ABSTRACT
The Murchison Widefield Array (MWA) team has derived new upper limits on the spherically averaged power spectrum of the 21-cm signal at six redshifts in the range z ≈ 6.5–8.7. We use these ...upper limits and a Bayesian inference framework to derive constraints on the ionization and thermal state of the intergalactic medium (IGM) as well as on the strength of a possible additional radio background. We do not find any constraints on the state of the IGM for z ≳ 7.8 if no additional radio background is present. In the presence of such a radio background, the 95 per cent credible intervals of the disfavoured models at redshift ≳ 6.5 correspond to an IGM with a volume-averaged fraction of ionized regions below 0.6 and an average gas temperature ≲ 103 K. In these models, the heated regions are characterized by a temperature larger than that of the radio background, and by a distribution with characteristic size $\lesssim 10\, h^{-1}$ Mpc and a full width at half maximum (FWHM) of $\lesssim 30\, h^{-1}$ Mpc. Within the same credible interval limits, we exclude an additional radio background of at least $0.008{{\ \rm per\ cent}}$ of the CMB at 1.42 GHz.
ABSTRACT
Using the latest upper limits on the 21-cm power spectrum at z ≈ 9.1 from the Low Frequency Array (LOFAR), we explore the regions of parameter space which are inconsistent with the data. We ...use 21cmmc, a Monte Carlo Markov chain sampler of 21cmfast which directly forward models the three dimensional (3D) cosmic 21-cm signal in a fully Bayesian framework. We use the astrophysical parametrization from 21cmfast, which includes mass-dependent star formation rates and ionizing escape fractions as well as soft-band X-ray luminosities to place limits on the properties of the high-z galaxies. Further, we connect the disfavoured regions of parameter space with existing observational constraints on the Epoch of Reionization such as ultra-violet (UV) luminosity functions, background UV photoionization rate, intergalactic medium (IGM) neutral fraction, and the electron scattering optical depth. We find that all models exceeding the 21-cm signal limits set by LOFAR at z ≈ 9.1 are excluded at ≳2σ by other probes. Finally, we place limits on the IGM spin temperature from LOFAR, disfavouring at 95 per cent confidence spin temperatures below ∼2.6 K across an IGM neutral fraction range of $0.15 \lesssim \bar{x}_{\rm H\, \rm {\small I}} \lesssim 0.6$. Note, these limits are only obtained from 141 h of data in a single redshift bin. With tighter upper limits, across multiple redshift bins expected in the near future from LOFAR, more viable models will be ruled out. Our approach demonstrates the potential of forward modelling tools such as 21cmmc in combining 21-cm observations with other high-z probes to constrain the astrophysics of galaxies.
The redshifted 21 cm line of neutral hydrogen is a promising probe of the epoch of reionization (EoR). However, its detection requires a thorough understanding and control of the systematic errors. ...We study two systematic biases observed in the Low-Frequency Array-EoR residual data after calibration and subtraction of bright discrete foreground sources. The first effect is a suppression in the diffuse foregrounds, which could potentially mean a suppression of the 21 cm signal. The second effect is an excess of noise beyond the thermal noise. The excess noise shows fluctuations on small frequency scales, and hence it cannot be easily removed by foreground removal or avoidance methods. Our analysis suggests that sidelobes of residual sources due to the chromatic point spread function (PSF) and ionospheric scintillation cannot be the dominant causes of the excess noise. Rather, both the suppression of diffuse foregrounds and the excess noise can occur due to calibration with an incomplete sky model containing predominantly bright discrete sources. The levels of the suppression and excess noise depend on the relative flux of sources which are not included in the model with respect to the flux of modelled sources. We predict that the excess noise will reduce with more observation time in the same way as the thermal noise does. We also discuss possible solutions such as using only long baselines to calibrate the interferometric gain solutions as well as simultaneous multifrequency calibration along with their benefits and shortcomings.
Context.
Faraday tomography of a field centred on the extragalactic point source 3C 196 with the LOw Frequency ARray (LOFAR) revealed an intertwined structure of diffuse polarised emission with ...straight depolarisation canals and tracers of the magnetised and multi-phase interstellar medium (ISM), such as dust and line emission from atomic hydrogen (HI).
