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The timeline of cosmic reionization remains uncertain despite sustained efforts to study how the ionizing output of early galaxies shaped the intergalactic medium (IGM). Using the semi-numerical code LIMFAST, we investigate the prospects for timing the reionization process by cross-correlating the 21 cm signal with the cosmic near-infrared background (NIRB) contributed by galaxies at $z>5$. Tracing opposite phases of the IGM on large scales during reionization, the two signals together serve as a powerful probe for the reionization history. However, because long-wavelength, line-of-sight Fourier modes -- the only modes probed by NIRB fluctuations -- are contaminated by 21 cm foregrounds and thus inevitably lost to foreground cleaning or avoidance, a direct cross-correlation of the two signals vanishes. We show that this problem can be circumvented by squaring the foreground-filtered 21 cm signal and cross-correlating the squared field with the NIRB. This statistic is related to the 21 cm--21 cm--NIRB cross-bispectrum and encodes valuable information regarding the reionization timeline. Particularly, the 21 cm$^2$ and NIRB signals are positively correlated during the early phases of reionization and negatively correlated at later stages. We demonstrate that this behavior is generic across several different reionization models and compare our simulated results with perturbative calculations. We show that this cross-correlation can be detected at high significance by forthcoming 21 cm and NIRB surveys such as SKA and SPHEREx. Our methodology is more broadly applicable to cross-correlations between line intensity mapping data and 2D tracers of the large-scale structure, including photometric galaxy surveys and CMB lensing mass maps, among others., Comment: 20 pages, 9 figures, submitted to AAS journals

The observed prevalence of galaxies exhibiting bursty star formation histories (SFHs) at $z\gtrsim6$ has created new challenges and opportunities for understanding their formation pathways. The degenerate effects of the efficiency and burstiness of star formation on the observed UV luminosity function are separable by galaxy clustering. However, quantifying the timescales of burstiness requires more than just the continuum UV measurements. Here we develop a flexible semi-analytic framework for modeling both the amplitude of star formation rate (SFR) variations and their temporal correlation, from which the luminosity function and clustering can be derived for SFR indicators tracing different characteristic timescales (e.g., UV continuum and H$\alpha$ luminosities). Based on this framework, we study the prospect of using galaxy summary statistics to distinguish models where SFR fluctuations are prescribed by different power spectral density (PSD) forms. Using the Fisher matrix approach, we forecast the constraints on parameters in our PSD-based model that can be extracted from mock JWST observations of the UV and H$\alpha$ luminosity functions and clustering bias factors at $z\sim6$. These constraints demonstrate the feasibility of constraining the burstiness of high-$z$ galaxies solely from their one-point and two-point statistics and underscore the importance of combining tracers of both long-term and short-term SFR variations. Our flexible framework can be readily extended to characterize the SFH of high-redshift galaxies with a wider range of observational diagnostics., Comment: 21 pages, 7 figures, submitted to JCAP

We introduce a new physically-motivated spectral template set for fitting the spectral energy distributions (SEDs) of high-z galaxies. We use the public galaxy formation code ARES to generate star formation histories of thirteen representative galaxies with diverse masses and generate their predicted spectra across a set of redshifts at z > 6. The model parameters are calibrated to reproduce the properties of z > 6 galaxies observed by HST. Motivated by the apparent importance of bursty star formation at high redshifts, we also include templates with recent starbursts. We use these templates with the SED-fitting code EAZY to analyze both an independent theoretical model and a public sample of JWST-observed galaxies from the JADES survey. The comparison with a semi-analytic model demonstrates that our fitting framework accurately measures the galaxy properties, even when the underlying assumptions of the model differ from ours. Our preliminary application to JWST data shows that galaxies at z > 8 are often bursty (especially at small galaxy masses), follow a star-forming main sequence similar to those at lower redshift (albeit with a higher normalization), and form stars earlier than expected in ares. Our SED-fitting framework is very fast (thanks to the efficiency of EAZY) but provides full inferred star formation histories for each source. Additionally, it enables a direct comparison to theoretical models and helps point toward improvements necessary in those models., Comment: 31 pages, 14 figures

The 21-cm background is a promising probe of early star formation and black hole activity. While a slew of experiments on the ground seek to detect the 21-cm monopole and spatial fluctuations on large $\sim 10$ arcminute scales, little work has been done on the prospects for detecting the 21-cm dipole signal or its utility as a probe of early galaxies. Though an intrinsically weak signal relative to the monopole, its direction is known well from the cosmic microwave background and wide-field surveys, plus as a relative measurement the dipole could help relax instrumental requirements. In order to understand the constraining power of the dipole, in this work we perform parameter inference on mock datasets that include the dipole, monopole, or both signals. We find that while the monopole does provide the best constraints for a given integration time, constraints from a dipole measurement are competitive, and can in principle constrain the cosmic star formation rate density and efficiency of X-ray photon production in early $z \sim 15$ galaxies to better than a factor of $\sim 2$. This result holds for most of the available prior volume, which is set by constraints on galaxy luminosity functions, the reionization history, and upper limits from 21-cm power spectrum experiments. We also find that predictions for the monopole from a dipole measurement are robust to different choices of signal model. As a result, the 21-cm dipole signal is a valuable target for future observations and offers a robust cross-check on monopole measurements., Comment: 17 pages, 10 figures, submitted to ApJ

New James Webb Space Telescope (JWST) observations are revealing the first galaxies to be prolific producers of ionizing photons, which we argue gives rise to a tension between different probes of reionization. Over the last two decades a consensus has emerged where star-forming galaxies are able to generate enough photons to drive reionization, given reasonable values for their number densities, ionizing efficiencies $\xi_{\rm ion}$ (per unit UV luminosity), and escape fractions $f_{\rm esc}$. However, some new JWST observations infer high values of $\xi_{\rm ion}$ during reionization and an enhanced abundance of earlier ($z\gtrsim 9$) galaxies, dramatically increasing the number of ionizing photons produced at high $z$. Simultaneously, recent low-$z$ studies predict significant escape fractions for faint reionization-era galaxies. Put together, we show that the galaxies we have directly observed ($M_{\rm UV} < -15$) not only can drive reionization, but would end it too early. That is, our current galaxy observations, taken at face value, imply an excess of ionizing photons and thus a process of reionization in tension with the cosmic microwave background (CMB) and Lyman-$\alpha$ forest. Considering galaxies down to $M_{\rm UV}\approx -11$, below current observational limits, only worsens this tension. We discuss possible avenues to resolve this photon budget crisis, including systematics in either theory or observations., Comment: 6+3 pages, 3+2 figures, 1 table, updated to match accepted version

