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Register a new userDipole of the Epoch of Reionization 21-cm signal
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Anv{z}e Slosar
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The motion of the solar system with respect to the cosmic rest frame modulates the monopole of the Epoch of Reionization 21-cm signal into a dipole. This dipole has a characteristic frequency dependence that is dominated by the frequency derivative of the monopole signal. We argue that although the signal is weaker by a factor of $sim100$, there are significant benefits in measuring the dipole. Most importantly, the direction of the cosmic velocity vector is known exquisitely well from the cosmic microwave background and is not aligned with the galaxy velocity vector that modulates the foreground monopole. Moreover, an experiment designed to measure a dipole can rely on differencing patches of the sky rather than making an absolute signal measurement, which helps with some systematic effects.
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It is predicted that sources emitting UV radiation in the Lyman band during the epoch of reionization (EoR) showed a series of discontinuities in their Ly-alpha flux radial profile as a consequence of the thickness of the Lyman line series in the primeval intergalactic medium. Through unsaturated Wouthuysen-Field coupling, these spherical discontinuities are also present in the 21 cm emission of the neutral IGM. In this article, we study the effects these discontinuities have on the differential brightness temperature of the 21 cm signal of neutral hydrogen in a realistic setting including all other sources of fluctuations. We focus on the early phases of the EoR, and we address the question of the detectability by the planned Square Kilometre Array. Such a detection would be of great interest, because these structures could provide an unambiguous diagnostic for the cosmological origin of the signal remaining after the foreground cleaning procedure. Also, they could be used as a new type of standard rulers. We determine the differential brightness temperature of the 21 cm signal in the presence of inhomogeneous Wouthuysen-Field effect using simulations which include (hydro)dynamics and both ionizing and Lyman lines 3D radiative transfer with the code LICORICE. We find that the Lyman horizons are clearly visible on the maps and radial profiles around the first sources of our simulations, but for a limited time interval, typically Delta z approx 2 at z sim 13. Stacking the profiles of the different sources of the simulation at a given redshift results in extending this interval to Delta z approx 4. When we take into account the implementation and design planned for the SKA (collecting area, sensitivity, resolution), we find that detection will be challenging. It may be possible with a 10 km diameter for the core, but will be difficult with the currently favored design of a 5 km core.
Using a suite of detailed numerical simulations we estimate the level of anisotropy generated by the time evolution along the light cone of the 21cm signal from the epoch of reionization. Our simulations include the physics necessary to model the signal during both the late emission regime and the early absorption regime, namely X-ray and Lyman-band 3D radiative transfer in addition to the usual dynamics and ionizing UV transfer. The signal is analysed using correlation functions perpendicular and parallel to the line of sight (LOS). We reproduce general findings from previous theoretical studies: the overall amplitude of the correlations and the fact that the light cone anisotropy is visible only on large scales (100 cMpc). However, the detailed behaviour is different. At 3 different epochs, the amplitude of the correlations along and perpendicular to the LOS differ from each other, indicating anisotropy. These 3 epochs are associated with 3 events of the global reionization history: the overlap of ionized bubbles, the onset of mild heating by X-rays in regions around the sources, and the onset of efficient Lyman-alpha coupling in regions around the sources. A 20x20 deg^2 survey area may be necessary to mitigate sample variance when we use the directional correlation functions. On a 100 cMpc scale the light cone anisotropy dominates over the anisotropy generated by peculiar velocity gradients computed in the linear regime. By modelling instrumental noise and limited resolution, we find that the anisotropy should be easily detectable by the SKA, assuming perfect foreground removal, the limiting factor being a large enough survey size. In the case of the LOFAR, it is likely that only first anisotropy episode will fall in the observing frequency range and will be detectable only if sample variance is much reduced (i.e. a larger than 20x20 deg^2 survey, which is not presently planned).
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.
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 $P({bf textit{k}})$. 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 $P({bf textit{k}})$. 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.
Heating of neutral gas by energetic sources is crucial for the prediction of the 21 cm signal during the epoch of reionization (EoR). To investigate differences induced on statistics of the 21 cm signal by various source types, we use five radiative transfer simulations which have the same stellar UV emission model and varying combinations of more energetic sources, such as X-ray binaries (XRBs), accreting nuclear black holes (BHs) and hot interstellar medium emission (ISM). We find that the efficient heating from the ISM increases the average global 21~cm signal, while reducing its fluctuations and thus power spectrum. A clear impact is also observed in the bispectrum in terms of scale and timing of the transition between a positive and a negative value. The impact of XRBs is similar to that of the ISM, although it is delayed in time and reduced in intensity because of the less efficient heating. Due to the paucity of nuclear BHs, the behaviour of the 21~cm statistics in their presence is very similar to that of a case when only stars are considered, with the exception of the latest stages of reionization, when the effect of BHs is clearly visible. We find that differences between the source scenarios investigated here are larger than the instrumental noise of SKA1-low at $z gtrsim 7-8$, suggesting that in the future it might be possible to constrain the spectral energy distribution of the sources contributing to the reionization process.
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