A Study of Fundamental Limitations to Statistical Detection of Redshifted H I from the Epoch of Reionization (original) (raw)
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arXiv: Cosmology and Nongalactic Astrophysics, 2019
The epoch of reionization, when photons from early galaxies ionized the intergalactic medium about a billion years after the Big Bang, is the last major phase transition in the Universe's history. Measuring the characteristics of the transition is important for understanding early galaxies and the cosmic web and for modeling dwarf galaxies in the later Universe. But such measurements require probes of the intergalactic medium itself. Here we describe how the 21-cm line of neutral hydrogen provides a powerful probe of the reionization process and therefore important constraints on both the galaxies and intergalactic absorbers at that time. While existing experiments will make precise statistical measurements over the next decade, we argue that improved 21-cm analysis techniques - allowing imaging of the neutral gas itself - as well as improved theoretical models, are crucial for testing our understanding of this important era.
21-cm signatures of residual H I inside cosmic H II regions during reionization
Monthly Notices of the Royal Astronomical Society, 2015
We investigate the impact of sinks of ionizing radiation on the reionization-era 21-cm signal, focusing on 1-point statistics. We consider sinks in both the intergalactic medium and inside galaxies. At a fixed filling factor of H II regions, sinks will have two main effects on the 21-cm morphology: (i) as inhomogeneous absorbers of ionizing photons they result in smaller and more widespread cosmic H II patches; and (ii) as reservoirs of neutral gas they contribute a non-zero 21-cm signal in otherwise ionized regions. Both effects damp the contrast between neutral and ionized patches during reionization, making detection of the epoch of reionization with 21-cm interferometry more challenging. Here we systematically investigate these effects using the latest semi-numerical simulations. We find that sinks dramatically suppress the peak in the redshift evolution of the variance, corresponding to the midpoint of reionization. As previously predicted, skewness changes sign at midpoint, but the fluctuations in the residual H I suppress a late-time rise. Furthermore, large levels of residual H I dramatically alter the evolution of the variance, skewness and power spectrum from that seen at lower levels. In general, the evolution of the large-scale modes provides a better, cleaner, higher signal-tonoise probe of reionization.
Radio views of cosmic reionization
Monthly Notices of the Royal Astronomical Society Letters, 2006
We use numerical simulations of cosmic reionization and radiative processes related to the HI 21-cm emission line to produce synthetic radio maps as seen by next-generation telescopes that will operate at low radio frequencies (e.g. the Low Frequency Array, LOFAR). Two different scenarios, in which the end of reionization occurs early (z~ 13) or late (z~ 8) depending on the initial mass function (IMF) of the first stars and ionizing photon escape fraction, have been explored. For each of these models we produce synthetic HI 21-cm emission maps by convolving the simulation outputs with the provisional LOFAR sampling function in the frequency range 76-140 MHz. If reionization occurs late, LOFAR will be able to detect individual HI structures on arcmin scales, emitting at a brightness temperature of ~35 mK as a 3σ signal in about 1000 h of observing time. In the case of early reionization, the detection would be unlikely, due to decreased sensitivity and increased sky temperatures. These results assume that ionospheric, interference and foreground issues are fully under control.
Observing the reionization epoch through 21-centimetre radiation
Monthly Notices of the Royal Astronomical Society, 2004
We study the observability of the reionization epoch through the 21-cm hyperfine transition of neutral hydrogen. We use a high-resolution cosmological simulation (including hydrodynamics) together with a fast radiative transfer algorithm to compute the evolution of 21-cm emission from the intergalactic medium (IGM) in several different models of reionization. We show that the mean brightness temperature of the IGM drops from δT b ∼ 25 mK to ∼10 −2 mK during overlap (over a frequency interval ν ∼ 25 MHz), while the root mean square fluctuations on small scales drop abruptly from δT 2 b 1/2 ∼ 10 mK to ∼10 −1 mK at the end of overlap. We show that 21-cm observations can efficiently discriminate models with a single early reionization epoch from models with two distinct reionization episodes.