The detection of the 21-cm signal at $z\gtrsim6$ will reveal insights into the properties of the first galaxies responsible for driving reionisation. To extract this information, we perform parameter inference which requires embedding 3D simulations of the 21-cm signal within a Bayesian inference pipeline. Presently, when performing inference we must choose which sources of uncertainty to sample and which to hold fixed. Since the astrophysics of galaxies are much more uncertain than those of the underlying halo-mass function (HMF), we usually parameterise and model the former while fixing the latter. However, in doing so we may bias our inference of the properties of these first galaxies. In this work, we explore the consequences of assuming an incorrect choice of HMF and quantify the relative biases in our inferred astrophysical model parameters when considering the wrong HMF. We then relax this assumption by constructing a generalised five parameter model for the HMF and simultaneously recover these parameters along with our underlying astrophysical model. For this analysis, we use 21cmFAST and perform Simulation-Based Inference by applying marginal neural ratio estimation to learn the likelihood-to-evidence ratio using Swyft. Using a mock 1000 hour observation of the 21-cm power spectrum from the forthcoming Square Kilometre Array, conservatively assuming foreground wedge avoidance, we find assuming the incorrect HMF can bias the recovered astrophysical parameters by up to $\sim3-4\sigma$ even when including independent information from observed luminosity functions. When considering our generalised HMF model, we recover constraints on our astrophysical parameters with a factor of $\sim2-4$ larger marginalised uncertainties. Importantly, these constraints are unbiased, agnostic to the underlying HMF and therefore more conservative., Comment: 27 pages, 14 figures, 3 tables and 3 appendices. Submitted to MNRAS, comments welcome

Upcoming experiments will map the spatial distribution of the 21-cm signal over three-dimensional volumes of space during the Epoch of Reionization (EoR). Several methods have been proposed to mitigate the issue of astrophysical foreground contamination in tomographic images of the 21-cm signal, one of which involves the excision of a wedge-shaped region in cylindrical Fourier space. While this removes the $k$-modes most readily contaminated by foregrounds, the concurrent removal of cosmological information located within the wedge considerably distorts the structure of 21-cm images. In this study, we build upon a U-Net based deep learning algorithm to reconstruct foreground wedge-removed maps of the 21-cm signal, newly incorporating light-cone effects. Adopting the Square Kilometre Array (SKA) as our fiducial instrument, we highlight that our U-Net recovery framework retains a reasonable level of reliability even in the face of instrumental limitations and noise. We subsequently evaluate the efficacy of recovered maps in guiding high-redshift galaxy searches and providing context to existing galaxy catalogues. This will allow for studies of how the high-redshift galaxy luminosity function varies across environments, and ultimately refine our understanding of the connection between the ionization state of the intergalactic medium (IGM) and galaxies during the EoR., Comment: v2: replaced with accepted MNRAS version (extra clarifying remarks and some demonstration of out-of-distribution performance). Results and conclusions unchanged

The high-redshift galaxy UV luminosity function (UVLF) has become essential for understanding the formation and evolution of the first galaxies. Yet, UVLFs only measure galaxy abundances, giving rise to a degeneracy between the mean galaxy luminosity and its stochasticity. Here, we show that upcoming clustering measurements with the James Webb Space Telescope (JWST), as well as with Roman, will be able to break this degeneracy, even at redshifts $z \gtrsim 10$. First, we demonstrate that current Subaru Hyper Suprime-Cam (HSC) measurements of the galaxy bias at $z\sim 4-6$ point to a relatively tight halo-galaxy connection, with low stochasticity. Then, we show that the larger UVLFs observed by JWST at $z\gtrsim 10$ can be explained with either a boosted average UV emission or an enhanced stochasticity. These two models, however, predict different galaxy biases, which are potentially distinguishable in JWST and Roman surveys. Galaxy-clustering measurements, therefore, will provide crucial insights into the connection between the first galaxies and their dark-matter halos, and identify the root cause of the enhanced abundance of $z \gtrsim 10$ galaxies revealed with JWST during its first year of operations., Comment: 6+3 pages, 3+3 figures. Published in MNRAS Lett

We report the most sensitive upper limits to date on the 21 cm epoch of reionization power spectrum using 94 nights of observing with Phase I of the Hydrogen Epoch of Reionization Array (HERA). Using similar analysis techniques as in previously reported limits (HERA Collaboration 2022a), we find at 95% confidence that $\Delta^2(k = 0.34$ $h$ Mpc$^{-1}$) $\leq 457$ mK$^2$ at $z = 7.9$ and that $\Delta^2 (k = 0.36$ $h$ Mpc$^{-1}) \leq 3,496$ mK$^2$ at $z = 10.4$, an improvement by a factor of 2.1 and 2.6 respectively. These limits are mostly consistent with thermal noise over a wide range of $k$ after our data quality cuts, despite performing a relatively conservative analysis designed to minimize signal loss. Our results are validated with both statistical tests on the data and end-to-end pipeline simulations. We also report updated constraints on the astrophysics of reionization and the cosmic dawn. Using multiple independent modeling and inference techniques previously employed by HERA Collaboration (2022b), we find that the intergalactic medium must have been heated above the adiabatic cooling limit at least as early as $z = 10.4$, ruling out a broad set of so-called "cold reionization" scenarios. If this heating is due to high-mass X-ray binaries during the cosmic dawn, as is generally believed, our result's 99% credible interval excludes the local relationship between soft X-ray luminosity and star formation and thus requires heating driven by evolved low-metallicity stars., Comment: 57 pages, 37 figures. Updated to match the accepted ApJ version. Corresponding author: Joshua S. Dillon

Many compelling dark matter (DM) scenarios feature Coulomb-like interactions between DM particles and baryons, in which the cross section for elastic scattering scales with relative particle velocity as $v^{-4}$. Previous studies have invoked such interactions to produce heat exchange between cold DM and baryons and alter the temperature evolution of hydrogen. In this study, we present a comprehensive study of the effects of Coulomb-like scattering on structure formation, in addition to the known effects on the thermal history of hydrogen. We find that interactions which significantly alter the temperature of hydrogen at Cosmic Dawn also dramatically suppress the formation of galaxies that source the Lyman-$\alpha$ background, further affecting the global 21-cm signal. In particular, an interaction cross section at the current observational upper limit leads to a decrease in the abundance of star-forming halos by a factor of $\sim 2$ at $z\sim 20$, relative to cold, collisionless DM. We also find that DM that is 100% millicharged cannot reproduce the depth and the timing of the reported EDGES anomaly in any part of the parameter space. These results critically inform modeling of the global 21-cm signal and structure formation in cosmologies with DM-baryon scattering, with repercussions for future and upcoming cosmological data analysis., Comment: 21 pages, 10 figures, submitted to PRD

