arXiv subjects

Rennan Barkana

Publications and source records attributed to Rennan Barkana.

At least 19 recordsLinked to original sources

arXiv2026

The Signature of Strong High-Redshift Radio Backgrounds on the Cosmic Dawn 21-cm Bispectrum

Measurements from the Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission 2 (ARCADE-2) reveal a strong radio background in the GHz frequency range. Since the cosmological 21-cm signal is measured relative to the background radiation temperature, the presence of a radio excess can significantly alter its characteristics. Previous studies have explored the impact of an inhomogeneous radio background on the global 21-cm signal and 21-cm power spectrum. This non-uniform radio background is also expected to introduce substantial non-Gaussianity. In this work, using the bispectrum, we analyze the non-Gaussianity in the 21-cm signal in the presence of an excess galactic radio background and investigate how line-of-sight radio fluctuations from early galaxies influence its nature. We find that even a moderate enhancement in radio efficiency in early galaxies significantly affects the small-scale 21-cm bispectrum. Furthermore, the delayed heating transition caused by a galactic radio background shifts the sign change in the squeezed-limit bispectrum to lower redshifts ($z\sim11$), providing a potential observational signature for distinguishing different radio background models. These results demonstrate that the 21-cm bispectrum, particularly in the squeezed limit, is highly sensitive to radio background fluctuations, making it a powerful tool for probing high-redshift radio-loud sources and the physics of the early cosmic epoch.

astro-ph.CO↗

arXiv2026

High-Redshift Signatures from the Cosmic Dawn and the Epoch of Reionization

In this chapter, we provide a comprehensive overview of the astrophysical and cosmological processes that shape the 21-cm signal during Cosmic Dawn and the Epoch of Reionization. We investigate both standard and exotic signatures potentially observable with SKA-Low. Standard signatures are those expected within the $Λ$CDM framework, including contributions from the first stars, galaxies, and black holes. Exotic signatures are more speculative indicating new physics, such as primordial black holes, modifications to the dark matter sector, non-standard primordial fluctuations, or strongly emitting radio galaxies. The effects of these different sources or scenarios are evaluated in the context of the expected sensitivity of SKA-Low, considering the AA* and AA4 configurations. The chapter aims to provide an overview of the theoretical landscape of 21-cm signatures and to highlight how the forthcoming SKA-Low observations will improve our understanding of astrophysical processes at early times and may open the door towards new physics beyond the $Λ$CDM framework.

astro-ph.CO↗

arXiv2026

Overview of 21cm Experiments at high redshift with SKAO

We provide an overview of the eight SKAO Science Book chapters that motivate the Epoch of Reionisation and Cosmic Dawn experiments with SKA-Low. We describe the individual SKA-Low experiments and expected sensitivity - power spectrum, tomography, 21-cm forest, cross-correlations, building on the broad observational plan laid out in the 2015 SKA Science Book. Finally, we outline features of the telescope that will be critical for the success of EoR/CD science, e.g., beam apodization, substations, and multi-beaming.

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arXiv2026

Impact of Stochastic Pop~III X-ray Binaries on the Cosmological 21-cm Signal

High-mass X-ray binaries are one of the primary drivers of the 21-cm signal from Cosmic Dawn and Reionization, playing a leading role in the thermal history of the intergalactic medium. In traditional semi-numerical simulations, a deterministic scaling relation between the total X-ray luminosity of high-mass X-ray binaries, $L_{\rm X}$, and star formation rate (SFR) is usually adopted. However, this assumption is inaccurate for high-redshift low-SFR regions hosting few sources. The spatial variation in the number and luminosity of these sources is expected to enhance fluctuations in the Cosmic Dawn 21-cm signal. Here we quantify this effect by introducing a stochastic $L_{\rm X}$ model sampled from a power-law X-ray luminosity function. Implementing this in 21cmSPACE, a large-scale simulation framework of Cosmic Dawn and Reionization, we find that the stochasticity leads to enhanced fluctuations in X-ray heating rate fields, and affects the 21-cm power spectrum on small scales ($k>0.3~ \mathrm{cMpc^{-1}}$). The impact of stochasticity on the global 21-cm signal and on the large-scale power spectrum is found to be negligible. Our results suggest these effects will remain undetected by the upcoming Square Kilometer Array. However, large-scale lunar-based experiments may be sensitive to the signatures of stochastic X-ray heating at $z\sim 25$. Quantifying these corrections is a vital step toward robust 21-cm modeling and ensuring that future precision data interpretation is free from astrophysical biases.

