The intergalactic medium is expected to be at its coldest point before the formation of the first stars in the universe. Motivated by recent results from the EDGES experiment, we revisit the standard calculation of the kinetic temperature of the neutral gas through this period. When the first ultraviolet (UV) sources turn on, photons redshift into the Lyman lines of neutral hydrogen and repeatedly scatter within the Lyman-$α$ line. They heat the gas via atomic recoils, and, through the Wouthuysen-Field effect, set the spin temperature of the 21-cm hyperfine (spin-flip) line of atomic hydrogen in competition with the resonant cosmic microwave background (CMB) photons. We show that the Lyman-$α$ photons also mediate energy transfer between the CMB photons and the thermal motions of the hydrogen atoms. In the absence of X-ray heating, this new mechanism is the major correction to the temperature of the adiabatically cooling gas ($\sim 10 \%$ at $z=17$), and is several times the size of the heating rate found in previous calculations. We also find that the effect is more dramatic in non-standard scenarios that either enhance the radio background above the CMB or invoke new physics to cool the gas in order to explain the EDGES results. The coupling with the radio background can reduce the depth of the 21-cm absorption feature by almost a factor of two relative to the case with no sources of heating, and prevent the feature from developing a flattened bottom. As an inevitable consequence of the UV background that generates the absorption feature, this heating should be accounted for in any theoretical prediction.
We present a detailed analysis of an astrophysical mechanism that generates cosmological magnetic fields during the Epoch of Reionization. It is based on the photoionization of the Intergalactic Medium by the first sources formed in the Universe. First the induction equation is derived, then the characteristic length and time scales of the mechanism are identified, and finally numerical applications are carried out for first stars, primordial galaxies and distant powerful quasars. In these simple examples, the strength of the generated magnetic fields varies between the order of $10^{-23}$ G o…
We study the formation and destruction of the first molecules at the epochs of the Dark Ages and Cosmic Dawn to evaluate the luminosity of the protogalaxy clumps (halos) in the molecular lines. The cosmological recombination is described using the model of an effective three-level atom, while the chemistry of the molecules is examined using the relevant basic kinetic equations. We then studied the effect of collisional and radiative excitation of molecules on the intensity of molecular emission in both warm and hot halos. Using the Planck data on the reionization of the intergalactic medium at…
We present an overview of the theory of high-redshift star and X-ray source formation, and how they affect the 21-cm background. Primary focus is given to Lyman alpha pumping and X-ray heating mechanisms at cosmic dawn, opening a new observational window for high-redshift astrophysics by generating sizable fluctuations in the 21-cm background. We describe observational prospects for power spectrum analysis and 3D tomography (imaging) of the signature of these early astrophysical sources by SKA1-LOW and SKA2.
The global 21-cm signal from the cosmic dawn is affected by a variety of heating and cooling processes. We investigate the impact of heating due to Lyman-$α$ (Ly~$α$) photons on the global 21-cm signal at cosmic dawn using an analytical expression of the spectrum around the Ly~$α$ resonance based on the so-called `wing approximation'. We derive a new expression for the scattering correction and for the first time give a simple close-form expression for the cooling due to injected Ly~$α$ photons. We perform a short parameter study by varying the Ly~$α$ background intensity by four orders of mag…
The resonance scattering of Ly$α$ photons with neutral hydrogen atoms in the intergalactic medium not only couples the spin temperature to the kinetic temperature but also leads to a heating of the gas. We investigate the impact of this heating on the average brightness temperature of the 21-cm signal from the Cosmic Dawn in the context of the claimed detection by the EDGES low-band experiment. We model the evolution of the global signal taking into account the Ly$α$ coupling and heating and a cooling which can be stronger than the Hubble cooling. Using the claimed detection of a strong absorp…
The upcoming radio interferometer Square Kilometre Array is expected to directly detect the redshifted 21-cm signal from the Cosmic Dawn for the first time. In this era temperature fluctuations from X-ray heating of the neutral intergalactic medium can impact this signal dramatically. Previously, in (Ross et al, 2017), we presented the first large-volume, 244 $h^{-1}$Mpc = 349 Mpc a side, fully numerical radiative transfer simulations of X-ray heating. This work is a follow-up where we now also consider QSO-like sources in addition to high mass X-ray binaries. Images of the two cases are clear…
Line Intensity Mapping Prediction from the Cosmic Dawn (CoDa) III Simulation for H$α$ from Galaxies and the Intergalactic Medium during the Epoch of Reionization
I review recent progress and challenges in studies of the earliest galaxies, seen when the Universe was less than 1 billion years old. Can they be used as reliable tracers of the physics of cosmic reionization thereby complementing other, more direct, probes of the evolving neutrality of the intergalactic medium? Were star-forming galaxies the primary agent in the reionization process and what are the future prospects for identifying the earliest systems devoid of chemical enrichment? Ambitious future facilities are under construction for exploring galaxies and the intergalactic medium in the …
Lyman-alpha photons from the first radiating sources in the Universe play a pivotal role in 21-cm radio detections of Cosmic Dawn and the Epoch of Reionization. Comments are provided on the effect of the hyperfine structure of hydrogen on the rate of heating or cooling of the Intergalactic Medium. It is shown that heating of the still neutral hydrogen by the Cosmic Microwave Background is negligible, with a characteristic heating time of 1e27 s/ (1+z) at redshift z.