The flux of high-energy astrophysical $\gamma$ rays is attenuated by the production of electron-positron pairs from scattering off of extragalactic background light (EBL). We use the most up-to-date information on galaxy populations to compute their contributions to the pair-production optical depth. We find that the optical depth inferred from $\gamma$-ray measurements exceeds that expected from galaxies at the $\sim2\sigma$ level. If the excess is modeled as a frequency-independent re-scaling of the standard contribution to the EBL from galaxies, then an excess (an overall $14-30\%$ increase of the EBL) over the null hypothesis of no excess at the $2.7\sigma$ level. If the frequency dependence of the excess is instead modeled as a two-photon decay of a dark-matter axion, then the excess is favored over the null hypothesis at the $2.1\sigma$ confidence level. While we find no evidence for a dark-matter signal, the analysis sets the strongest current bounds on the photon-axion coupling over the $8-25$ eV mass range. This work highlights the sensitivity of $\gamma$-ray optical depth measurements to ALPs, which is expected to improve with new observatories and better EBL determinations from future observations., Comment: 5 pages + 3 pages of appendix (+5 pages of references), 6 figures. Discussions and figures slightly modified, an appendix to discuss the likelihood added, conclusions unchanged. Matches the published version

Over the last three decades, photometric galaxy selection using the Lyman-break technique has transformed our understanding of the high-z Universe, providing large samples of galaxies at 3 < z < 8 with relatively small contamination. With the advent of the James Webb Space Telescope, the Lyman-break technique has now been extended to z ~ 17. However, the purity of the resulting samples has not been tested. Here we use a simple model, built on the robust foundation of the dark matter halo mass function, to show that the expected level of contamination rises dramatically at z > 10, especially for luminous galaxies, placing stringent requirements on the selection process. The most luminous sources at z > 12 are likely at least ten thousand times rarer than potential contaminants, so extensive spectroscopic followup campaigns may be required to identify a small number of target sources., Comment: 6 pages, 2 figures, submitted to MNRAS

Early observations with JWST indicate an over-abundance of bright galaxies at redshifts $z \gtrsim 10$ relative to Hubble-calibrated model predictions. More puzzling still is the apparent lack of evolution in the abundance of such objects between $z \sim 9$ and the highest redshifts yet probed, $z \sim 13$-$17$. In this study, we first show that, despite a poor match with JWST LFs, semi-empirical models calibrated to UVLFs and colours at $4 \lesssim z \lesssim 8$ are largely consistent with constraints on the properties of individual JWST galaxies, including their stellar masses, ages, and rest-ultraviolet spectral slopes. We then show that order-of-magnitude scatter in the star formation rate of galaxies (at fixed halo mass) can indeed boost the abundance of bright galaxies, provided that star formation is more efficient than expected in low-mass halos. However, this solution to the abundance problem introduces tension elsewhere: because it relies on the up-scattering of low-mass halos into bright magnitude bins, one expects typical ages, masses, and spectral slopes to be much lower than constraints from galaxies observed thus far. This tension can be alleviated by non-negligible reddening, suggesting that -- if the first batch of photometrically-selected candidates are confirmed -- star formation \textit{and} dust production could be more efficient than expected in galaxies at $z \gtrsim 10$., Comment: 12 pages, 5 figures, MNRAS accepted

The reionization of the intergalactic medium at redshifts $z\gtrsim 6$ is expected to have a lasting impact on galaxies residing in low-mass dark matter halos. Unable to accrete or retain gas photo-heated to temperatures $T \gtrsim 10^4$ K, the star formation histories of faint galaxies in the early Universe are expected to decline as they exhaust their gas supply, resulting in a turn-over in the galaxy luminosity function (LF) and a potential solution to the missing satellites problem in the local group. Unfortunately, there are several challenges to constraining `reionization feedback' empirically, most notably that galaxies in low-mass halos are intrinsically faint, and that there are other physical mechanisms capable of inducing a turn-over in the LF. In this work, we investigate a new signature of reionization feedback that is in principle distinct from other processes: as faint galaxies are quenched by reionization, their stellar populations passively age and grow redder while the brighter galaxies nearby continue to form stars at an increasing rate and so remain relatively blue. We find that this contrast, between quenched and un-quenched galaxies induces a scale and colour-dependent signature in the present-day near-infrared background comparable to the expected sensitivity of NASA's upcoming SPHEREx mission. Whereas models with pure mass suppression largely affect the signal at wavelengths $\lesssim 2 \mu\rm{m}$, $\sim 5$%-level differences in the background persist out to $\sim 5 \mu\rm{m}$ for reionization feedback models. Finally, the power spectra of intensity ratio maps exhibit larger variations, and may thus be a promising target for future analyses., Comment: 13 pages, 7 figures, submitted to MNRAS

The cross-correlation between the 21 cm field and the galaxy distribution is a potential probe of the Epoch of Reionization (EoR). The 21 cm signal traces neutral gas in the intergalactic medium and, on large spatial scales, this should be anti-correlated with the high-redshift galaxy distribution which partly sources and tracks the ionized gas. In the near future, interferometers such as the Hydrogen Epoch of Reionization Array (HERA) are projected to provide extremely sensitive measurements of the 21 cm power spectrum. At the same time, the Nancy Grace Roman Space Telescope (Roman) will produce the most extensive catalog to date of bright galaxies from the EoR. Using semi-numeric simulations of reionization, we explore the prospects for measuring the cross-power spectrum between the 21 cm and galaxy fields during the EoR. We forecast a 14$\sigma$ detection between HERA and Roman, assuming an overlapping survey area of 500 deg$^2$, redshift uncertainties of $\sigma_z = 0.01$ (as expected for the high-latitude spectroscopic survey of Ly$\alpha$-emitting galaxies), and an effective Ly$\alpha$ emitter duty cycle of $f_\mathrm{LAE} = 0.1$. Thus the HERA-Roman cross-power spectrum may be used to help verify 21 cm detections from HERA. We find that the shot-noise in the galaxy distribution is a limiting factor for detection, and so supplemental observations using Roman should prioritize deeper observations, rather than covering a wider field of view., Comment: 26 pages, 15 figures, accepted at ApJ

