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ABSTRACT: 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 ∼10−4 M⊙ yr−1 Mpc−3 between z ∼ 10 and z ∼ 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 $140 \, \mathrm{M}_\odot$.

The Experiment to Detect the Global Epoch of Reionization Signature (EDGES)recently reported a strong 21-cm absorption signal relative to the cosmicmicrowave background at $z \sim 18$. While its anomalous amplitude may indicatenew physics, in this work we focus on the timing of the signal, as it aloneprovides an important constraint on galaxy formation models. Whereas rest-frameultraviolet luminosity functions (UVLFs) over a broad range of redshifts arewell fit by simple models in which galaxy star formation histories track theassembly of dark matter halos, we find that these same models, with reasonableassumptions about X-ray production in star-forming galaxies, cannot generate anarrow absorption trough at $z \sim 18$. If verified, the EDGES signaltherefore requires the fundamental inputs of galaxy formation models to evolverapidly at $z \gtrsim 10$. Unless extremely faint sources residing in halosbelow the atomic cooling threshold are responsible for the EDGES signal, starformation in $\sim 10^8$-$10^{10} \ M_{\odot}$ halos must be more efficientthan expected, implying that the faint-end of the UVLF at $M_{\mathrm{UV}}\lesssim -12$ must steepen at the highest redshifts. This steepening provides aconcrete test for future galaxy surveys with the James Webb Space Telescope andongoing efforts in lensed fields, and is required regardless of whether theamplitude of the EDGES signal is due to new cooling channels or a strong radiobackground in the early Universe. However, the radio background solutionrequires 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 challengefor models of low-frequency photon production in the early Universe.

Detecting the 21-cm signal at |$z\gtrsim 6$| will reveal insights into the properties of the first galaxies responsible for driving reionization. To extract this information, we perform parameter inference with three-dimensional simulations of the 21-cm signal embedded 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 is much more uncertain than that of the underlying halo-mass function (HMF), we typically parametrize and model the former while fixing the latter. However, doing so may bias our inference of the galaxy properties. In this work, we explore the consequences of assuming an incorrect HMF and quantify the relative biases on our inferred astrophysical model parameters when considering the wrong HMF. We then relax this assumption by constructing a generalized five parameter HMF model and simultaneously recover it with our underlying astrophysical model. For this, we use 21cmfast and perform simulation-based inference using marginal neural ratio estimation to learn the likelihood-to-evidence ratio with Swyft. Using a mock 1000-h observation of the 21-cm power spectrum from the forthcoming Square Kilometre Array, conservatively assuming foreground wedge avoidance, we find that assuming the incorrect HMF can bias the recovered astrophysical parameters by up to |$\sim 3{\!-\!}4\sigma$| even when including independent information from observed luminosity functions. Using our generalized HMF model, although we recover our astrophysical parameters with a factor of |$\sim 2{\!-\!}4$| larger marginalized uncertainties, the constraints are unbiased, agnostic to the underlying HMF and therefore more conservative. [ABSTRACT FROM AUTHOR]

We present a semi-analytic model of star formation in the early universe,beginning with the first metal-free stars. By employing a completelyfeedback-limited star formation prescription, stars form at maximum efficiencyuntil the self-consistently calculated feedback processes halt formation. Weaccount for a number of feedback processes including a meta-galacticLyman-Werner background, supernovae, photoionization, and chemical feedback.Halos are evolved combining mass accretion rates found through abundancematching with our feedback-limited star formation prescription, allowing for avariety of Population III (Pop III) initial mass functions (IMFs). We findthat, for a number of models, massive Pop III star formation can continue onuntil 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 starsform in isolation. At this point Lyman-Werner feedback pushes the minimum halomass for star formation above the atomic cooling threshold, cutting off theformation of massive Pop III stars. We find that, in most models, Pop II andPop III star formation co-exist over cosmological time-scales, with the totalstar formation rate density and resulting radiation background stronglydominated by the former before Pop III star formation finally ends. These halosform at most $\sim 10^3$ M$_\odot$ of massive Pop III stars during this phaseand typically have absolute magnitudes in the range of $M_\text{AB} = -5 $ to $-10$. We also briefly discuss how future observations from telescopes such asJWST or WFIRST and 21-cm experiments may be able to constrain unknownparameters in our model such as the IMF, star formation prescription, or thephysics of massive Pop III stars.