The sky-averaged (global) 21-cm signal is a very promising probe of the Cosmic Dawn, when the first luminous sources were formed and started to shine in a substantially neutral intergalactic medium. I here report on the status and early result of the Large-Aperture Experiment to Detect the Dark Age that focuses on observations of the global 21-cm signal in the $16 \lesssim z \lesssim 30$ range.
Relativistic charged particles (CR for cosmic-rays) produced by Supernova explosion of the first generation of massive stars that are responsible for the re-ionization of the universe escape into the intergalactic medium, carrying an electric current. Charge imbalance and induction give rise to a return current, $\vec j_t$, carried by the cold thermal plasma which tends to cancel the CR current. The electric field, $\vec E=η\vec j_t$, required to draw the collisional return current opposes the outflow of low energy cosmic rays and ohmically heats the cold plasma. Owing to inhomogeneities in th…
Upcoming measurements of the highly redshifted 21cm line with next-generation radio telescopes such as HERA and SKA will provide the intriguing opportunity to probe dark matter (DM) physics during the Epoch of Reionization (EoR), Cosmic Dawn, and the Dark Ages. With HERA already under construction, there is a pressing need to thoroughly understand the impact of DM physics on the intergalactic medium (IGM) during these epochs. We present first results of a hydrodynamic simulation suite with $2 \times 512^3$ particles in a $(100 \ h^{-1} \ \text{Mpc})^3$ box with DM annihilation and baryonic coo…
Upcoming observations of the 21-cm signal from the Epoch of Reionization will soon provide the first direct detection of this era. This signal is influenced by many astrophysical effects, including long range X-ray heating of the intergalactic gas. During the preceding Cosmic Dawn era the impact of this heating on the 21-cm signal is particularly prominent, especially before spin temperature saturation. We present the largest-volume (349\,Mpc comoving=244~$h^{-1}$Mpc) full numerical radiative transfer simulations to date of this epoch that include the effects of helium and multi-frequency heat…
Miniati and Bell (2011) proposed a mechanism for the generation of magnetic seeds that is based the finite resistivity of the low temperature IGM in the high redshift universe. In this model, cosmic-ray protons generated by the first generation of galaxies, escape into the intergalactic medium carrying an electric current that induces return currents, $j_t$, and associated electric fields, $\vec E=η\vec j_t$ there. Because the resistivity, $η$, depends on the IGM temperature, which is highly inhomogeneous due to adiabatic contraction and shocks produced by structure formation, a non-vanishing …
Allowing for enhanced Ly$α$ photon line emission from Population III dominated stellar systems in the first forming galaxies, we show the 21-cm cosmic dawn signal at $10<z<30$ may substantially differ from standard scenarios. Energy transfer by Ly$α$ photons emerging from galaxies may heat intergalactic gas if HII regions within galaxies are recombination bound, or cool the gas faster than by adiabatic expansion if reddened by winds internal to the haloes. In some cases, differential 21-cm antenna temperatures near $-500$ mK may be achieved at $15<z<25$, similar to the signature detected by th…
The "cosmic dawn" refers to the period of the Universe's history when stars and black holes first formed and began heating and ionizing hydrogen in the intergalactic medium (IGM). Though exceedingly difficult to detect directly, the first stars and black holes can be constrained indirectly through measurements of the cosmic 21-cm background, which traces the ionization state and temperature of intergalactic hydrogen gas. In this white paper, we focus on the science case for such observations, in particular those targeting redshifts z $\gtrsim$ 10 when the IGM is expected to be mostly neutral. …
The upcoming radio interferometer Square Kilometre Array (SKA) is expected to directly detect the redshifted 21-cm signal from the neutral hydrogen present during the Cosmic Dawn. Temperature fluctuations from X-ray heating of the neutral intergalactic medium can dominate the fluctuations in the 21-cm signal from this time. This heating depends on the abundance, clustering, and properties of the X-ray sources present, which remain highly uncertain. We present a suite of three new large-volume, 349\,Mpc a side, fully numerical radiative transfer simulations including QSO-like sources, extending…
We make an analytical estimate of the maximum 21 cm absorption signal from the cosmic dawn, taking into account the inhomogeneity of gas distribution in the intergalactic medium (IGM) due to non-linear structure formation. The gas located near halos is over-dense but adiabatically heated, while the gas far from halos is under-dense and hence cooler. The cumulative effect of adiabatic heating and cooling from this gas inhomogeneity results in a reduction in the maximum global 21 cm absorption depth by about 40% as compared with homogeneous IGM model, assuming saturated coupling between the spin…