Upper limits from the current generation of interferometers targeting the 21-cm signal from high redshifts have recently begun to rule out physically realistic, though still extreme, models of the Epoch of Reionization (EoR). While inferring the detailed properties of the first galaxies is one of the most important motivations for measuring the high-$z$ 21-cm signal, they can also provide useful constraints on the properties of the intergalactic medium (IGM). Motivated by this, we build a simple, phenomenological model for the 21-cm power spectrum that works directly in terms of IGM properties, which bypasses the computationally expensive 3-D semi-numerical modeling generally employed in inference pipelines and avoids explicit assumptions about galaxy properties. The key simplifying assumptions are that (i) the ionization field is binary, and composed of spherical bubbles with an abundance described well by a parametric bubble size distribution, and (ii) that the spin temperature of the ``bulk'' IGM outside bubbles is uniform. Despite the simplicity of the model, the mean ionized fraction and spin temperature of the IGM recovered from mock 21-cm power spectra generated with \textsc{21cmfast} are generally in good agreement with the true input values. This suggests that it is possible to obtain comparable constraints on the IGM using models with very different assumptions, parameters, and priors. Our approach will thus be complementary to semi-numerical models as upper limits continue to improve in the coming years., Comment: 23 pages, 14 figures, accepted for publication in MNRAS, code publicly available at https://github.com/mirochaj/micro21cm

Upcoming ground and space-based experiments may have sufficient accuracy to place significant constraints upon high-redshift star formation, Reionization, and dark matter (DM) using the global 21-cm signal of the intergalactic medium. In the early universe, when the relative abundance of low-mass DM halos is important, measuring the global signal would place constraints on the damping of structure formation caused by DM having a higher relic velocity (warm dark matter, or WDM) than in cold dark matter (CDM). Such damping, however, can be mimicked by altering the star formation efficiency (SFE) and difficult to detect because of the presence of Pop III stars with unknown properties. We study these various cases and their degeneracies with the WDM mass parameter $m_X$ using a Fisher matrix analysis. We study the $m_X = 7$ keV case and a star-formation model that parametrizes the SFE as a strong function of halo mass and include several variations of this model along with three different input noise levels for the likelihood; we also use a minimum halo virial temperature for collapse near the molecular cooling threshold. We find that when the likelihood includes only Pop II stars, $m_X$ is constrained to an uncertainty of $\sim 0.4$ keV for all models and noise levels at 68$\%$ CI. When the likelihood includes weak Pop III stars, $m_X \sim 0.3$ keV, and if Pop III star formation is relatively efficient, $m_X \sim 0.1$ keV uncertainty, with tight Pop III star-formation parameter constraints. Our results show that the global 21-cm signal is a promising test-bed for WDM models, even in the presence of strong degeneracies with astrophysical parameters., Comment: 22 pages, 9 figures, updated to version accepted by ApJ

In recent years, several analytic models have demonstrated that simple assumptions about halo growth and feedback-regulated star formation can match the (limited) existing observational data on galaxies at z>6. By extending such models, we demonstrate that imposing a time delay on stellar feedback (as inevitably occurs in the case of supernova explosions) induces burstiness in small galaxies. Although supernova progenitors have short lifetimes (~5-30 Myr), the delay exceeds the dynamical time of galaxies at such high redshifts. As a result, star formation proceeds unimpeded by feedback for several cycles and "overshoots" the expectations of feedback-regulated star formation models. We show that such overshoot is expected even in atomic cooling halos, with masses up to ~10^10.5 Msun at z>6. However, these burst cycles damp out quickly in massive galaxies, because large haloes are more resistant to feedback so retain a continuous gas supply. Bursts in small galaxies - largely beyond the reach of existing observations - induce a scatter in the luminosity of these haloes (of ~1 mag) and increase the time-averaged star formation efficiency by up to an order of magnitude. This kind of burstiness can have substantial effects on the earliest phases of star formation and reionization., Comment: 15 pages, 10 figures, submitted to MNRAS

Recently, the Hydrogen Epoch of Reionization Array (HERA) collaboration has produced the experiment's first upper limits on the power spectrum of 21-cm fluctuations at z~8 and 10. Here, we use several independent theoretical models to infer constraints on the intergalactic medium (IGM) and galaxies during the epoch of reionization (EoR) from these limits. We find that the IGM must have been heated above the adiabatic cooling threshold by z~8, independent of uncertainties about the IGM ionization state and the nature of the radio background. Combining HERA limits with galaxy and EoR observations constrains the spin temperature of the z~8 neutral IGM to 27 K < T_S < 630 K (2.3 K < T_S < 640 K) at 68% (95%) confidence. They therefore also place a lower bound on X-ray heating, a previously unconstrained aspects of early galaxies. For example, if the CMB dominates the z~8 radio background, the new HERA limits imply that the first galaxies produced X-rays more efficiently than local ones (with soft band X-ray luminosities per star formation rate constrained to L_X/SFR = { 10^40.2, 10^41.9 } erg/s/(M_sun/yr) at 68% confidence), consistent with expectations of X-ray binaries in low-metallicity environments. The z~10 limits require even earlier heating if dark-matter interactions (e.g., through millicharges) cool down the hydrogen gas. Using a model in which an extra radio background is produced by galaxies, we rule out (at 95% confidence) the combination of high radio and low X-ray luminosities of L_{r,\nu}/SFR > 3.9 x 10^24 W/Hz/(M_sun/yr) and L_X/SFR<10^40 erg/s/(M_sun/yr). The new HERA upper limits neither support nor disfavor a cosmological interpretation of the recent EDGES detection. The analysis framework described here provides a foundation for the interpretation of future HERA results., Comment: 40 pages, 19 figures, accepted to ApJ

The cosmic near-infrared background (NIRB) offers a powerful integral probe of radiative processes at different cosmic epochs, including the pre-reionization era when metal-free, Population III (Pop III) stars first formed. While the radiation from metal-enriched, Population II (Pop II) stars likely dominates the contribution to the observed NIRB from the reionization era, Pop III stars -- if formed efficiently -- might leave characteristic imprints on the NIRB thanks to their strong Ly$\alpha$ emission. Using a physically-motivated model of first star formation, we provide an analysis of the NIRB mean spectrum and anisotropy contributed by stellar populations at $z>5$. We find that in circumstances where massive Pop III stars persistently form in molecular cooling haloes at a rate of a few times $10^{-3}\,M_\odot \ \mathrm{yr}^{-1}$, before being suppressed towards the epoch of reionization (EoR) by the accumulated Lyman-Werner background, a unique spectral signature shows up redward of $1\,\mu$m in the observed NIRB spectrum sourced by galaxies at $z>5$. While the detailed shape and amplitude of the spectral signature depend on various factors including the star formation histories, IMF, LyC escape fraction and so forth, the most interesting scenarios with efficient Pop III star formation are within the reach of forthcoming facilities such as the Spectro-Photometer for the History of the Universe, Epoch of Reionization and Ices Explorer (SPHEREx). As a result, new constraints on the abundance and formation history of Pop III stars at high redshifts will be available through precise measurements of the NIRB in the next few years., Comment: 19 pages, 10 figures, accepted for publication in MNRAS