Aims.
This study aims at extending the multi-tracer analysis of LOFAR data to three additional fields in the surroundings of the 3C 196 field. For the first time, we study the three-dimensional structure of the LOFAR emission by determining the distance to the depolarisation canals.
Methods.
We used the rolling Hough transform to compare the orientation of the depolarisation canals with that of the filamentary structure seen in HI, and based on starlight and dust polarisation data, with that of the plane-of-the-sky magnetic field. Stellar parallaxes from
Gaia
complemented the starlight polarisation with the corresponding distances.
Results.
Faraday tomography of the three fields shows a rich network of diffuse polarised emission at Faraday depths between − 10 and + 15 rad m
−2
. A complex system of straight depolarisation canals resembles that of the 3C 196 field. The depolarisation canals align both with the HI filaments and with the magnetic field probed by dust. The observed alignment suggests that an ordered magnetic field organises the multiphase ISM over a large area (~20°). In one field, two groups of stars at distances below and above 200 pc, respectively, show distinct magnetic field orientations. These are both comparable with the orientations of the depolarisation canals in the same field. We conclude that the depolarisation canals likely trace the same change in the magnetic field as probed by the stars, which corresponds to the edge of the Local Bubble.
We introduce a novel technique, called 'granulometry', to characterize and recover the mean size and the size distribution of HII regions from 21-cm tomography. The technique is easy to implement, ...but places the previously not very well-defined concept of morphology on a firm mathematical foundation. The size distribution of the cold spots in 21-cm tomography can be used as a direct tracer of the underlying probability distribution of HII region sizes. We explore the capability of the method using large-scale reionization simulations and mock observational data cubes while considering capabilities of SquareKilometreArray 1 (SKA1) low and a future extension to SKA2. We show that the technique allows the recovery of the HII region size distribution with a moderate signal-to-noise ratio from wide-field imaging (SNR less than or similar to 3), for which the statistical uncertainty is sample variance dominated. We address the observational requirements on the angular resolution, the field of view, and the thermal noise limit for a successful measurement. To achieve a full scientific return from 21-cm tomography and to exploit a synergy with 21-cm power spectra, we suggest an observing strategy using widefield imaging (several tens of square degrees) by an interferometric mosaicking/multibeam observation with additional intermediate baselines (similar to 2-4 km) in an SKA phase 2.
There are only a few ways to constrain the era of reionization and the properties of high-redshift (z 6) stars through observations. Here, we discuss one of these observables - the spectrum of the ...near-infrared background - and how it is potentially affected by the transition from Population III to Population II stars. The stronger Lyman α emission expected from massive Population III stars could result in a 'bump' in the spectrum of the near-infrared background (referred to in this work as the Lyman α bump). The strength and shape of this bump can reveal properties of Population III stars. The Lyman α bump is predicted to be higher if Population III stars are more massive and present at lower redshifts. The shape of the bump is governed by the star formation rate and the time it takes Population III stars to transition to Population II stars. If Population III stars are indeed massive, a bump is predicted as long as Population III stars exist at z 15, even if their star formation rate is as low as
. This means that there may be some observational signature in the near-infrared background of small pockets of metal-free gas forming Population III stars at z ∼ 6, even if they are quite rare.
A one-dimensional radiative transfer code is developed to track the ionization and heating pattern around the first miniquasars and Population III stars. The code follows the evolution of the ...ionization of the species of hydrogen and helium and the intergalactic medium temperature profiles as a function of redshift. The radiative transfer calculations show that the ionization signature of the first miniquasars and stars is very similar yet the heating pattern around the two is very different. Furthermore, the first massive miniquasars (≳105M⊙) do produce large ionized bubbles around them, which can potentially be imaged directly using future radio telescopes. It is also shown that the ionized bubbles not only stay ionized for considerable time after the switching off of the source, but also continue to expand for a short while due to secondary collisions prompted by the X-ray part of their spectra. Varying spectral shapes also produced sizable variations in ionized fraction and temperature profile. We also compare the radiative transfer results with the analytical approximation usually adopted for heating by miniquasars and find that, because of the inadequate treatment of the He species, the analytical approach leads to an underestimation of the temperature in the outer radii by a factor of ≈5. Population III stars – with masses in the range of 10–1000 M⊙ and modelled as blackbodies at a temperature of 50 000 K – are found to be efficient in ionizing their surroundings. The lack of very high energy photons limits the extent of heating of these first stars and has a distinctly different signature from that of the miniqsos. Observational effects on the 21-cm brightness temperature, the thermal and kinetic Sunyaev–Ze'ldovich effects, are also studied in the context of the upcoming radio and microwave telescopes like LOFAR and SPT.