The Square Kilometre Array (SKA) will offer an unprecedented view onto the early Universe, using interferometric observations of the redshifted 21cm line. The 21cm line probes the thermal and ionization state of the cosmic gas, which is governed by the birth and evolution of the first structures in our Universe. Here we show how the evolution of the 21cm signal will allow us to study when the first generations of galaxies appeared, what were their properties, and what was the structure of the intergalactic medium. We highlight qualitative trends which will offer robust insights into the early …
TeV-blazars potentially heat the intergalactic medium (IGM) as their gamma rays interact with photons of the extragalactic background light to produce electron-positron pairs, which lose their kinetic energy to the surrounding medium through plasma instabilities. This results in a heating mechanism that is only weakly sensitive to the local density, and therefore approximately spatially uniform, naturally producing an inverted temperature-density relation in underdense regions. In this paper we go beyond the approximation of uniform heating and quantify the heating rate fluctuations due to the…
The intergalactic medium (IGM) prior to the epoch of reionization consists mostly of neutral hydrogen gas. Ly-alpha photons produced by early stars resonantly scatter off hydrogen atoms, causing energy exchange between the radiation field and the gas. This interaction results in moderate heating of the gas due to the recoil of the atoms upon scattering, which is of great interest for future studies of the pre-reionization IGM in the HI 21 cm line. We investigate the effect of this Ly-alpha heating in the IGM with linear density, temperature, and velocity perturbations. Perturbations smaller th…
In this review article, we briefly outline our current understanding of the physics associated with the HI 21-cm signal from cosmic dawn. We discuss different phases of cosmic dawn as the ambient gas and the background radiations evolve with the redshift. We address the consequences of several possible heating sources and radiation background on the global 21-cm signal. We further review our present perspective of other important aspects of the HI 21-cm signal such as the power spectrum and imaging. Finally, we highlight the future key measurements of the Square Kilometre Array and other ongoi…
It is extremely important to understand the processes through which the thermal state of the inter-galactic medium (IGM) evolved in the early universe in order to study the evolution of HI 21-cm signal during cosmic dawn. Here, we consider the heating of the IGM due to cosmic ray protons generated by the supernovae from both early Pop III and Pop II stars. The low energy cosmic ray protons from Pop III supernovae can escape from minihalos and heat the IGM via collision and ionization of hydrogen. Furthermore, high energy protons generated in Pop II supernovae can escape the hosting halos and h…
Current and upcoming radio interferometric experiments are aiming to make a statistical characterization of the high-redshift 21cm fluctuation signal spanning the hydrogen reionization and X-ray heating epochs of the universe. However, connecting 21cm statistics to underlying physical parameters is complicated by the theoretical challenge of modeling the relevant physics at computational speeds quick enough to enable exploration of the high dimensional and weakly constrained parameter space. In this work, we use machine learning algorithms to build a fast emulator that mimics expensive simulat…
The dark age of the universe, when no luminous object had existed, ended with the birth of the first stars, galaxies, and blackholes. This epoch is called cosmic dawn. Cosmic reionization is the major transition of the intergalactic medium (IGM) in the universe driven by ionizing photons emitted from luminous objects. Although the epoch through the dark age to reionization is a milestone in the universe, our knowledge of this epoch has not been sufficient yet. Cosmic 21cm signal, which is emitted from neutral hydrogen, is expected to open a new window for this epoch. In this review paper, we f…
21 cm cosmology, the statistical observation of the high redshift universe using the hyperfine transition of neutral hydrogen, has the potential to revolutionize our understanding of cosmology and the astrophysical processes that underlie the formation of the first stars, galaxies, and black holes during the "Cosmic Dawn." By making tomographic maps with low frequency radio interferometers, we can study the evolution of the 21 cm signal with time and spatial scale and use it to understand the density, temperature, and ionization evolution of the intergalactic medium over this dramatic period i…