Superconducting cosmic strings emit electromagnetic waves between the times of recombination and reionization. Hence, they have an effect on the global 21cm signal. We compute the resulting absorption features, focusing on strings with critical current, study their dependence on the string tension $\mu$, and compare with observational results. For string tensions in the range of $G \mu = 10^{-10}$, where $G$ is Newton's gravitational constant, there is an interesting amplification of the two characteristic absorption features, one during the cosmic dawn, $z \lesssim 30$, and the other during the cosmic dark age, $z \sim 80$, the former being comparable in amplitude to what was observed by the EDGES experiment., Comment: 8 pages, 3 figures

An array of low-frequency dipole antennas on the lunar farside surface will probe a unique, unexplored epoch in the early Universe called the Dark Ages. It begins at Recombination when neutral hydrogen atoms formed, first revealed by the cosmic microwave background. This epoch is free of stars and astrophysics, so it is ideal to investigate high energy particle processes including dark matter, early Dark Energy, neutrinos, and cosmic strings. A NASA-funded study investigated the design of the instrument and the deployment strategy from a lander of 128 pairs of antenna dipoles across a 10 kmx10 km area on the lunar surface. The antenna nodes are tethered to the lander for central data processing, power, and data transmission to a relay satellite. The array, named FARSIDE, would provide the capability to image the entire sky in 1400 channels spanning frequencies from 100 kHz to 40 MHz, extending down two orders of magnitude below bands accessible to ground-based radio astronomy. The lunar farside can simultaneously provide isolation from terrestrial radio frequency interference, the Earth's auroral kilometric radiation, and plasma noise from the solar wind. It is thus the only location within the inner solar system from which sky noise limited observations can be carried out at sub-MHz frequencies. Through precision calibration via an orbiting beacon and exquisite foreground characterization, the farside array would measure the Dark Ages global 21-cm signal at redshifts z~35-200. It will also be a pathfinder for a larger 21-cm power spectrum instrument by carefully measuring the foreground with high dynamic range., Comment: 14 pages, 3 figures, 1 table, response to DOE request for information on lunar farside radio telescope to explore the early universe. arXiv admin note: substantial text overlap with arXiv:1911.08649

Upcoming galaxy surveys and 21-cm experiments targeting high redshifts $z\gtrsim 6$ are highly complementary probes of galaxy formation and reionization. However, in order to expedite the large volume simulations relevant for 21-cm observations, many models of galaxies within reionization codes are entirely subgrid and/or rely on halo abundances only. In this work, we explore the extent to which resolving and modeling individual galaxy formation histories affects predictions both for the galaxy populations detectable by upcoming surveys and the signatures of reionization accessible to upcoming 21-cm experiments. We find that a common approach, in which galaxy luminosity is assumed to be a function of halo mass only, is biased with respect to models in which galaxy properties are evolved through time via semi-analytic modeling and thus reflective of the diversity of assembly histories that naturally arise in $N$-body simulations. The diversity of galaxy formation histories also results in scenarios in which the brightest galaxies do \textit{not} always reside in the centers of large ionized regions, as there are often relatively low-mass halos undergoing dramatic, but short-term, growth. This has clear implications for attempts to detect or validate the 21-cm background via cross correlation. Finally, we show that a hybrid approach -- in which only halos hosting galaxies bright enough to be detected in surveys are modeled in detail, with the rest modeled as an unresolved field of halos with abundance related to large-scale overdensity -- is a viable way to generate large-volume `simulations` well-suited to wide-area surveys and current-generation 21-cm experiments targeting relatively large $k \lesssim 1 \ h \ \text{Mpc}^{-1}$ scales., Comment: 17 pages, 14 figures, accepted for publication in MNRAS

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 stellar population synthesis (SPS) model. We find that cosmological uncertainties affect the Thomson scattering optical depth at the few percent level and the amplitude of the global 21-cm signal at the $\sim$5-10 mK level. The differences brought about by choice of HMF and SPS models are more dramatic, comparable to the $1 \sigma$ error-bar on $\tau_e$ and a $\sim 20$ mK effect on the global 21-cm signal amplitude. Finally, we jointly fit galaxy luminosity functions and global 21-cm signals for all HMF/SPS combinations and find that (i) doing so requires additional free parameters to compensate for modeling systematics and (ii) the spread in constraints on parameters of interest for different HMF and SPS choices, assuming $5$ mK noise in the global signal, is comparable to those obtained when adopting the "true" HMF and SPS with $\gtrsim 20$ mK errors. Our work highlights the need for dedicated efforts to reduce modeling uncertainties in order to enable precision inference with future datasets., Comment: 10 pages, 8 figures, in press

Prior to the epoch of reionisation, the 21-cm signal of the cosmic dawn is dominated by the Lyman-$\alpha$ coupling and gas temperature fluctuations caused by the first sources of radiation. While early efforts to model this epoch relied on analytical techniques, the community quickly transitioned to more expensive semi-numerical models. Here, we re-assess the viability of simpler approaches that allow for rapid explorations of the vast astrophysical parameter space. We propose a new analytical method to calculate the 21-cm power spectrum based on the framework of the halo model. Both the Lyman-$\alpha$ coupling and temperature fluctuations are described by overlapping radiation flux profiles that include spectral red-shifting and source attenuation due to look-back (light-cone) effects. The 21-cm halo model is compared to the semi-numerical code 21cmFAST exhibiting generally good agreement, i.e., the power spectra differ by less than a factor of three over a large range of $k$-modes and redshifts. We show that the remaining differences between the two methods are comparable to the expected variations from modelling uncertainties associated with the abundance, bias, and accretion rates of haloes. While these current uncertainties must be reduced in the future, our work suggests that inference at acceptable accuracy will become feasible with very efficient halo models of the cosmic dawn., Comment: Accepted version (Phys.Rev.D)