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arXiv2026

Probing small-scale dark matter clumping with the large-scale 21-cm power spectrum

The 21-cm line of hydrogen is the most promising probe of the Dark Ages and Cosmic Dawn. We combine hydrodynamical simulations with a large-scale grid in order to calculate the effect of non-linear structure formation on the large-scale 21-cm power spectrum, focusing on redshifts $z=20-40$. As the clumping effect arises from small-scale density fluctuations, it offers a unique opportunity to probe the standard cold dark matter model in a new regime and thus potentially investigate the properties of dark matter. To this end, we also study a warm dark matter $-$ like model with a Gaussian cutoff on a scale of 50 kpc. We find that clumping has a significant impact on the large-scale 21-cm power spectrum. For example, for the Dark Ages case at $z=30$ and wavenumber $k=0.05$ Mpc$^{-1}$, small-scale clustering enhances the 21-cm power spectrum by $13\%$. Once Lyman-$α$ coupling kicks in due to the first stars, the 21-cm signal strengthens, and the effect of clumping grows; it suppresses the observable power spectrum at $z=20$ by a factor of two, while the cutoff model has less than half the clumping impact. The clumping effect is significantly higher than the sensitivity of the planned Square Kilometre Array (SKA) AA$^\star$ configuration, by up to a factor of 20 for standard cold dark matter, though detection will require separation from foregrounds and from astrophysical contributions to the 21-cm power spectrum.

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arXiv2026

The effect of photon re-scattering due to cosmic reionization on 21-cm images and power spectra

The 21-cm signal from neutral hydrogen serves as a critical tool for unraveling the astrophysical processes that shaped cosmic dawn and the epoch of reionization. We explore the usually overlooked impact of re-scattering of 21-cm photons during and after reionization, similarly to cosmic microwave background photons. This scattering affects the observed brightness temperature by mixing the original signal with light scattered into the line of sight from other regions, effectively at the mean 21-cm brightness temperature. This gives a small but significant effect. We show that it attenuates the fluctuations in a 21-cm image by $4-7\%$, while reducing the 21-cm power spectrum by a scale-independent $7-13\%$ during cosmic dawn and reionization. Incorporating this correction is vital for precisely comparing theoretical predictions with observations from experiments such as NenuFAR, LOFAR, and HERA, and the upcoming Square Kilometre Array.

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arXiv2025

First Results from HERA Phase II

We report the first upper limits on the power spectrum of 21-cm fluctuations during the Epoch of Reionization and Cosmic Dawn from Phase II of the Hydrogen Epoch of Reionization Array (HERA) experiment. HERA Phase II constitutes several significant improvements in the signal chain compared to Phase I, most notably resulting in expanded frequency bandwidth, from 50-250 MHz. In these first upper limits, we investigate a small two-week subset of the available Phase II observations, with a focus on identifying new systematic characteristics of the instrument, and establishing an analysis pipeline to account for them. We report 2$σ$ upper limits in eight spectral bands, spanning $5.6 \leq z \leq 24.4$ that are consistent with thermal noise at the $2σ$ level for $k \gtrsim 0.6-0.9 h{\rm Mpc}^{-1}$ (band dependent). Our tightest limit during Cosmic Dawn ($z>12$) is $1.13\times 10^6 {\rm mK}^2$ at ($k=0.55 h{\rm Mpc}^{-1}, z=16.78$), and during the EoR ($5.5<z<12$) it is $1.78\times 10^3 {\rm mK}^2$ at ($k=0.70 h{\rm Mpc}^{-1}, z=7.05$). We find that mutual coupling has become our dominant systematic, leaking foreground power that strongly contaminates the low-$k$ modes, resulting in the loss of modes from $k=0.35-0.55$ compared to Phase I data.