Redshifted 21cm signal is a promising tool to investigate the state of intergalactic medium (IGM) in the Cosmic Dawn (CD) and Epoch of Reionization(EoR). In our previous work (Shimabukuro et al 2015), we studied the variance and skewness to give a clear interpretation of 21cm power spectrum and found that skewness is a good indicator of the epoch when X-ray heating becomes effective. Thus, the non-Gaussian feature of the spatial distribution of the 21cm signal is expected to be useful to investigate the astrophysical effects in the CD and EoR. In this paper, in order to investigate such a non-…
We study prospects of constraining the primordial magnetic field (PMF) and its evolution during the dark ages and cosmic dawn in light of EDGES 21-cm signal. Our analysis has been carried out on a `colder IGM' background which is one of the promising avenues to interpret the EDGES signal. We consider the dark matter-baryon interactions for the excess cooling. We find that the colder IGM suppresses both the residual free electron fraction and the coupling coefficient between the ionised and neutral components. The Compton heating also gets affected in colder IGM background. Consequently, the IG…
Global signal in the redshifted hydrogen 21-cm line from the Dark Ages and Cosmic Dawn: Dependence on the nature of dark matter and modeling of first light
We present a study of the 21-cm signal bispectrum (which quantifies the non-Gaussianity in the signal) from the Cosmic Dawn (CD). For our analysis, we have simulated the 21-cm signal using radiative transfer code GRIZZLY, while considering two types of sources (mini-QSOs and HMXBs) for Ly$α$ coupling and the X-ray heating of the IGM. Using this simulated signal, we have, for the first time, estimated the CD 21-cm bispectra for all unique $k$-triangles and for a range of $k$ modes. We observe that the redshift evolution of the bispectra magnitude and sign follow a generic trend for both source …
Dark Matter annihilations after recombination and during the epoch of structure formation deposit energy in the primordial intergalactic medium, producing reionization and heating. We investigate the constraints that are imposed by the observed optical depth of the Universe and the measured temperature of the intergalactic gas. We find that the bounds are significant, and have the power to rule out large portions of the `DM mass/cross section' parameter space. The optical depth bound is generally stronger and does not depend significantly on the history of structure formation. The temperature …
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 21cm line from the earliest phases of star and galaxy formation in the Universe. This 21cm signal provides a new and unique window on both the formation of the first stars and accreting black holes and the later 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 …
The 21-cm signal from the Cosmic Dawn (CD) is likely to contain large fluctuations, with the most extreme astrophysical models on the verge of being ruled out by observations from radio interferometers. It is therefore vital that we understand not only the astrophysical processes governing this signal, but also other inherent processes impacting the signal itself, and in particular line-of-sight effects. Using our suite of fully numerical radiative transfer simulations, we investigate the impact on the redshifted 21-cm from the CD from one of these processes, namely the redshift-space distorti…
The process of heating and reionization of the Universe at high redshift links small scale structures/galaxy formation and large scale inter-galactic medium properties. Even if the first one is difficult to observe, an observation window is opening on the second one, with the promising development of current and future radio telescopes. They will permit to observe the 21cm brightness temperature global signal and fluctuations. The need of large scale simulations is therefore strong to understand the properties of the IGM that will be observed. But at the same time the urge to resolve the struc…
Significant historic cosmic evolution for the formation rate of stellar black holes is inferred from current theoretical models of the evolution of massive stars, the multiple observations of compact stellar remnants in the near and distant universe, and the cosmic chemical evolution. The mean mass of stellar black holes, the fraction of black holes/neutron stars, and the fraction of black hole high mass X-ray binaries (BH-HMXBs)/solitary black holes increase with redshift. The energetic feedback from large populations of BH-HMXBs form in the first generations of star burst galaxies has been o…
Modern cosmological simulations predict that the first generation of stars formed with a mass scale around 100 solar masses about 300-400 million years after the Big Bang. When the first stars reached the end of their lives, many of them might have died as energetic supernovae that could have significantly affected the early Universe via injecting large amounts of energy and metals into the primordial intergalactic medium. In this paper, we review the current models of the first supernovae by discussing on the relevant background physics, computational methods, and the latest results.