Many semi-empirical galaxy formation models have recently emerged to interpret high-$z$ galaxy luminosity functions and make predictions for future galaxy surveys. A common approach assumes a "universal" star formation efficiency, $f_{\ast}$, independent of cosmic time but strongly dependent on the masses of dark matter halos. Though this class of models has been very successful in matching observations over much of cosmic history, simple stellar feedback models do predict redshift evolution in $f_{\ast}$, and are commonly used in semi-analytic models. In this work, we calibrate a set of universal $f_{\ast}$ and feedback-regulated models to the same set of rest-ultraviolet $z \gtrsim 4$ observations, and find that a rapid, $\sim (1+z)^{-3/2}$ decline in both the efficiency of dust production and duty cycle of star formation are needed to reconcile feedback-regulated models with current observations. By construction, these models remain nearly identical to universal $f_{\ast}$ models in rest-ultraviolet luminosity functions and colours. As a result, the only way to distinguish these competing scenarios is either via (i) improved constraints on the clustering of galaxies -- universal and feedback-regulated models differ in predictions for the galaxy bias by $0.1 \lesssim \Delta \langle b \rangle \lesssim 0.3$ over $4 \lesssim z \lesssim 10$ -- or (ii) independent constraints on the dust contents and/or duty cycle of star formation. This suggests that improved constraints on the `dustiness' and `burstiness' of high-$z$ galaxies will not merely add clarity to a given model of star formation in high-$z$ galaxies, but rather fundamentally determine our ability to identify the correct model in the first place., Comment: 12 pages, 7 figures, accepted for publication in MNRAS

Connecting the observed rest-ultraviolet (UV) luminosities of high-$z$ galaxies to their intrinsic luminosities (and thus star formation rates) requires correcting for the presence of dust. We bypass a common dust-correction approach that uses empirical relationships between infrared (IR) emission and UV colours, and instead augment a semi-empirical model for galaxy formation with a simple -- but self-consistent -- dust model and use it to jointly fit high-$z$ rest-UV luminosity functions (LFs) and colour-magnitude relations ($M_{\mathrm{UV}}$-$\beta$). In doing so, we find that UV colours evolve with redshift (at fixed UV magnitude), as suggested by observations, even in cases without underlying evolution in dust production, destruction, absorption, or geometry. The observed evolution in our model arises due to the reduction in the mean stellar age and rise in specific star formation rates with increasing $z$. The UV extinction, $A_{\mathrm{UV}}$, evolves similarly with redshift, though we find a systematically shallower relation between $A_{\mathrm{UV}}$ and $M_{\mathrm{UV}}$ than that predicted by IRX-$\beta$ relationships derived from $z \sim 3$ galaxy samples. Finally, assuming that high $1600 \r{A}$ transmission ($\gtrsim 0.6$) is a reliable LAE indicator, modest scatter in the effective dust surface density of galaxies can explain the evolution both in $M_{\mathrm{UV}}$-$\beta$ and LAE fractions. These predictions are readily testable by deep surveys with the James Webb Space Telescope., Comment: 14+4 pages, 11+5 figures, accepted for publication in MNRAS

We present the completion of a data analysis pipeline that self-consistently separates global 21-cm signals from large systematics using a pattern recognition technique. In the first paper of this series, we obtain optimal basis vectors from signal and foreground training sets to linearly fit both components with the minimal number of terms that best extracts the signal given its overlap with the foreground. In this second paper, we utilize the spectral constraints derived in the first paper to calculate the full posterior probability distribution of any signal parameter space of choice. The spectral fit provides the starting point for a Markov Chain Monte Carlo (MCMC) engine that samples the signal without traversing the foreground parameter space. At each MCMC step, we marginalize over the weights of all linear foreground modes and suppress those with unimportant variations by applying priors gleaned from the training set. This method drastically reduces the number of MCMC parameters, augmenting the efficiency of exploration, circumvents the need for selecting a minimal number of foreground modes, and allows the complexity of the foreground model to be greatly increased to simultaneously describe many observed spectra without requiring extra MCMC parameters. Using two nonlinear signal models, one based on EDGES observations and the other on phenomenological frequencies and temperatures of theoretically expected extrema, we demonstrate the success of this methodology by recovering the input parameters from multiple randomly simulated signals at low radio frequencies (10-200 MHz), while rigorously accounting for realistically modeled beam-weighted foregrounds., Comment: 19 pages, 11 figures, 5 tables. Submitted to ApJ. Code available at https://bitbucket.org/ktausch/pylinex

This is the final report submitted to NASA for a Probe-class concept study of the "Farside Array for Radio Science Investigations of the Dark ages and Exoplanets" (FARSIDE), a low radio frequency interferometric array on the farside of the Moon. The design study focused on the instrument, a deployment rover, the lander and base station, and delivered an architecture broadly consistent with the requirements for a Probe mission. This notional architecture consists of 128 dipole antennas deployed across a 10 km area by a rover, and tethered to a base station for central processing, power and data transmission to the Lunar Gateway, or an alternative relay satellite. FARSIDE would provide the capability to image the entire sky each minute in 1400 channels spanning frequencies from 150 kHz to 40 MHz, extending down two orders of magnitude below bands accessible to ground-based radio astronomy. The lunar farside can simultaneously provide isolation from terrestrial radio frequency interference, auroral kilometric radiation, and plasma noise from the solar wind. This would enable near-continuous monitoring of the nearest stellar systems in the search for the radio signatures of coronal mass ejections and energetic particle events, and would also detect the magnetospheres for the nearest candidate habitable exoplanets. Simultaneously, FARSIDE would be used to characterize similar activity in our own solar system, from the Sun to the outer planets. Through precision calibration via an orbiting beacon, and exquisite foreground characterization, FARSIDE would also measure the Dark Ages global 21-cm signal at redshifts from 50-100. It will also be a pathfinder for a larger 21-cm power spectrum instrument by carefully measuring the foreground with high dynamic range., Comment: 50 pages, NASA Probe final study report. arXiv admin note: text overlap with arXiv:1907.05407

This white paper submitted for the 2020 Canadian Long-Range Planning process (LRP2020) presents the prospects for Canada and the Square Kilometre Array (SKA) from 2020-2030, focussing on the first phase of the project (SKA1) scheduled to begin construction early in the next decade. SKA1 will make transformational advances in our understanding of the Universe across a wide range of fields, and Canadians are poised to play leadership roles in several. Canadian key SKA technologies will ensure a good return on capital investment in addition to strong scientific returns, positioning Canadian astronomy for future opportunities well beyond 2030. We therefore advocate for Canada's continued scientific and technological engagement in the SKA from 2020-2030 through participation in the construction and operations phases of SKA1., Comment: 14 pages, 4 figures, 2020 Canadian Long-Range Plan (LRP2020) white paper

