The N-body approx- imation is valid on the length and halo mass scales studied. We show that LFDM and ULA models predict an increase in the HI power spectrum from the epoch of reionization by a factor between 2–10 for a range of scales 0.1 < k < 4 Mpc −1 . Assuming a fiducial model where a neutral hydrogen fraction x ̄ HI = 0.5 must be achieved by z = 8, the reionization process allows us to put approximate bounds on the redshift of dark matter formation z f > 4 × 10 5 (for LFDM) and the axion mass m a > 2.6 × 10 −23 eV (for ULA). The comparison of the collapsed mass fraction inferred from damped Lyman-α observations to the theoretical predictions of our models lead to the weaker bounds: z f > 2 × 10 5 and m a > 10 −23 eV. These bounds are consistent with other constraints in the literature using different observables and, in the case of ULAs, are also consistent with a solution to the cusp-core problem of CDM. Keywords: cosmology: theory - Epoch of Rei
The non-Gaussian nature of the Epoch of Reionization (EoR) 21-cm signal has a significant impact ... more The non-Gaussian nature of the Epoch of Reionization (EoR) 21-cm signal has a significant impact on the error variance of its power spectrum ${P_{\rm b}}({\bf k})$ (Mondal et al., 2015). Building on the previous work, we have used a large ensemble of semi-numerical simulations and an analytical model to estimate the effect of this non-Gaussianity on the entire error covariance matrix ${\mathcal{C}}_{ij}$. Our analytical model shows that ${\mathcal{C}}_{ij}$ has contributions from two sources. One is the usual variance for a Gaussian random field which scales inversely of the number of modes that goes into the estimation of ${P_{\rm b}}({\bf k})$. The other is the trispectrum of the signal. Using the simulated 21-cm signal ensemble, an ensemble of the randomized signal and ensembles of Gaussian random ensembles we have quantified the effect of the trispectrum on the error variance ${\mathcal{C}}_{ij}$. We find that its relative contribution is comparable to or larger than that of the Gaussian term for the $k$ range $0.3 \leq k \leq 1.0 \,{\rm Mpc}^{-1}$, and can be even $\sim 200$ times larger at $k \sim 5\, {\rm Mpc}^{-1}$. We also establish that the off-diagonal terms of ${\mathcal{C}}_{ij}$ have statistically significant non-zero values which arise purely from the trispectrum. This further signifies that the error in different $k$ modes are not independent. We find a strong correlation between the errors at large $k$ values $(\ge 0.5 \,{\rm Mpc}^{-1})$, and a weak correlation between the smallest and largest $k$ values. There is also a small anti-correlation between the errors in the smallest and intermediate $k$ values. These results are relevant for the $k$ range that will be probed by the current and upcoming EoR 21-cm experiments.
The EoR 21-cm signal is expected to become increasingly non-Gaussian as reionization proceeds. We... 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.
The EoR 21-cm signal is expected to become increasingly non-Gaussian as reionization proceeds. We... 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 $SNR=\sqrt{N_k}$ for a Gaussian random field where $N_k$ 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 $N_k$ is increased. The value of $[SNR]_l$ falls as reionization proceeds, dropping from $\sim 500$ at $\bar{x}_{HI} = 0.8-0.9$ to $\sim 10$ at $\bar{x}_{HI} = 0.15 $ for a $[150.08\, {\rm Mpc}]^3$ simulation. We show that it is possible to interpret $[SNR]_l$ in terms of the trispectrum, and we expect $[SNR]_l \propto \sqrt{V}$ if the volume is increased. For $SNR \ll [SNR]_l$ we find $SNR = \sqrt{N_k}/A $ with $A \sim 0.95 - 1.75$, roughly consistent with the Gaussian prediction. We present a fitting formula for the $SNR$ as a function of $N_k$, 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.
Studying the cosmic dawn and the epoch of reionization through the redshifted 21 cm line are amon... 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
The line of sight direction in the redshifted 21-cm signal coming from the cosmic dawn and the ep... 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.
The EoR 21-cm signal is expected to become highly non-Gaussian as reionization progresses. This s... 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.