At low densities the standard ionisation history of the intergalactic medium (IGM) predicts a decreasing temperature of the IGM with decreasing density once hydrogen (and helium) reionisation is complete. Heating the high-redshift, low-density IGM above the temperature expected from photo-heating is difficult, and previous claims of high/rising temperatures in low density regions of the Universe based on the probability density function (PDF) of the opacity in Lyman-$α$ forest data at $2<z<4$ have been met with considerable scepticism, particularly since they appear to be in tension with other…
We review some of the fundamental physics necessary for computing the highly-redshifted spin-flip background. We first discuss the radiative transfer of the 21-cm line and define the crucial quantities of interest. We then review the processes that set the spin temperature of the transition, with a particular focus on Wouthuysen-Field coupling, which is likely to be the most important process during and after the Cosmic Dawn. Finally, we discuss processes that heat the intergalactic medium during the Cosmic Dawn, including the scattering of Lyman-alpha, cosmic microwave background, and X-ray p…
Potential small-scale discrepancies in the picture of galaxy formation painted by the $Λ$CDM paradigm have led to considerations of modified dark matter models. One such dark matter model that has recently attracted much attention is fuzzy dark matter (FDM). In FDM models, the dark matter is envisaged to be an ultra-light scalar field with a particle mass $m_{\rm FDM} \sim 10^{-22} $ eV. This yields astronomically large de Broglie wavelengths which can suppress small-scale structure formation and give rise to the observed kpc-sized density cores in dwarf galaxies. We investigate the evolution …
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 which is known as the "light-cone effect". We model this effect by post-processing a dark matter $N-$body simulation with an 1-D radiative transfer code. We find that the effect is much stronger and dramatic in presence of inhomogeneous heating and Ly$α$ coupling compared to the case where these processes are not accounted for. One…
We study the heating of the intergalactic medium (IGM) surrounding high redshift star forming galaxies due to cosmic rays (CR). We take into account the diffusion of low energy cosmic rays and study the patchiness of the resulting heating. We discuss the case of IGM heating around a high redshift minihalo ($z\sim 10\hbox{--}20$, M$\sim 10^5\hbox{--}10^7$ M$_\odot$),and put an upper limit on the diffusion coefficient $D\le 1\times 10^{26}$ cm$^2$ s$^{-1}$ for the heating to be inhomogeneous at $z\sim 10$ and $D\le 5\hbox{--}6 \times 10^{26}$ cm$^2$ s$^{-1}$ at $z\sim 20$. For typical values of …
The recent EDGES measurements of the global 21-cm signal from the cosmic dawn suggest that the kinetic temperature of the inter-galactic medium (IGM) might be significantly lower compared to its expected value. The colder IGM directly affects the hydrogen recombination of the universe during the cosmic dawn and dark ages by enhancing the rate of recombinations. Here, we study and quantify, the impact of the colder IGM scenario on the recombination history of the universe in the context of DM-baryonic interaction model which is widely used to explain the depth of the EDGES 21-cm signal. We find…
The redshifted 21-cm line of hydrogen holds great potential for the study of cosmology, as it can probe otherwise unobservable cosmic epochs. In particular, measurements of the 21-cm power spectrum during cosmic dawn---the era when the first stars were formed---will provide us with a wealth of information about the astrophysics of stellar formation, as well as the origin of fluctuations in our Universe. In addition to their usually considered density fluctuations, dark matter and baryons possess large relative velocities with respect to each other, due to the baryon acoustic oscillations (BAOs…