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arXiv2025

Narrowing the discovery space of the cosmological 21-cm signal using multi-wavelength constraints

The cosmic 21-cm signal is a promising probe of the early Universe, owing to its sensitivity to the thermal state of the neutral intergalactic medium (IGM) and properties of the first luminous sources. Here, we constrain the 21-cm signal and infer IGM properties using the Population II galaxy parameters derived in a previous study through multi-wavelength synergies. This includes high-redshift UV luminosity functions (UVLFs) from Hubble Space Telescope (HST) and James Webb Space Telescope (JWST), cosmic X-ray and radio backgrounds (CXB and CRB), the SARAS 3 global 21-cm signal non-detection, and HERA 21-cm power spectrum upper limits. From CXB and HERA data, we infer the IGM kinetic temperature to be $T_\text{K}(z=15)\lesssim 7.7~\text{K}$, $2.5~\text{K} \lesssim T_\text{K}(z=10) \lesssim 66~\text{K}$, and $20~\text{K}\lesssim T_\text{K}(z=6) \lesssim 2078~\text{K}$ at 95% credible interval (C.I.). Similarly, CRB and HERA data limit the radio emission efficiency of galaxies, giving $T_\text{rad}(z=15) \lesssim 47~\text{K}$, $T_\text{rad}(z=10)\lesssim 51~\text{K}$, and $T_\text{rad}(z=6)\lesssim 101~\text{K}$. These constraints, strengthened by UVLFs from HST and JWST, enable the first $\textit{lower bound}$ on the cosmic 21-cm signal. We infer an absorption trough of depth ${-201~\text{mK}\lesssim T_\text{21,min} \lesssim -68~\text{mK}}$ at $z_\text{min}\approx10-16$, and a power spectrum of $8.7~\text{mK}^2 < Δ_{21}^2(z=15) < 197~\text{mK}^2$ at $k=0.35~h\text{Mpc}^{-1}$. Our results highlight the power of multi-wavelength synergies in constraining the early Universe. While promising for upcoming 21-cm experiments, the results depend on our assumption of a redshift-independent X-ray and radio efficiency of galaxies, and the exclusion of a flexible model for Population III stars.

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arXiv2025

Classification of Radio Backgrounds at Cosmic Dawn and 21 cm Signal Confirmation Using Neural Networks

Several ongoing and upcoming radio telescopes aim to detect either the global 21 cm signal or the 21 cm power spectrum. The extragalactic radio background, as detected by ARCADE-2 and LWA-1, suggests a strong radio background from cosmic dawn, which can significantly alter the cosmological 21 cm signal, enhancing both the global signal amplitude and the 21 cm power spectrum. In this paper, we employ an artificial neural network (ANN) to check if there is a radio excess over the cosmic microwave background in mock data, and if present, we classify its type into one of two categories, a background from high-redshift radio galaxies or a uniform exotic background from the early Universe. Based on clean data (without observational noise), the ANN can predict the background radiation type with $96\%$ accuracy for the power spectrum and $90\%$ for the global signal. Although observational noise reduces the accuracy, the results remain quite useful. We also apply ANNs to map the relation between the 21 cm power spectrum and the global signal. By reconstructing the global signal using the 21 cm power spectrum, an ANN can estimate the global signal range consistent with an observed power spectrum from SKA-like experiments. Conversely, we show that an ANN can reconstruct the 21 cm power spectrum over a wide range of redshifts and wavenumbers given the global signal over the same redshifts. Such trained networks can potentially serve as a valuable tool for cross-confirmation of the 21 cm signal.

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arXiv2025

The Signature of Sub-galactic Dark Matter Clumping in the Global 21-cm Signal of Hydrogen

It is thought that the Universe went through an early period known as the Dark Ages, during which primeval density fluctuations grew to form the first luminous objects, marking the beginning of Cosmic Dawn around 100 million years after the Big Bang. The 21-cm line of hydrogen atoms is the most promising probe of these epochs, with extensive observational efforts underway. We combine hydrodynamical simulations with a large-scale grid in order to precisely calculate the effect of non-linear structure formation on the global (sky-averaged) 21-cm radio intensity. We show that it presents a potential opportunity to probe the properties of dark matter in a new regime, corresponding to a length-scale of only 150,000 light years and a mass-scale of 20 million Solar masses. This effect can in principle be detected unambiguously during the Dark Ages, where the weak signal requires an array of global signal antennae. During Cosmic Dawn, when stellar radiation boosts the signal, a single global antenna suffices, but the clumping effect must then be separated from the effect of the stars. Our findings open new avenues for testing the nature of dark matter as well as non-standard cosmological models.