We investigate the effects of Population III (Pop III) stars and their remnants on the cosmological 21-cm global signal. By combining a semi-analytic model of Pop III star formation with a global 21-cm simulation code, we investigate how X-ray and radio emission from accreting Pop III black holes may affect both the timing and depth of the 21-cm absorption feature that follows the initial onset of star formation during the Cosmic Dawn. We compare our results to the findings of the EDGES experiment, which has reported the first detection of a cosmic 21-cm signal. In general, we find that our fiducial Pop III models, which have peak star formation rate densities of $\sim 10^{-4}$ M$_\odot$ yr$^{-1}$ Mpc$^{-3}$ between $z \sim 10$ and $z \sim 15$, are able to match the timing of the EDGES signal quite well, in contrast to models that ignore Pop III stars. To match the unexpectedly large depth of the EDGES signal without recourse to exotic physics, we vary the parameters of emission from accreting black holes (formed as Pop III remnants) including the intrinsic strength of X-ray and radio emission as well as the local column density of neutral gas. We find that models with strong radio emission and relatively weak X-ray emission can self-consistently match the EDGES signal, though this solution requires fine-tuning. We are only able to produce signals with sharp features similar to the EDGES signal if the Pop~III IMF is peaked narrowly around $140 \, M_\odot$., Comment: 11 pages, 8 figures, submitted to MNRAS

This chapter describes the astrophysics encoded by the 21-cm background. We begin with a brief introduction to the radiative transfer and ionization chemistry relevant to the high-z intergalactic medium. Then, we will provide a review of the most plausible sources of ionization and heating in the early Universe. Finally, we will explore a variety of current 21-cm predictions, and illustrate the sensitivity of the global 21-cm signal and power spectrum to parameters of interest., Comment: To appear as a book chapter in "The Cosmic 21-cm Revolution: Charting the first billion years of our Universe", Ed Andrei Mesinger (Bristol: IOP Publishing Ltd) AAS-IOP ebooks http://www.iopscience.org/books/aas

Despite significant progress both observationally and theoretically, the origin of high-ionization nebular HeII emission in galaxies dominated by stellar photoionization remains unclear. Accretion-powered radiation from high-mass X-ray binaries (HMXBs) is still one of the leading proposed explanations for the missing $\mathrm{He^+}$-ionizing photons, but this scenario has yet to be conclusively tested. In this paper, we present nebular line predictions from a grid of photoionization models with input SEDs containing the joint contribution of both stellar atmospheres and a multi-color disk model for HMXBs. This grid demonstrates that HMXBs are inefficient producers of the photons necessary to power HeII, and can only boost this line substantially in galaxies with HMXB populations large enough to power X-ray luminosities of $10^{42}$ erg/s per unit star formation rate (SFR). To test this, we assemble a sample of eleven low-redshift star-forming galaxies with high-quality constraints on both X-ray emission from Chandra and HeII emission from deep optical spectra, including new observations with the MMT. These data reveal that the HMXB populations of these nearby systems are insufficient to account for the observed HeII strengths, with typical X-ray luminosities or upper limits thereon of only $10^{40}$-$10^{41}$ erg/s per SFR. This indicates that HMXBs are not the dominant source of $\mathrm{He^+}$ ionization in these metal-poor star-forming galaxies. We suggest that the solution may instead reside in revisions to stellar wind predictions, softer X-ray sources, or very hot products of binary evolution at low metallicity., Comment: 18 pages, 9 figures, published in MNRAS

In this white paper, we lay out a US roadmap for high-redshift 21 cm cosmology (30 < z < 6) in the 2020s. Beginning with the currently-funded HERA and MWA Phase II projects and advancing through the decade with a coordinated program of small-scale instrumentation, software, and analysis projects targeting technology development, this roadmap incorporates our current best understanding of the systematics confronting 21 cm cosmology into a plan for overcoming them, enabling next-generation, mid-scale 21 cm arrays to be proposed late in the decade. Submitted for consideration by the Astro2020 Decadal Survey Program Panel for Radio, Millimeter, and Submillimeter Observations from the Ground as a Medium-Sized Project., Comment: 10 pages (plus a cover page and references), 6 figures. Submitted as a APC White Paper for Astro2020

In addition to being a probe of Cosmic Dawn and Epoch of Reionization astrophysics, the 21cm line at $z>6$ is also a powerful way to constrain cosmology. Its power derives from several unique capabilities. First, the 21cm line is sensitive to energy injections into the intergalactic medium at high redshifts. It also increases the number of measurable modes compared to existing cosmological probes by orders of magnitude. Many of these modes are on smaller scales than are accessible via the CMB, and moreover have the advantage of being firmly in the linear regime (making them easy to model theoretically). Finally, the 21cm line provides access to redshifts prior to the formation of luminous objects. Together, these features of 21cm cosmology at $z>6$ provide multiple pathways toward precise cosmological constraints. These include the "marginalizing out" of astrophysical effects, the utilization of redshift space distortions, the breaking of CMB degeneracies, the identification of signatures of relative velocities between baryons and dark matter, and the discovery of unexpected signs of physics beyond the $\Lambda$CDM paradigm at high redshifts., Comment: Science white paper submitted to Decadal 2020 survey

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. 21-cm observations provide a unique window into this epoch and are thus critical to advancing first star and black hole science in the next decade., Comment: White paper submitted to the Astro2020 Decadal Survey

The Dark Ages are the period between the last scattering of the cosmic microwave background and the appearance of the first luminous sources, spanning approximately 1100 < z < 30. The only known way to measure fluctuations in this era is through the 21-cm line of neutral hydrogen. Such observations have enormous potential for cosmology, because they span a large volume while the fluctuations remain linear even on small scales. Observations of 21-cm fluctuations during this era can therefore constrain fundamental aspects of our Universe, including inflation and any exotic physics of dark matter. While the observational challenges to these low-frequency 21-cm observations are enormous, especially from the terrestrial environment, they represent an important goal for cosmology., Comment: 10 pages, 4 figures

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., Comment: 8 pages, 4 figures

The early phases of galaxy formation constitute one of the most exciting frontiers in astrophysics. It is during this era that the first luminous sources reionize the intergalactic medium - the moment when structure formation affects every baryon in the Universe. Here we argue that we will obtain a complete picture of this era by combining observations of galaxies with direct measurements of the reionization process: the former will provide a detailed understanding of bright sources, while the latter will constrain the (substantial) faint source population. We further describe how optimizing the comparison of these two measurements requires near-infrared galaxy surveys covering large volumes and retaining redshift information and also improvements in 21-cm analysis, moving those experiments into the imaging regime., Comment: 9 pages, 3 figures