Imaging neutral hydrogen on large scales during the Epoch of Reionization with LOFAR
Monthly Notices of the Royal Astronomical Society, 2012
The first generation of redshifted 21 cm detection experiments, carried out with arrays like LOFAR, MWA and GMRT, will have a very low signal-to-noise ratio per resolution element ( < ∼ 0.2). In addition, whereas the variance of the cosmological signal decreases on scales larger than the typical size of ionization bubbles, the variance of the formidable galactic foregrounds increases, making it hard to disentangle the two on such large scales. The poor sensitivity on small scales on the one hand, and the foregrounds effect on large scales on the other hand, make direct imaging of the Epoch of Reionization of the Universe very difficult, and detection of the signal therefore is expected to be statistical. Despite these hurdles, in this paper we argue that for many reionization scenarios low resolution images could be obtained from the expected data. This is because at the later stages of the process one still finds very large pockets of neutral regions in the IGM, reflecting the clustering of the large-scale structure, which stays strong up to scales of ≈ 120 h −1 comoving Mpc (≈ 1 • ). The coherence of the emission on those scales allows us to reach sufficient S/N ( > ∼ 3) so as to obtain reionization 21 cm images. Such images will be extremely valuable for answering many cosmological questions but above all they will be a very powerful tool to test our control of the systematics in the data. The existence of this typical scale (≈ 120 h −1 comoving Mpc) also argues for designing future EoR experiments, e.g., with SKA, with a field of view of at least 4 • .
Reionization history from coupled cosmic microwave background/21-cm line data
Monthly Notices of The Royal Astronomical Society, 2005
We study cosmic microwave background (CMB) secondary anisotropies produced by inhomogeneous reionization by means of cosmological simulations coupled with the radiative transfer code crash. The reionization history is consistent with the Wilkinson Microwave Anisotropy Probe Thomson optical depth determination. We find that the signal arising from this process dominates over the primary CMB component for l≳ 4000 and reaches a maximum amplitude of l(l+ 1)Cl/2π≃ 1.6 × 10−13 on arcmin scales (i.e. l as large as several thousands). We then cross-correlate secondary CMB anisotropy maps with neutral hydrogen 21-cm line emission fluctuations obtained from the same simulations. The two signals are highly anticorrelated on angular scales corresponding to the typical size of H ii regions (including overlapping) at the 21-cm map redshift. We show how the CMB/21-cm cross-correlation can be used: (i) to study the nature of the reionization sources; (ii) to reconstruct the cosmic reionization history; (iii) to infer the mean cosmic ionization level at any redshift. We discuss the feasibility of the proposed experiment with forthcoming facilities.
Monthly Notices of the Royal Astronomical Society, 2006
We present the first large-scale radiative transfer simulations of cosmic reionization, in a simulation volume of (100 h −1 Mpc) 3 . This is more than a 2 orders of magnitude improvement over previous simulations. We achieve this by combining the results from extremely large, cosmological, N-body simulations with a new, fast and efficient code for 3D radiative transfer, C 2 -Ray, which we have recently developed. These simulations allow us to do the first numerical studies of the large-scale structure of reionization which at the same time, and crucially, properly take account of the dwarf galaxy ionizing sources which are primarily responsible for reionization. In our realization, reionization starts around z ∼ 21, and final overlap occurs by z ∼ 11. The resulting electron-scattering optical depth is in good agreement with the firstyear WMAP polarization data. We show that reionization clearly proceeded in an inside-out fashion, with the high-density regions being ionized earlier, on average, than the voids. Ionization histories of smaller-size (5 to 10 comoving Mpc) subregions exabit a large scatter about the mean and do not describe the global reionization history well. This is true even when these subregions are at the mean density of the universe, which shows that small-box simulations of reionization have little predictive power for the evolution of the mean ionized fraction. The minimum reliable volume size for such predictions is ∼ 30 Mpc. We derive the power-spectra of the neutral, ionized and total gas density fields and show that there is a significant boost of the density fluctuations in both the neutral and the ionized components relative to the total at arcminute and larger scales. We find two populations of H II regions according to their size, numerous, mid-sized (∼ 10 Mpc) regions and a few, rare, very large regions tens of Mpc in size. Thus, local overlap on fairly large scales of tens of Mpc is reached by z ∼ 13, when our volume is only about 50% ionized, and well before the global overlap. We derive the statistical distributions of the ionized fraction and ionized gas density at various scales and for the first time show that both distributions are clearly non-Gaussian. All these quantities are critical for predicting and interpreting the observational signals from reionization from a variety of observations like 21-cm emission, Ly-α emitter statistics, Gunn-Peterson optical depth and small-scale CMB secondary anisotropies due to patchy reionization.
