Kanan K. Datta

Reionization and the Cosmic Dawn with the Square Kilometre Array

The Square Kilometre Array (SKA) will have a low frequency component (SKA-low) which has as one of its main science goals the study of the redshifted 21 cm line from the earliest phases of star and galaxy formation in the Universe. This 21 cm signal provides a new and unique window both on the time of the formation of the first stars and accreting black holes and the subsequent period of substantial ionization of the intergalactic medium. The signal will teach us fundamental new things about the earliest phases of structure formation, cosmology and even has the potential to lead to the discovery of new physical phenomena. Here we present a white paper with an overview of the science questions that SKA-low can address, how we plan to tackle these questions and what this implies for the basic design of the telescope.

The multifrequency angular power spectrum of the epoch of reionization 21-cm signal

Observations of redshifted 21cm radiation from neutral hydrogen (HI) at high redshifts is an important future probe of reionization. We consider the Multi-frequency Angular Power Spectrum (MAPS) to quantify the statistics of the HI signal as a joint function of the angular multipole l and frequency separation �ν. The signal at two different frequencies is expected to decorrelate as �ν is increased, and quantifying this is particularly importa nt in deciding the frequency resolution for future HI observations. This is al so expected to play a very crucial role in extracting the signal from foregrounds as the signal is expected to decorrelate much faster than the foregrounds (which are largely continuum sources) with increasing �ν. In this paper we develop formulae relating MAPS to different components of the three dimensional HI power spectrum taking into account HI peculiar velocities. We show that the flat-sky approximation provides a very good representation over the angular scales of interest, and a final expression which is very simple to calculate and in terpret. We present results for z = 10 assuming a neutral hydrogen fraction of 0.6 considering two models for the HI distribution, namely, (i) DM: where the HI traces the dark matter and (ii) PR: where the effects of patchy reionization are incorporated through two parameters which are the bubble size and the clustering of the bubble centers relative to the dark mat ter (bias) respectively. We find that while the DM signal is largely featureless, the PR signal peaks at the angular scales of the individual bubbles where it is Poisson fluctuation dominate d, and the signal is considerably enhanced for large bubble size. For most cases of interest at l � 100 the signal is uncorrelated beyond �ν � 1MHz or even less, whereas this occurs around � 0.1MHz at l � 10 3 . The �ν dependence also carries an imprint of the bubble size and the bias, and is expected to be an important probe of the reionization scenario. Finally we find that the l range 10 3 10 4 is optimum for separating out the cosmological HI signal from the foregrounds, while this will be extremely demanding at l < 100 where it is necessary to characterize the �ν dependence of the foreground MAPS to an accuracy better than 1%.

H i as a probe of the large-scale structure in the post-reionization universe

Simulated maps of the HI distribution in the post-reionization era are used to study the prospects for detection with existing and upcoming radio telescopes. We consider detection in the redshifted radiation from the hyperfine transition with a rest frame frequency of 1420 MHz. Possibility of a statistical detection using visibility correlations is discussed. We show that the MWA (Murchison Widefield Array) and the GMRT (Giant Meterwave Radio Telescope) can potentially detect signal from the HI distribution at high redshifts. MWA can detect visibility correlations at large angular scales at all redshifts accessible to it in the post-reionization era. The GMRT can detect visibility correlations at lower redshifts, specifically there is a strong case for a survey at z = 1.3. We also discuss prospects for direct detection of rare peaks in the HI distribution using the GMRT. We show that direct detection should be possible with integration time that is comparable to, or even less than, the time required for a statistical detection. Specifically, it is possible to make a statistical detection of the HI distribution by measuring the visibility correlation, and, direct detection of rare peaks in the HI distribution using the GMRT in less than 1000 hours of observations.

Light-cone effect on the reionization 21-cm power spectrum

Observations of redshifted 21-cm radiation from neutral hydrogen during the epoch of reionization are considered to constitute the most promising tool to probe that epoch. One of the major goals of the first generation of low-frequency radio telescopes is to measure the 3D 21-cm power spectrum. However, the 21-cm signal could evolve substantially along the line-of-sight (LOS) direction of an observed 3D volume, since the received signal from different planes transverses to the LOS originated from different look-back times and could therefore be statistically different. Using numerical simulations we investigate this so-called light-cone effect on the spherically averaged 3D 21-cm power spectrum. For this version of the power spectrum, we find that the effect mostly ‘averages out’ and observe a smaller change in the power spectrum compared to the amount of evolution in the mean 21-cm signal and its rms variations along the LOS direction. Nevertheless, changes up to ∼50 per cent at large scales are possible. In general, the power is enhanced/suppressed at large/small scales when the effect is included. The cross-over mode below/above which the power is enhanced/suppressed moves towards larger scales as reionization proceeds. When considering the 3D power spectrum we find it to be anisotropic at the late stages of reionization and on large scales. The effect is dominated by the evolution of the ionized fraction of hydrogen during reionization and including peculiar velocities hardly changes these conclusions. We present simple analytical models which explain qualitatively all the features we see in the simulations

Probing reionization with LOFAR using 21-cm redshift space distortions

One of the most promising ways to study the epoch of reionization (EoR) is through radio observations of the redshifted 21-cm line emission from neutral hydrogen. These observations are complicated ...