Cosmic dawn and the Epoch of Reionization (EoR) are among the least explored observational eras in cosmology: a time at which the first galaxies and supermassive black holes formed and reionized the cold, neutral Universe of the post-recombination era. With current instruments, only a handful of the brightest galaxies and quasars from that time are detectable as individual objects, due to their extreme distances. Fortunately, a multitude of multi-wavelength intensity mapping measurements, ranging from the redshifted 21 cm background in the radio to the unresolved X-ray background, contain a plethora of synergistic information about this elusive era. The coming decade will likely see direct detections of inhomogenous reionization with CMB and 21 cm observations, and a slew of other probes covering overlapping areas and complementary physical processes will provide crucial additional information and cross-validation. To maximize scientific discovery and return on investment, coordinated survey planning and joint data analysis should be a high priority, closely coupled to computational models and theoretical predictions., Comment: 5 pages, 1 figure, submitted to the Astro2020 Decadal Survey Science White Paper call

The cosmic dawn and epoch of reionization mark the time period in the universe when stars, galaxies, and blackhole seeds first formed and the intergalactic medium changed from neutral to an ionized one. Despite substantial progress with multi-wavelength observations, astrophysical process during this time period remain some of the least understood with large uncertainties on our existing models of galaxy, blackhole, and structure formation. This white paper outlines the current state of knowledge and anticipated scientific outcomes with ground and space-based astronomical facilities in the 2020s. We then propose a number of scientific goals and objectives for new facilities in late 2020s to mid 2030s that will lead to definitive measurements of key astrophysical processes in the epoch of reionization and cosmic dawn., Comment: Science White paper submitted to Astro2020 Decadal Survey

The Experiment to Detect the Global Epoch of Reionization Signature (EDGES) recently reported a strong 21-cm absorption signal relative to the cosmic microwave background at $z \sim 18$. While its anomalous amplitude may indicate new physics, in this work we focus on the timing of the signal, as it alone provides an important constraint on galaxy formation models. Whereas rest-frame ultraviolet luminosity functions (UVLFs) over a broad range of redshifts are well fit by simple models in which galaxy star formation histories track the assembly of dark matter halos, we find that these same models, with reasonable assumptions about X-ray production in star-forming galaxies, cannot generate a narrow absorption trough at $z \sim 18$. If verified, the EDGES signal therefore requires the fundamental inputs of galaxy formation models to evolve rapidly at $z \gtrsim 10$. Unless extremely faint sources residing in halos below the atomic cooling threshold are responsible for the EDGES signal, star formation in $\sim 10^8$-$10^{10} \ M_{\odot}$ halos must be more efficient than expected, implying that the faint-end of the UVLF at $M_{\mathrm{UV}} \lesssim -12$ must steepen at the highest redshifts. This steepening provides a concrete test for future galaxy surveys with the James Webb Space Telescope and ongoing efforts in lensed fields, and is required regardless of whether the amplitude of the EDGES signal is due to new cooling channels or a strong radio background in the early Universe. However, the radio background solution requires that galaxies at $z > 15$ emit 1-2 GHz photons with an efficiency $\sim 10^3$ times greater than local star-forming galaxies, posing a challenge for models of low-frequency photon production in the early Universe., Comment: 13 pages, 5 figures, submitted to MNRAS

We investigate the effects of Population III stars on the sky-averaged 21-cm background radiation, which traces the collective emission from all sources of ultraviolet and X-ray photons before reionization is complete. While UV photons from PopIII stars can in principle shift the onset of radiative coupling of the 21-cm transition -- and potentially reionization -- to early times, we find that the remnants of PopIII stars are likely to have a more discernible impact on the 21-cm signal than PopIII stars themselves. The X-rays from such sources preferentially heat the IGM at early times, which elongates the epoch of reheating and results in a more gradual transition from an absorption signal to emission. This gradual heating gives rise to broad, asymmetric wings in the absorption signal, which stand in contrast to the relatively sharp, symmetric signals that arise in models treating PopII sources only. A stronger signature of PopIII, in which the position of the absorption minimum becomes inconsistent with PopII-only models, requires extreme star-forming events that may not be physically plausible, lending further credence to predictions of relatively high frequency absorption troughs, $\nu_{\min} \sim 100$ MHz. As a result, though the trough location alone may not be enough to indicate the presence of PopIII, the asymmetric wings should arise even if only a few PopIII stars form in each halo before the transition to PopII star formation occurs, provided that the PopIII IMF is sufficiently top-heavy and at least some PopIII stars form in binaries., Comment: 17 pages, 8 figures, submitted to MNRAS

We present a semi-analytic model of star formation in the early universe, beginning with the first metal-free stars. By employing a completely feedback-limited star formation prescription, stars form at maximum efficiency until the self-consistently calculated feedback processes halt formation. We account for a number of feedback processes including a meta-galactic Lyman-Werner background, supernovae, photoionization, and chemical feedback. Halos are evolved combining mass accretion rates found through abundance matching with our feedback-limited star formation prescription, allowing for a variety of Population III (Pop III) initial mass functions (IMFs). We find that, for a number of models, massive Pop III star formation can continue on until at least $z \sim 20$ and potentially past $z \sim 6$ at rates of around $10^{-4}$ to $10^{-5}$ M$_\odot$ yr$^{-1}$ Mpc$^{-3}$, assuming these stars form in isolation. At this point Lyman-Werner feedback pushes the minimum halo mass for star formation above the atomic cooling threshold, cutting off the formation of massive Pop III stars. We find that, in most models, Pop II and Pop III star formation co-exist over cosmological time-scales, with the total star formation rate density and resulting radiation background strongly dominated by the former before Pop III star formation finally ends. These halos form at most $\sim 10^3$ M$_\odot$ of massive Pop III stars during this phase and typically have absolute magnitudes in the range of $M_\text{AB} = -5 $ to $ -10$. We also briefly discuss how future observations from telescopes such as JWST or WFIRST and 21-cm experiments may be able to constrain unknown parameters in our model such as the IMF, star formation prescription, or the physics of massive Pop III stars., Comment: 16 pages, 13 figures, submitted to MNRAS

The redshifted 21-cm monopole is expected to be a powerful probe of the epoch of the first stars and galaxies ($10