astro-ph.CO↗

arXiv2025

Exploiting synergies between JWST and cosmic 21-cm observations to uncover star formation in the early Universe

In the current era of JWST, we continue to uncover a wealth of information about the Universe deep into the Epoch of Reionization. In this work, we use a suite of simulations with 21cmSPACE, to explore the astrophysical properties of early galaxies and their imprint on high-redshift observables. Our analysis incorporates a range of multi-wavelength datasets including UV luminosity functions (UVLFs) from HST and JWST spanning $z=6-14.5$, the 21-cm global signal and power spectrum limits from SARAS 3 and HERA respectively, as well as present-day diffuse X-ray and radio backgrounds. We constrain a flexible halo-mass and redshift dependent model of star-formation efficiency (SFE), defined as the fraction of gas converted into stars, and find that it is best described by minimal redshift evolution at $z\approx 6-10$, followed by rapid evolution at $z\approx10-15$. Using Bayesian inference, we derive functional posteriors of the SFE, inferring that halos of mass $M_h=10^{10}\,\mathrm{M}_\odot$ have efficiencies of $\approx 1 - 2\%$ at $z\lesssim10$, $\approx8\%$ at $z=12$ and $\approx21\%$ at $z=15$. We also highlight the synergy between UVLFs and global 21-cm signal from SARAS 3 in constraining the minimum virial conditions required for star-formation in halos. In parallel, we find the X-ray and radio efficiencies of early galaxies to be $f_X = 0.8^{+9.7}_{-0.4}$ and $f_r \lesssim 16.9$ respectively, improving upon previous works that exclude UVLF data. Our results underscore the critical role of UVLFs in constraining early galaxy properties, and their synergy with 21-cm and other multi-wavelength observations.

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arXiv2025

Effects of chemically homogeneous evolution of the first stars on the 21-cm signal and reionization

The first generation of stars, known as Population III (Pop III), played a crucial role in the early Universe through their unique formation environment and metal-free composition. These stars can undergo chemically homogeneous evolution (CHE) due to fast rotation, becoming more compact and hotter/bluer than their (commonly assumed) non-rotating counterparts. In this study, we investigate the impact of Pop III CHE on the 21-cm signal and cosmic reionization under various assumptions on Pop III star formation, such as their formation efficiency, initial mass function, and transition to metal-enriched star formation. We combine stellar spectra computed by detailed atmosphere models with semi-numerical simulations of Cosmic Dawn and the Epoch of Reionization ($z\sim 6-30$). The key effect of CHE arises from the boosted ionizing power of Pop III stars, which reduces the Pop III stellar mass density required to reproduce the observed Thomson scattering optical depth by a factor of $\sim 2$. Meanwhile, the maximum 21-cm global absorption signal is shallower by up to $\sim 15$ mK (11%), partly due to the reduced Lyman-band emission from CHE, and the large-scale ($k\sim 0.2\ \rm cMpc^{-1}$) power drops by a factor of a few at $z\gtrsim 25$. In general, the effects of CHE can be comparable to those of Pop III star formation parameters, showing an interesting interplay with distinct features in different epochs. These results highlight the importance of metal-free/poor stellar evolution in understanding the early Universe and suggest that future studies should consider joint constraints on the physics of star/galaxy formation and stellar evolution.