On the use of seminumerical simulations in predicting the 21-cm signal from the epoch of reionization

We present a detailed comparison of three different simulations of the epoch of reionization (EoR). The radiative transfer simulation (C-2-RAY) among them is our benchmark. Radiative transfer codes can produce realistic results, but are computationally expensive. We compare it with two seminumerical techniques: one using the same haloes as C-2-RAY as its sources (Sem-Num), and one using a conditional Press-Schechter scheme (CPS+GS). These are vastly more computationally efficient than C-2-RAY, but use more simplistic physical assumptions. We evaluate these simulations in terms of their ability to reproduce the history and morphology of reionization. We find that both Sem-Num and CPS+GS can produce an ionization history and morphology that is very close to C-2-RAY, with Sem-Num performing slightly better compared to CPS+GS. 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 seminumerical 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. The CPS+GS scheme, however, has the advantage that it is not restricted by the mass resolution of the dark matter density field.

Detecting ionized bubbles in redshifted 21-cm maps

The reionization of the Universe, it is believed, occurred b y the growth of ionized regions (bubbles) in the neutral intergalactic medium (IGM). We study the possibility of detecting these bubbles in radio-interferometric observations of re dshifted neutral hydrogen (HI) 21 cm radiation. The signal (< 1 mJy) will be buried in noise and foregrounds, the latter being at least a few orders of magnitude stronger than the signal. We develop a visibility based formalism that uses a filter to optimally combine the entire signal f rom a bubble while minimizing the noise and foreground contributions. This formalism makes definite predictions on the ability to detect an ionized bubble or conclusively rule out its p resence in a radio-interferometric observation. We make predictions for the currently functioning GMRT and a forthcoming instrument, the MWA at a frequency of 150 MHz (corresponding to a redshift of 8.5). For both instruments, we show that a 3 σ detection will be possible for a bubble of comoving radius Rb � 40 Mpc (assuming it to be spherical) in 100 hrs of observation and Rb � 22 Mpc in 1000 hrs of observation, provided the bubble is at the center of the fie ld of view. In both these cases the filter effectively removes the expected foregroun d contribution so that it is below the signal, and the system noise is the deciding criteria. We find that there is a fundamental limitation on the smallest bubble that can be detected arisi ng from the statistical fluctuations in the HI distribution. Assuming that the HI traces the dark matter we find that it will not be possible to detect bubbles with Rb < 8 Mpc using the GMRT and Rb < 16 Mpc using the MWA, however large be the integration time.

Prospects of observing a quasar HII region during the Epoch of Reionization with redshifted 21cm

We present a study of the impact of a bright quasar on the redshifted 21cm signal during the Epoch of Reionization (EoR). Using three different cosmological radiative transfer simulations, we investigate if quasars are capable of substantially changing the size and morphology of the H II regions they are born in. We choose stellar and quasar luminosities in a way that is favourable to seeing such an effect. We find that even the most luminous of our quasar models is not able to increase the size of its native H II region substantially beyond those of large H II regions produced by clustered stellar sources alone. However, the quasar H II region is found to be more spherical. We next investigate the prospects of detecting such H II regions in the redshifted 21cm data from the Low Frequency Array (LOFAR) by means of a matched filter technique. We find that H II regions with radii ~ 25 comoving Mpc or larger should have a sufficiently high detection probability for 1200 hours of integration time. Although the matched filter can in principle distinguish between more and less spherical regions, we find that when including realistic system noise this distinction can no longer be made. The strong foregrounds are found not to pose a problem for the matched filter technique. We also demonstrate that when the quasar position is known, the redshifted 21cm data can still be used to set upper limits on the ionizing photon rate of the quasar. If both the quasar position and its luminosity are known, the redshifted 21 cm data can set new constrains on quasar lifetimes.

Light cone effect on the reionization 21-cm signal - II. Evolution, anisotropies and observational implications

Measurements of the H I 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 seminumerical simulations of reionization in a cubic volume of 607 Mpc across to study this so-called light-cone effect on the H I 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 per cent and other when it is ∼80 per cent 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 per cent error due to the light-cone effect, the optimum frequency bandwidth for k = 0.056 Mpc−1 is ∼7.5 MHz. For k = 0.15 and 0.41 Mpc−1, the optimum bandwidth is ∼11 and ∼16 MHz, respectively.

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. We model this light-cone effect by post-processing a dark matter N body simulation with a 1-D radiative transfer code. We find that the effect is much stronger and dramatic in presence of inhomogeneous heating and Lyα coupling compared to that where these processes are not accounted for. One finds increase (decrease) in the coeval power spectrum up to a factor of 3 (0.6) at large scales (k � 0.05Mpc 1 ), though these numbers are highly dependent on the source model. Consequently, the peak and trough-like features in the evolution of the large-scale power spectrum can be smoothed out to a large extent if the width of the frequency bands 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.