The detection of Lyman-$α$ emitting galaxies (LAEs) puts severe constraints on the reionization history. In this Paper we derive the properties of very high-$z$ LAEs predicted in the only two reionization scenarios shown in a previous Paper to be consistent with current data on 15 independent evolving global (or averaged) cosmic properties regarding luminous objects and the intergalactic medium and the optical depth to electron scattering of ionized hydrogen to CMB photons: one with a monotonic behavior, which is completed by $z=6$, as commonly considered, and another one with a non-monotonic …
The Universe is opaque to extragalactic very high-energy gamma rays (VHEGRs, E>100 GeV) because they annihilate and pair produce on the extragalactic background light. The resulting ultra-relativistic pairs are assumed to lose energy through inverse Compton scattering of CMB photons. In Broderick et al. (2011, Paper I of this three paper series), we argued that instead powerful plasma instabilities in the ultra-relativistic pair beam dissipate the kinetic energy of the TeV-generated pairs locally, heating the intergalactic medium (IGM). Here, we explore the effect of this heating upon the ther…
The 21 cm brightness temperature $δT_{\rm b}$ fluctuations from reionization promise to provide information on the physical processes during that epoch. We present a formalism for generating the $δT_{\rm b}$ distribution using dark matter simulations and an one-dimensional radiative transfer code. Our analysis is able to account for the spin temperature $T_{\rm S}$ fluctuations arising from inhomogeneous X-ray heating and Ly$α$ coupling during cosmic dawn. The $δT_{\rm b}$ power spectrum amplitude at large scales ($k \sim 0.1$ Mpc$^{-1}$) is maximum when $\sim 10\%$ of the gas (by volume) is h…
We present the evolution of the 21-cm signal from cosmic dawn and the epoch of reionization (EoR) in an upgraded model including three subtle effects of Ly-a radiation: Ly-a heating, CMB heating (mediated by Ly-a photons), and multiple scattering of Ly-a photons. Taking these effects into account we explore a wide range of astrophysical models and quantify the impact of these processes on the global 21-cm signal and its power spectrum at observable scales and redshifts. We find that, in agreement with the literature, Ly-a and CMB heating raise the gas temperature by up to $\mathcal{O}(100)$ de…
The distribution of diffuse gas in the intergalactic medium (IGM) imprints a series of hydrogen absorption lines on the spectra of distant background quasars known as the Lyman-$α$ forest. Cosmological hydrodynamical simulations predict that IGM density fluctuations are suppressed below a characteristic scale where thermal pressure balances gravity. We measured this pressure-smoothing scale by quantifying absorption correlations in a sample of close quasar pairs. We compared our measurements to hydrodynamical simulations, where pressure smoothing is determined by the integrated thermal history…
Models of the reionization and reheating of the intergalactic medium (IGM) at redshifts $z \gtrsim 6$ continue to grow more sophisticated in anticipation of near-future 21-cm, cosmic microwave background, and galaxy survey measurements. However, there are many potential sources of systematic uncertainty in models that could bias and/or degrade upcoming constraints if left unaccounted for. In this work, we examine three commonly-ignored sources of uncertainty in models for the mean reionization and thermal histories of the IGM: the underlying cosmology, halo mass function (HMF), and choice of s…
We compute the expected X-ray diffuse background and radiative feedback on the intergalactic medium (IGM) from X-ray binaries prior and during the epoch of reionization. The cosmic evolution of compact binaries is followed using a population synthesis technique that treats separately neutron stars and black hole binaries in different spectral states and is calibrated to reproduce the observed X-ray properties of galaxies at z<4. Together with an updated empirical determination of the cosmic history of star formation, recent modeling of the stellar mass-metallicity relation, and a scheme for ab…