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arXiv2025

Rapid and Late Cosmic Reionization Driven by Massive Galaxies: a Joint Analysis of Constraints from 21-cm, Lyman Line & CMB Data Sets

Observations of the Epoch of Reionization (EoR) have the potential to answer long-standing questions of astrophysical interest regarding the nature of the first luminous sources and their effects on the intergalactic medium (IGM). We present astrophysical constraints from a Neural Density Estimation-Accelerated Bayesian joint analysis of constraints deriving from Cosmic Microwave Background power spectrum measurements from Planck and SPT, IGM neutral fraction measurements from Lyman-line-based data sets and 21-cm power spectrum upper limits from HERA, LOFAR and the MWA. In the context of the model employed, the data is found to be consistent with galaxies forming from predominantly atomic-cooled hydrogen gas in dark matter halos, with masses $M_\mathrm{min} \gtrsim 2.6 \times 10^{9}~M_{\odot} ((1+z)/10)^{\frac{1}{2}}$ at 95% credibility ($V_\mathrm{c} \gtrsim 50~\mathrm{km~s^{-1}}$) being the dominant galactic population driving reionization. These galaxies reionize the neutral hydrogen in the IGM over a narrow redshift interval ($Δz_\mathrm{re} < 1.8$ at 95% credibility), with the midpoint of reionization (when the sky-averaged IGM neutral fraction is 50%) constrained to $z_{50} = 7.16^{+0.15}_{-0.12}$. Given the parameter posteriors from our joint analysis, we find that the posterior predictive distribution of the global 21-cm signal is reduced in amplitude and shifted to lower redshifts relative to the model prior. We caution, however, that our inferences are model-dependent. Future work incorporating updated, mass-dependent star formation efficiencies in atomic cooling halos, informed by the latest UV luminosity function constraints from the James Webb Space Telescope, promises to refine these inferences further and enhance our understanding of cosmic reionization.

astro-ph.CO↗

arXiv2024

Constraining the clustering and 21-cm signature of radio galaxies at cosmic dawn

The efficiency of radio emission is an important unknown parameter of early galaxies at cosmic dawn, as models with high efficiency have been shown to modify the cosmological 21-cm signal substantially, deepening the absorption trough and boosting the 21-cm power spectrum. Such models have been previously directly constrained by the overall extragalactic radio background as observed by ARCADE-2 and LWA-1. In this work, we constrain the clustering of high redshift radio sources by utilizing the observed upper limits on arcminute-scale anisotropy from the VLA at 4.9~GHz and ATCA at 8.7~GHz. Using a semi-numerical simulation of a plausible astrophysical model for illustration, we show that the clustering constraints on the radio efficiency are much stronger than those from the overall background intensity, by a factor that varies from 12 at redshift 7 to 30 at redshift 22. As a result, the predicted maximum depth of the global 21-cm signal is lowered by a factor of 5 (to 1700~mK), and the maximum 21-cm power spectrum peak at cosmic dawn is lowered by a factor of 24 (to $2\times 10^5$~mK$^2$). We conclude that the observed clustering is the strongest current direct constraint on such models, but strong early radio emission from galaxies remains viable for producing a strongly enhanced 21-cm signal from cosmic dawn.

astro-ph.CO↗

arXiv2024

Emulation of the Cosmic Dawn 21-cm Power Spectrum and Classification of Excess Radio Models Using an Artificial Neural Network

The cosmic 21-cm line of hydrogen is expected to be measured in detail by the next generation of radio telescopes. The enormous dataset from future 21-cm surveys will revolutionize our understanding of early cosmic times. We present a machine learning approach based on an Artificial Neural Network that uses emulation in order to uncover the astrophysics in the epoch of reionization and cosmic dawn. Using a seven-parameter astrophysical model that covers a very wide range of possible 21-cm signals, over the redshift range 6 to 30 and wavenumber range $0.05$ to $1 \ \rm{Mpc}^{-1}$ we emulate the 21-cm power spectrum with a typical accuracy of $10 - 20\%$. As a realistic example, we train an emulator using the power spectrum with an optimistic noise model of the Square Kilometre Array (SKA). Fitting to mock SKA data results in a typical measurement accuracy of $2.8\%$ in the optical depth to the cosmic microwave background, $34\%$ in the star-formation efficiency of galactic halos, and a factor of 9.6 in the X-ray efficiency of galactic halos. Also, with our modeling we reconstruct the true 21-cm power spectrum from the mock SKA data with a typical accuracy of $15 - 30\%$. In addition to standard astrophysical models, we consider two exotic possibilities of strong excess radio backgrounds at high redshifts. We use a neural network to identify the type of radio background present in the 21-cm power spectrum, with an accuracy of $87\%$ for mock SKA data.