The particle nature of dark matter remains a mystery. In this paper, we consider two dark matter
... more The particle nature of dark matter remains a mystery. In this paper, we consider two dark matter models—Late Forming Dark Matter (LFDM) and Ultra-Light Axion (ULA) models—where the matter power spectra show novel effects on small scales. The high redshift universe offers a powerful probe of their pa- rameters. In particular, we study two cosmological observables: the neutral hydrogen (HI) redshifted 21-cm signal from the epoch of reionization, and the evolution of the collapsed fraction of HI in the redshift range 2 < z < 5. We model the theoretical predictions of the models using CDM-like N-body simulations with modified initial conditions, and generate reionization fields using an excursion-set model. The N-body approx- imation is valid on the length and halo mass scales studied. We show that LFDM and ULA models predict an increase in the HI power spectrum from the epoch of reionization by a factor between 2–10 for a range of scales 0.1 < k < 4 Mpc −1 . Assuming a fiducial model where a neutral hydrogen fraction x ̄ HI = 0.5 must be achieved by z = 8, the reionization process allows us to put approximate bounds on the redshift of dark matter formation z f > 4 × 10 5 (for LFDM) and the axion mass m a > 2.6 × 10 −23 eV (for ULA). The comparison of the collapsed mass fraction inferred from damped Lyman-α observations to the theoretical predictions of our models lead to the weaker bounds: z f > 2 × 10 5 and m a > 10 −23 eV. These bounds are consistent with other constraints in the literature using different observables and, in the case of ULAs, are also consistent with a solution to the cusp-core problem of CDM. Keywords: cosmology: theory - Epoch of Rei
The non-Gaussian nature of the Epoch of Reionization (EoR) 21-cm signal has a significant impact ... more The non-Gaussian nature of the Epoch of Reionization (EoR) 21-cm signal has a significant impact on the error variance of its power spectrum ${P_{\rm b}}({\bf k})$ (Mondal et al., 2015). Building on the previous work, we have used a large ensemble of semi-numerical simulations and an analytical model to estimate the effect of this non-Gaussianity on the entire error covariance matrix ${\mathcal{C}}_{ij}$. Our analytical model shows that ${\mathcal{C}}_{ij}$ has contributions from two sources. One is the usual variance for a Gaussian random field which scales inversely of the number of modes that goes into the estimation of ${P_{\rm b}}({\bf k})$. The other is the trispectrum of the signal. Using the simulated 21-cm signal ensemble, an ensemble of the randomized signal and ensembles of Gaussian random ensembles we have quantified the effect of the trispectrum on the error variance ${\mathcal{C}}_{ij}$. We find that its relative contribution is comparable to or larger than that of the Gaussian term for the $k$ range $0.3 \leq k \leq 1.0 \,{\rm Mpc}^{-1}$, and can be even $\sim 200$ times larger at $k \sim 5\, {\rm Mpc}^{-1}$. We also establish that the off-diagonal terms of ${\mathcal{C}}_{ij}$ have statistically significant non-zero values which arise purely from the trispectrum. This further signifies that the error in different $k$ modes are not independent. We find a strong correlation between the errors at large $k$ values $(\ge 0.5 \,{\rm Mpc}^{-1})$, and a weak correlation between the smallest and largest $k$ values. There is also a small anti-correlation between the errors in the smallest and intermediate $k$ values. These results are relevant for the $k$ range that will be probed by the current and upcoming EoR 21-cm experiments.
The EoR 21-cm signal is expected to become increasingly non-Gaussian as reionization proceeds. We... 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.
The EoR 21-cm signal is expected to become increasingly non-Gaussian as reionization proceeds. We... 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 $SNR=\sqrt{N_k}$ for a Gaussian random field where $N_k$ 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 $N_k$ is increased. The value of $[SNR]_l$ falls as reionization proceeds, dropping from $\sim 500$ at $\bar{x}_{HI} = 0.8-0.9$ to $\sim 10$ at $\bar{x}_{HI} = 0.15 $ for a $[150.08\, {\rm Mpc}]^3$ simulation. We show that it is possible to interpret $[SNR]_l$ in terms of the trispectrum, and we expect $[SNR]_l \propto \sqrt{V}$ if the volume is increased. For $SNR \ll [SNR]_l$ we find $SNR = \sqrt{N_k}/A $ with $A \sim 0.95 - 1.75$, roughly consistent with the Gaussian prediction. We present a fitting formula for the $SNR$ as a function of $N_k$, 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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Papers by Rajesh Mondal
models—Late Forming Dark Matter (LFDM) and Ultra-Light Axion (ULA) models—where the matter power
spectra show novel effects on small scales. The high redshift universe offers a powerful probe of their pa-
rameters. In particular, we study two cosmological observables: the neutral hydrogen (HI) redshifted 21-cm
signal from the epoch of reionization, and the evolution of the collapsed fraction of HI in the redshift range
2 < z < 5. We model the theoretical predictions of the models using CDM-like N-body simulations with
modified initial conditions, and generate reionization fields using an excursion-set model. The N-body approx-
imation is valid on the length and halo mass scales studied. We show that LFDM and ULA models predict
an increase in the HI power spectrum from the epoch of reionization by a factor between 2–10 for a range of
scales 0.1 < k < 4 Mpc −1 . Assuming a fiducial model where a neutral hydrogen fraction x
̄ HI = 0.5 must be
achieved by z = 8, the reionization process allows us to put approximate bounds on the redshift of dark matter
formation z f > 4 × 10 5 (for LFDM) and the axion mass m a > 2.6 × 10 −23 eV (for ULA). The comparison
of the collapsed mass fraction inferred from damped Lyman-α observations to the theoretical predictions of
our models lead to the weaker bounds: z f > 2 × 10 5 and m a > 10 −23 eV. These bounds are consistent with
other constraints in the literature using different observables and, in the case of ULAs, are also consistent with
a solution to the cusp-core problem of CDM.
Keywords: cosmology: theory - Epoch of Rei
models—Late Forming Dark Matter (LFDM) and Ultra-Light Axion (ULA) models—where the matter power
spectra show novel effects on small scales. The high redshift universe offers a powerful probe of their pa-
rameters. In particular, we study two cosmological observables: the neutral hydrogen (HI) redshifted 21-cm
signal from the epoch of reionization, and the evolution of the collapsed fraction of HI in the redshift range
2 < z < 5. We model the theoretical predictions of the models using CDM-like N-body simulations with
modified initial conditions, and generate reionization fields using an excursion-set model. The N-body approx-
imation is valid on the length and halo mass scales studied. We show that LFDM and ULA models predict
an increase in the HI power spectrum from the epoch of reionization by a factor between 2–10 for a range of
scales 0.1 < k < 4 Mpc −1 . Assuming a fiducial model where a neutral hydrogen fraction x
̄ HI = 0.5 must be
achieved by z = 8, the reionization process allows us to put approximate bounds on the redshift of dark matter
formation z f > 4 × 10 5 (for LFDM) and the axion mass m a > 2.6 × 10 −23 eV (for ULA). The comparison
of the collapsed mass fraction inferred from damped Lyman-α observations to the theoretical predictions of
our models lead to the weaker bounds: z f > 2 × 10 5 and m a > 10 −23 eV. These bounds are consistent with
other constraints in the literature using different observables and, in the case of ULAs, are also consistent with
a solution to the cusp-core problem of CDM.
Keywords: cosmology: theory - Epoch of Rei