Motivated by recent progress in studies of the high-$z$ Universe, we build anew model for the global 21-cm signal that is explicitly calibrated tomeasurements of the galaxy luminosity function (LF) and further tuned to matchthe Thomson scattering optical depth of the cosmic microwave background,$\tau_e$. Assuming that the $z \lesssim 8$ galaxy population can be smoothlyextrapolated to higher redshifts, the recent decline in best-fit values of$\tau_e$ and the inefficient heating induced by X-ray binaries (HMXBs; thepresumptive sources of the X-ray background at high-$z$) imply that theentirety of cosmic reionization and reheating occurs at redshifts $z \lesssim12$. In contrast to past global 21-cm models, whose $z \sim 20$ ($u \sim 70$MHz) absorption features and strong $\sim 25$ mK emission features were drivenlargely by the assumption of efficient early star-formation and X-ray heating,our new fiducial model peaks in absorption at $u \sim 110$ MHz at a depth of$\sim -160$ mK and has a negligible emission component. As a result, a strongemission signal would provide convincing evidence that HMXBs are not the onlydrivers of cosmic reheating. Shallow absorption troughs should accompany strongheating scenarios, but could also be caused by a low escape fraction ofLyman-Werner photons. Generating signals with troughs at $u \lesssim 95$ MHzrequires a floor in the star-formation efficiency in halos below $\sim 10^{9}M_{\odot}$, which is equivalent to steepening the faint-end of the galaxy LF.These findings demonstrate that the global 21-cm signal is a powerfulcomplement to current and future galaxy surveys and efforts to betterunderstand the interstellar medium in high-$z$ galaxies.

Early observations with JWST indicate an overabundance of bright galaxies at redshifts z ≳ 10 relative to Hubble -calibrated model predictions. More puzzling still is the apparent lack of evolution in the abundance of such objects between z ∼ 9 and the highest redshifts yet probed, z ∼ 13–17. In this study, we first show that, despite a poor match with JWST luminosity functions (LFs), semi-empirical models calibrated to rest-ultraviolet LFs and colours at 4 ≲ z ≲ 8 are largely consistent with constraints on the properties of individual JWST galaxies, including their stellar masses, ages, and 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 haloes. However, this solution to the abundance problem introduces tension elsewhere: because it relies on the upscattering of low-mass haloes 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 and dust production could be more efficient than expected in galaxies at z ≳ 10. [ABSTRACT FROM AUTHOR]

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 3D 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 21 cm  fast 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. [ABSTRACT FROM AUTHOR]

One approach to extracting the global 21-cm signal from total-power measurements at low radio frequencies is to parametrize the different contributions to the data and then fit for these parameters. We examine parametrizations of the 21-cm signal itself, and propose one based on modelling the Ly α background, intergalactic medium temperature and hydrogen ionized fraction using tanh functions. This captures the shape of the signal from a physical modelling code better than an earlier parametrization based on interpolating between maxima and minima of the signal, and imposes a greater level of physical plausibility. This allows less biased constraints on the turning points of the signal, even though these are not explicitly fit for. Biases can also be alleviated by discarding information which is less robustly described by the parametrization, for example by ignoring detailed shape information coming from the covariances between turning points or from the high-frequency parts of the signal, or by marginalizing over the high-frequency parts of the signal by fitting a more complex foreground model. The fits are sufficiently accurate to be usable for experiments gathering 1000 h of data, though in this case it may be important to choose observing windows which do not include the brightest areas of the foregrounds. Our assumption of pointed, single-antenna observations and very broad-band fitting makes these results particularly applicable to experiments such as the Dark Ages Radio Explorer, which would study the global 21-cm signal from the clean environment of a low lunar orbit, taking data from the far side. [ABSTRACT FROM AUTHOR]