STUDY OF REDSHIFTED H I FROM THE EPOCH OF REIONIZATION WITH DRIFT SCAN
The Astrophysical Journal, 2014
The detection of the Epoch of Reionization (EoR) in the redshifted 21-cm line is a challenging task. Here we formulate the detection of the EoR signal using the drift scan strategy. This method potentially has better instrumental stability as compared to the case where a single patch of sky is tracked. We demonstrate that the correlation time between measured visibilities could extend up to 1 − 2 hr for an interferometer array such as the Murchison Widefield Array (MWA), which has a wide primary beam. We estimate the EoR power based on cross-correlation of visibilities across time and show that the drift scan strategy is capable of the detection of the EoR signal with * sourabh@rri.res.in † sethi@rri.res.in arXiv:1407.4620v1 [astro-ph.IM] 17 Jul 2014 comparable/better signal-to-noise as compared to the tracking case. We also estimate the visibility correlation for a set of bright point sources and argue that the statistical inhomogeneity of bright point sources might allow their separation from the EoR signal.
arXiv (Cornell University), 2017
By using N-body hydrodynamical cosmological simulations in which the chemistry of major metals and molecules is consistently solved for, we study the interaction of metallic fine-structure lines with the cosmic microwave background radiation (CMB). Our analysis shows that the collisional induced emissions in the OI 145 µm and CII 158 µm lines during the epoch of reionization (z > 5) introduce a distortion of the CMB black-body spectrum at low frequencies (ν < 300 GHz) with amplitudes up to ∆I ν /B ν (T CMB) ∼ 10 −8-10 −7 , i.e., at the ∼ 0.1 percent level of FIRAS upper limits. Shorter wavelength fine-structure transitions (like OI 63 µm, FeII 26 µm, SiII 35 µm, FeII 35 µm, and FeII 51 µm) typically sample the reionization epoch at higher observing frequencies (ν ∈ [400, 1000] GHz). This corresponds to the Wien tail of the CMB black-body spectrum and thus the distortion level induced by those lines may be as high as ∆I ν /B ν (T CMB) ∼ 10 −4. Consequently, the brightness temperature anisotropy produced by these lines should be more relevant at higher frequencies: while practically negligible at ν = 145 GHz, signatures from CII 158 µm and OI 145 µm should amount to 1 %-5 % of the anisotropy power measured at l ∼ 5000 and ν = 220 GHz by the ACT and SPT collaborations (after taking ∆ν obs /ν obs 0.005 for the line observations). More importantly, our simulations indicate that anisotropy maps from different lines (e.g., OI 145 µm and CII 158 µm) at the same redshift show a very high degree (> 0.8) of spatial correlation, allowing for the use of observations at different frequencies (under different systematic and noise contributions) to unveil the same snapshot of the reionization epoch. Finally, our simulations also demonstrate that line-emission anisotropies extracted in narrow frequency/redshift shells are practically uncorrelated in frequency space, thus enabling standard methods for removal of foregrounds that vary smoothly in frequency, just as in HI 21 cm studies.