astro-ph.CO↗

arXiv2023

Joint analysis constraints on the physics of the first galaxies with low frequency radio astronomy data

The first billion years of cosmic history remains largely unobserved. We demonstrate, using a novel machine learning technique, how combining upper limits on the spatial fluctuations in the 21-cm signal with observations of the sky-averaged 21-cm signal from neutral hydrogen can improve our understanding of this epoch. By jointly analysing data from SARAS3 (redshift $z\approx15-25$) and limits from HERA ($z\approx8$ and $10$), we show that such a synergetic analysis provides tighter constraints on the astrophysics of galaxies 200 million years after the Big Bang than can be achieved with the individual data sets. Although our constraints are weak, this is the first time data from a sky-averaged 21-cm experiment and power spectrum experiment have been analysed together. In synergy, the two experiments leave only $64.9^{+0.3}_{-0.1}$% of the explored broad theoretical parameter space to be consistent with the joint data set, in comparison to $92.3^{+0.3}_{-0.1}$% for SARAS3 and $79.0^{+0.5}_{-0.2}$% for HERA alone. We use the joint analysis to constrain star formation efficiency, minimum halo mass for star formation, X-ray luminosity of early emitters and the radio luminosity of early galaxies. The joint analysis disfavours at 68% confidence a combination of galaxies with X-ray emission that is $\lesssim 33$ and radio emission that is $\gtrsim 32$ times as efficient as present day galaxies. We disfavour at $95$% confidence scenarios in which power spectra are $\geq126$ mK$^{2}$ at $z=25$ and the sky-averaged signals are $\leq-277$ mK.

astro-ph.CO↗

arXiv2023

Constraining exotic dark matter models with the dark ages 21-cm signal

The dark ages 21-cm signal is a powerful tool for precision cosmology and probing new physics. We study two non-standard models: an excess radio background (ERB) model (possibly generated by dark matter decay) and the millicharged dark matter (mDM) model. These models were inspired by the possible EDGES detection of a strong global 21-cm absorption during cosmic dawn, but more generally they provide a way to anticipate the potential discovery space. During the dark ages the 21-cm global signal in the ERB model reaches a saturated form for an amplitude $A_{\rm r}=0.4$, where $A_{\rm r}$ is the radio background intensity at cosmic dawn relative to the cosmic microwave background. This amplitude is one-fifth of the minimum required to explain the EDGES signal, and corresponds to just 0.1% of the observed extragalactic background; it would give a signal that can be detected at 5.9$σ$ significance (compared to $4.1\,σ$ for the standard signal) and can be distinguished from the standard (no ERB) signal at $8.5\,σ$, all with a 1,000 hr global signal measurement. The 21-cm power spectrum has potentially more information, but far greater resources would be required for comparable constraints. For the mDM model, over a range of viable parameters, the global signal detection significance would be $4.7-7.2\,σ$, and it could be distinguished from the standard at $2.2-9.3\,σ$. With an array of global signal antennas achieving an effective 100,000 hr integration, the significance would be $10\,\times$ better. Our analysis helps motivate the development of lunar and space-based dark ages experiments.

astro-ph.CO↗

arXiv2023

Prospects for precision cosmology with the 21 cm signal from the dark ages

The 21 cm signal from the dark ages provides a potential new probe of fundamental cosmology. While exotic physics could be discovered, here we quantify the expected benefits within the standard cosmology. A measurement of the global (sky-averaged) 21 cm signal to the precision of thermal noise from a 1,000 h integration would yield a measurement within 10% of a combination of cosmological parameters. A 10,000 h integration would improve this measurement to 3.2% and constrain the cosmic helium fraction to 9.9%. Precision cosmology with 21 cm fluctuations requires a collecting area of 10 km$^2$ (corresponding to 400,000 stations), which, with a 1,000 h integration, would exceed the same global case by a factor of $\sim2$. Enhancing the collecting area or integration time by an order of magnitude would yield a 0.5% parameter combination, a helium measurement five times better than Planck and a constraint on the neutrino mass as good as Planck. Our analysis sets a baseline for upcoming lunar and space-based dark-ages experiments.

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