With improving telescopes, it may now be possible to observe the Epoch of Reionization in multiple ways. We examine two of these observables - the excess light in the near-infrared background that ...may be due to high-redshift stars and ionized Hii bubbles, and the 21 cm emission from neutral hydrogen. Because these two forms of emission should result from different, mutually exclusive regions, an anticorrelation should exist between them. We discuss the strengths of using cross-correlations between these observations to learn more about high-redshift star formation and reionization history. In particular, we create simulated maps of emission from both the near-infrared background and 21 cm emission. We find that these observations are anticorrelated, with the strongest anticorrelation originating from times when the universe is half ionized. This result is robust and does not depend on the properties of the stars themselves. Rather, it depends on the ionization history. Cross-correlations can provide redshift information, which the near-infrared background cannot provide alone. In addition, cross-correlations can help separate foreground emission from the true high-redshift component, making it possible to say with greater certainty that we are indeed witnessing the Epoch of Reionization.
One of the aims of the Low Frequency Array (LOFAR) Epoch of Reionization (EoR) project is to measure the power spectrum of variations in the intensity of redshifted 21-cm radiation from the EoR. The ...sensitivity with which this power spectrum can be estimated depends on the level of thermal noise and sample variance, and also on the systematic errors arising from the extraction process, in particular from the subtraction of foreground contamination. We model the extraction process using realistic simulations of the cosmological signal, the foregrounds and noise, and so estimate the sensitivity of the LOFAR EoR experiment to the redshifted 21-cm power spectrum. Detection of emission from the EoR should be possible within 360 h of observation with a single station beam. Integrating for longer, and synthesizing multiple station beams within the primary (tile) beam, then enables us to extract progressively more accurate estimates of the power at a greater range of scales and redshifts. We discuss different observational strategies which compromise between depth of observation, sky coverage and frequency coverage. A plan in which lower frequencies receive a larger fraction of the time appears to be promising. We also study the nature of the bias which foreground fitting errors induce on the inferred power spectrum and discuss how to reduce and correct for this bias. The angular and line-of-sight power spectra have different merits in this respect, and we suggest considering them separately in the analysis of LOFAR data.
We report the direct detection of the kinetic Sunyaev-Zel’dovich (kSZ) effect in galaxy clusters with a 3.5
σ
significance level. The measurement was performed by stacking the
Planck
map at 217 GHz ...at the positions of galaxy clusters from the Wen-Han-Liu (WHL) catalog. To avoid the cancelation of positive and negative kSZ signals, we used the large-scale distribution of the Sloan Digital Sky Survey (SDSS) galaxies to estimate the peculiar velocities of the galaxy clusters along the line of sight and incorporated the sign in the velocity-weighted stacking of the kSZ signals. Using this technique, we were able to measure the kSZ signal around galaxy clusters beyond 3 ×
R
500
. Assuming a standard
β
-model, we also found that the gas fraction within
R
500
is
f
gas, 500
= 0.12 ± 0.04 for the clusters with the mass of
M
500
∼ 1.0 × 10
14
h
−1
M
⊙
. We compared this result to predictions from the Magneticum cosmological hydrodynamic simulations as well as other kSZ and X-ray measurements, most of which show a lower gas fraction than the universal baryon fraction for the same mass of clusters. Our value is statistically consistent with results from the measurements and simulations and also with the universal value within our measurement uncertainty.
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